rotation of earth essay

• Celestial Bodies
• Rotation And Revolution

Rotation and Revolution

We have heard the terms rotation and revolution associated with celestial objects. Let us know more about rotation and revolution and the difference between rotation and revolution.

What is Rotation?

What is revolution.

Rotation of the Earth

Earth rotates on its axis from west to east, and the Sun and the Moon appear to move from east to west across the sky. The spinning of the Earth around its axis is called ‘rotation’. The axis has an angle of 23 1/2º and is perpendicular to the plane of Earth’s orbit. This means the Earth is tilted on its axis, and because of this tilt, the northern and southern hemispheres lean in a direction away from the Sun. The rotation of the Earth divides it into a lit-up half and a dark half, which gives rise to day and night. The direction of the Earth’s rotation depends on the direction of viewing. When viewed looking down from the North Pole, Earth spins counterclockwise. On the contrary, when viewed looking down from the south pole, the earth spins in the clockwise direction.

Importance of Earth Rotation

Some of the importance of the rotation of the Earth are listed below:

• The Earth’s rotation creates the diurnal cycle of lightness and darkness, temperature and humidity changes.
• The Earth’s rotation causes tides in the oceans and seas.

Revolution of the Earth

The movement of the Earth around the Sun in a fixed path is called a revolution. The Earth revolves from west to east, i.e., in the anticlockwise direction. The one revolution of the Earth around the Sun takes around one year or precisely 365.242 days. The revolution speed of the earth is 30 km/s -1 .

Importance of Revolution

• Revolution causes seasons.
• Revolution creates perihelion and aphelion. Perihelion occurs when the Earth is closest to the Sun. Aphelion occurs when the Earth is far from the Sun.
• Revolution has a direct influence on the varied length of day and night time. The duration of days and nights are the same at the equator. This is known as the equinox. The duration of days and nights vary in the Northern and Southern hemispheres. This is known as solstices.

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Rotation and Revolution of Planets

Difference Between Rotation and Revolution

The table given below provides the basic differences between rotation and revolution.

Watch the video below to understand what would happen if the Earth stopped spinning

Frequently Asked Questions – FAQs

What is meant by rotation, what is meant by revolution, do earthquakes affect the earth’s rotation.

Using the data from the Indonesian Earthquake, NASA calculated that the earthquake affected Earth’s rotation, decreased the length of the day, shifted the North Pole by centimetres and slightly changed the planet’s shape. The earthquake that created a huge tsunami also changed the Earth’s rotation.

Has the Earth’s rotation ever speeded up in the past?

Probably, but in the last 900 million years, any speed-ups have been superimposed on a more or less steady slow down in spin rate. Even today, we can identify how the Earth’s rotation rate changes fast and slow by milliseconds per day, depending on how the mass distribution of the Earth and its atmosphere change from earthquakes and the movement of water and air.

Is it possible to slow down the Earth’s rotation artificially?

It is said that humans have made a measurable change in the Earth’s rotation period by several microseconds by accumulating vast reservoirs with trillions of tons of water. There may be a weak interaction between this activity and the weather over the long term, and possibly even in the strength of the Earth’s magnetic field, which is very sensitive to the Earth’s rotation rate.

What is the angle made by the axis of the earth with its orbital plane?

The angle made by the axis of the earth, which is an imaginary line with the orbital plane, is 66 degrees.

What is an equinox?

An equinox is defined as the time when the sun crosses the celestial equator such that the length of the day and night are equal. Every year has two equinoxes. Also, the length of nights at latitudes L degree north and L degree south are equal.

In this video, we have provided important questions and concepts of Rotation for JEE Advanced 2023

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Earth and Space Science

Earth’s rotation and revolution explained.

• The Albert Team
• Last Updated On: March 6, 2024

Have you ever wondered what keeps the day transitioning into the night and back again? This daily cycle is all thanks to Earth’s rotation, which affects everything from the time zones we follow to the weather patterns we experience. This post will review Earth’s rotation and revolution and illustrate how they impact our daily lives. Understanding Earth’s motion isn’t just about knowing why the sun rises and sets but also about comprehending a basic principle that underpins life on our planet.

What We Review

Rotation vs Revolution: What’s the Difference?

Often, the terms “rotation” and “revolution” are used interchangeably. However, they describe distinct movements of Earth that profoundly affect our environment. So, what is the difference between rotation and revolution?

Rotation refers to the Earth spinning around its axis. Imagine an invisible line running through the Earth from the North Pole to the South Pole; this is Earth’s axis. Every 24 hours, Earth completes one full rotation, which is why we have day and night.

Revolution , on the other hand, is the Earth’s journey around the sun. This path isn’t a perfect circle but rather an elliptical orbit, and it takes about 365.25 days to complete. This revolution is the reason we have seasons.

Understanding Earth’s Rotation

Earth’s rotation is how our planet spins around its axis. Picture Earth like a giant top spinning in space, completing a full turn every 24 hours . This rotation gives us the cycle of day and night—daytime when your location faces the sun and nighttime when it turns away.

Historically, the realization that Earth rotates came from observing the sun’s and stars’ apparent movement. Ancient astronomers initially thought that Earth was the center of the universe and everything revolved around it. However, figures like Copernicus and Galileo challenged this view, providing evidence that the Earth spins on its axis. The Foucault pendulum , introduced in the 19th century, offered direct, observable proof of the Earth’s rotation. As you can see , it swings in a consistent direction while the Earth turns beneath it.

Earth’s Revolution Around the Sun

Earth’s revolution around the sun describes our planet’s yearly journey in space. This orbit isn’t a perfect circle; it’s slightly stretched, causing Earth’s distance from the sun to vary throughout the year.

This elliptical path brings us closer to the sun at perihelion in early January and farther away at aphelion in early July. This slight variation doesn’t significantly impact our climate. The primary influence on seasons is Earth’s axial tilt. As Earth revolves, its tilted axis leads to varying sunlight angles across the globe, resulting in seasonal changes. For example, when the Northern Hemisphere tilts towards the sun, it experiences summer. Meanwhile, the Southern Hemisphere faces away, experiencing winter, and vice versa.

Earth completes this orbit in about 365.25 days, a fact that defines the length of our year. This is why we have a leap day every four years to keep our calendar in sync with Earth’s orbit.

Real-Life Implications of Earth’s Rotation and Revolution

Impact on navigation.

Historically, Earth’s rotation has been crucial for navigation, especially before the advent of modern technology. Mariners relied on the position of the stars in the night sky to find their way. The postition of the stars change predictably as the Earth rotates, an d seafarers use this to determine their latitude and longitude at sea. Even today, with GPS technology, understanding Earth’s rotation is vital as it influences the positioning of satellites and the accuracy of GPS signals.

Influence on Timekeeping

The concept of a day is directly tied to Earth’s rotation. Our 24-hour day is Earth’s period to complete one full rotation on its axis. Time zones are determined by the Earth’s rotation relative to the sun, dividing the planet into different time zones based on longitudinal degrees. As Earth rotates, different parts of the world experience sunrise and sunset at different times, leading to various time zones.

Effects on Technology

Earth’s rotation and revolution have significant impacts on technology, particularly in the fields of telecommunications and space exploration. Satellite communication systems must account for Earth’s movement to maintain alignment and provide consistent coverage. Astronomical observatories and space launch facilities must also consider Earth’s rotation when planning observations and launches.

Frequently Asked Questions About Earth’s Rotation

What causes the earth’s rotation.

Earth’s rotation is due to how it formed and the conservation of angular momentum. When our solar system formed, the cloud of gas and dust that became the Earth was spinning, and as it condensed to form the planet, it retained this spinning motion.

What is Earth’s rotational speed?

Earth’s rotational speed is fastest at the equator, reaching about 1,670 kilometers per hour (or around 1,040 miles per hour). It decreases as you move toward the poles, where it is effectively zero. This variation affects phenomena such as weather patterns and the Coriolis effect, which influences the direction of ocean currents and wind.

Why can’t we feel Earth’s rotation?

Earth’s rotation is very consistent and doesn’t change speed abruptly, so we don’t feel any acceleration or deceleration. Plus, the Earth is so large that its curvature is difficult to perceive, making the rotation feel even less apparent.

What would happen if the Earth stopped rotating?

If the Earth stopped rotating, it would be catastrophic. One side of the Earth would be in constant sunlight, while the other side would be in perpetual darkness. This would cause extreme temperature changes, disrupt weather patterns, and eliminate the day-night cycle as we know it.

How does Earth’s revolution around the sun influence seasons?

The seasons are influenced by Earth’s axis tilt and elliptical orbit around the sun. As Earth revolves, different parts of the planet receive varying amounts of sunlight, leading to seasonal changes.

Why is a year 365.25 days long and not exactly 365 days?

A year is 365.25 days long. This is the time it takes for Earth to complete one full orbit around the sun. The extra 0.25 day adds on over four years to an additional day. So, we have a leap year every four years to keep our calendar in sync with Earth’s orbit.

How does Earth’s elliptical orbit affect our distance from the sun?

Earth’s elliptical orbit means its distance from the sun varies throughout the year. We are closest to the sun (perihelion) around early January and farthest (aphelion) around early July, but this difference in distance doesn’t significantly affect our planet’s climate.

In this post, we’ve reviewed how Earth’s rotation and its orbit around the sun shape our world. The rotation of Earth creates the cycle of day and night, setting a natural rhythm for all life forms. Even though we don’t feel it, this rotation influences our weather and the environment every single day.

Then there’s Earth’s journey around the sun, which, combined with its tilt, gives us the seasons. This revolution impacts everything from the weather we experience to the crops farmers grow.

Earth’s spinning on its axis and its path around the sun do more than just mark time; they define our days, bring about the seasons, and shape the world we call home.

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What is the Rotation of the Earth?

What if someone were to tell you that at any given moment, you were traveling at speeds well in excess of the speed of sound? You might think they were crazy, given that – as best as you could tell – you were standing on solid ground, and not in the cockpit of a supersonic jet. Nevertheless, the statement is correct. At any given moment, we are all moving at a speed of about 1,674 kilometers an hour, thanks to the Earth’s rotation,

By definition, the Earth’s rotation is the amount of time that it takes to rotate once on its axis. This is, apparently, accomplished once a day – i.e. every 24 hours. However, there are actually two different kinds of rotation that need to be considered here. For one, there’s the amount of time it take for the Earth to turn once on its axis so that it returns to the same orientation compared to the rest of the Universe. Then there’s how long it takes for the Earth to turn so that the Sun returns to the same spot in the sky.

Solar vs. Sidereal Day:

As we all know, it takes exactly 24 hours for the Sun to return to the same spot in the sky, which would seem obvious. 24 hours is what we think of as being a complete day, and the time it takes to transition from day to night and back again. But in truth, it actually takes the Earth 23 hours, 56 minutes, and 4.09 seconds to turn rotate once on its axis compared to the background stars.

Why the difference? Well, that would be because the Earth is orbiting around the Sun, completing one orbit in just over 365 days. If you divide 24 hours by 365 days, you’ll see that you’re left with about 4 minutes per day. In other words, the Earth rotates on its axis, but it’s also orbiting around the Sun, so the Sun’s position in the sky catches up by 4 minutes each day.

The amount of time it takes for the Earth to rotate once on its axis is known as a sidereal day – which is 23.9344696 hours. Because this type of day-measurement is based on the Earth’s position relative to the stars, astronomers use it as a time-keeping system to keep track of where stars will appear in the night sky, mainly so they will know which direction to point their telescopes in.

The amount of time it takes for the Sun to return to the same spot in the sky is called a solar day , which is 24 hours. However, this varies through the year, and the accumulated effect produces seasonal deviations of up to 16 minutes from the average. This is caused by two factors, which include the Earth’s elliptical orbit around the Sun and it’s axial tilt.

Orbit and Axial Tilt:

As Johannes Kepler stated in his Astronomia Nova (1609), the Earth and Solar planets do not rotate about the Sun in perfect circles. This is known as Kepler’s First Law , which states that “the orbit of a planet about the Sun is an ellipse with the Sun’s center of mass at one focus”. At perihelion (i.e. its closest) it is 147,095,000 km (91,401,000 mi) from the Sun; whereas at aphelion, it is 152,100,000 km (94,500,000 mi).

This change in distance means that the Earth’s orbital speed increases when it is closest to the Sun. While its speed averages out to about 29.8 km/s (18.5 mps) or 107,000 km/h (66487 mph), it actually ranges by a full km per second during the course of the year – between 30.29 km/s and 29.29 km/s (109,044 – 105,444 km/h; 67,756.8 – 65,519.864 mph).

At this rate, it takes the Sun the equivalent of 24 hours – i.e. one solar day – to complete a full rotation about the Earth’s axis and return to the meridian (a point on the globe that runs from north to south through the poles). Viewed from the vantage point above the north poles of both the Sun and Earth, Earth orbits in a counterclockwise direction about the Sun.

This Earth’s rotation around the Sun, or the precession of the Sun through the equinoxes, is the reason a year lasts approximately 365.2 days. It is also for this reason that every four years, an extra day is required (a February 29th during every Leap Year). Also, Earth’s rotation about the Sun is subject to a slight eccentricity of (0.0167°), which means that it is periodically closer or farther from the Sun at certain times of the year.

Earth’s axis is also inclined at approximately 23.439° towards the ecliptic. This means that when the Sun crosses the equator at both equinoxes, it’s daily shift relative to the background stars is at an angle to the equator. In June and December, when the Sun is farthest from the celestial equator, a given shift along the ecliptic corresponds to a large shift at the equator.

So apparent solar days are shorter in March and September than in June or December. In northern temperate latitudes, the Sun rises north of true east during the summer solstice, and sets north of true west, reversing in the winter. The Sun rises south of true east in the summer for the southern temperate zone, and sets south of true west.

Rotational Velocity:

As stated earlier, the Earth’s is spinning rather rapidly. In fact, scientists have determined that Earth’s rotational velocity at the equator is 1,674.4 km/h. This means that just by standing on the equator, a person would already be traveling at a speed in excess of the speed of sound in a circle. But much like measuring a day, the Earth’s rotation can be measured in one of two different ways.

Earth’s rotation period relative to the fixed stars is known as a “stellar day”, which is 86,164.098903691 seconds of mean solar time (or 23 hours, 56 minutes and 4.0989 seconds). Earth’s rotation period relative to the precessing or moving mean vernal equinox, meanwhile, is 23 hours 56 minutes and 4.0905 seconds of mean solar time. Not a major difference, but a difference nonetheless.

However, the planet is slowing slightly with the passage of time, due to the tidal effects the Moon has on Earth’s rotation. Atomic clocks show that a modern day is longer by about 1.7 milliseconds than a century ago, slowly increasing the rate at which UTC is adjusted by leap seconds. The Earth’s rotation also goes from the west towards east, which is why the Sun rises in the east and sets in the west.

Earth’s Formation:

Another interesting thing about the Earth’s rotation is how it all got started. Basically, the planet’s rotation is due to the angular momentum of all the particles that came together to create our planet 4.6 billion years ago. Before that, the Earth, the Sun and the rest of the Solar System were part of a giant molecular cloud of hydrogen, helium, and other heavier elements.

As the cloud collapsed down, the momentum of all the particles set the cloud spinning. The current rotation period of the Earth is the result of this initial rotation and other factors, including tidal friction and the hypothetical impact of Theia – a collision with a Mars-sized object that is thought to have taken place approx. 4.5 billion years ago and formed the Moon.

This rapid rotation is also what gives the Earth it’s shape, flattening it out into an oblate spheroid (or what looks like a squished ball). This special shape of our planet means that points along the equator are actually further from the center of the Earth than at the poles.

History of Study:

In ancient times, astronomers naturally believed that the Earth was a fixed body in the cosmos, and that the Sun, the Moon, the planets and stars all rotating around it. By classical antiquity, this became formalized into cosmological systems by philosophers and astronomers like Aristotle and Ptolemy – which later came to be known as the Ptolemaic Model (or Geocentric Model ) of the universe.

However, there were those during Antiquity that questioned this convention. One point of contention was the fact that the Earth was not only fixed in place, but that it did not rotate. For instance, Aristarchus of Samos (ca. 310 – 230 BCE) published writings on the subject that were cited by his contemporaries (such as Archimedes). According to Archimedes, Aristarchus espoused that the Earth revolved around the Sun and that the universe was many times greater than previously thought.

And then there was Seleucis of Seleucia (ca. 190 – 150 BCE), a Hellenistic astronomer who lived in the Near-Eastern Seleucid empire. Seleucus was a proponent of the heliocentric system of Aristarchus, and may have even proven it to be true by accurately computing planetary positions and the revolution of the Earth around the Earth-Moon ‘center of mass’.

The geocentric model of the universe would also be challenged by medieval Islamic and Indian scholars. For instance, In 499 CE, Indian astronomer Aaryabhata published his magnum opus Aryabhatiya , in which he proposed a model where the Earth was spinning on its axis and the periods of the planets were given with respect to the Sun.

The 10th-century Iranian astronomer Abu Sa’id al-Sijzi contradicted the Ptolemaic model by asserting that the Earth revolved on its axis, thus explaining the apparent diurnal cycle and the rotation of the stars relative to Earth. At about the same time, Abu Rayhan Biruni  973 – 1048) discussed the possibility of Earth rotating about its own axis and around the Sun – though he considered this a philosophical issue and not a mathematical one.

At the Maragha and the Ulugh Beg (aka. Samarkand) Observatory, the Earth’s rotation was discussed by several generations of astronomers between the 13th and 15th centuries, and many of the arguments and evidence put forward resembled those used by Copernicus. It was also at this time that Nilakantha Somayaji published the Aryabhatiyabhasya (a commentary on the Aryabhatiya ) in which he advocated a partially heliocentric planetary model. This was followed in 1500 by the Tantrasangraha, in which Somayaji incorporated the Earth’s rotation on its axis.

In the 14th century, aspects of heliocentricism and a moving Earth began to emerge in Europe. For example, French philosopher Bishop Nicole Oresme (ca. 1320-1325 to 1382 CE) discussed the possibility that the Earth rotated on its axis. However, it was Polish astronomer Nicolaus Copernicus who had the greatest impact on modern astronomy when, in 1514, he published his ideas about a heliocentric universe in a short treatise titled Commentariolus (“Little Commentary”).

Like others before him, Copernicus built on the work of Greek astronomer Atistarchus, as well as paying homage to the Maragha school and several notable philosophers from the Islamic world (see below). Intrinsic to his model was the fact that the Earth, and all the other planets, rolved around the Sun, but also that the Earth revolved on its axis and was orbited by the Moon.

In time, and thanks to scientists such as Galileo and Sir Isaac Newton , the motion and revolution of our planet would become an accepted scientific convention. With the advent of the Space Age, the deployment of satellites and atomic clocks, we have not only confirmed that it is in constant motion, but have been able to measure the its orbit and rotation with incredibly accuracy.

In short, the world has been spinning since its inception. And, contrary to what some might say, it actually is slowing down, albeit at an incredibly slow rate. But of course, by the time it slows significantly, we will have likely ceased to exist, or slipped its “surly bonds” and become an interplanetary species.

We have written many interesting articles about the motions of the Earth here at Universe Today. Here’s How Fast Does The Earth Rotate? , Earth’s Orbit Around The Sun , How Fast Does The Earth Rotate? ,  Why Does The Earth Spin? , What Would Happen If The Earth Stopped Spinning? , and What Is The Difference Between the Heliocentric and Geocentric Models Of The Solar System?

If you’d like more information on the Earth’s rotation, check out NASA’s Solar System Exploration Guide on Earth . And here’s a link to NASA’s Earth Observatory .

We’ve also recorded an episode of Astronomy Cast all about Earth . Listen here, Episode 51: Earth .

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6 Replies to “What is the Rotation of the Earth?”

Uh, no, we are not “all moving at a speed of about 1,674 kilometers an hour, thanks to the Earth’s rotation.” Our speed attributable to the Earth’s rotation varies depending upon our latitude. At the poles, a person would not be moving laterally at all; the person would just be moving in a circle very, very slowly.

And you would be moving faster with respects to the Sun at night than during the day. You’ll have to add in our orbiting velocity around the Sun at about midnight and subtract that velocity at about noon. The exact timing will vary depending on your location in a time zone and daylight savings time or standard time.

I love this topic. Another interesting implication of the different kinds of day is that the Earth rotates 366 times in a 365-day year. In fact I wrote a very similar article to yours called “Our Whirling World” for Mercury magazine all about this in 2007 (vol. 36, no. 4, pp. 28-31.). You can check it out here: http://philosophicalastronomy.blogspot.com/2007/12/our-whirling-world.html

I’m glad you added earth’s orbital velocity to its rotational velocity, since that tremendously increases our actual speed. Some reference ought to be mentioned as to the fact that the sun is also orbiting the Milky Way, which further augments our actual speed. And since the Milky Way itself is also moving along at a good clip, both headed toward Andromeda, as well as moving outward with the expanding universe, what then is our actual speed?

PS8 – you can read the answer to your suggestion regarding all the things that make up our actual total speed at the RASC page: http://calgary.rasc.ca/howfast.htm

Hey, thx! That’s a sweet site. Amazing answers, too.

Comments are closed.

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Earth's revolution and rotation around the Sun, explained

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Earth’s Rotation: Observations and Relation to Deep Interior

• Open access
• Published: 15 November 2021
• Volume 43 , pages 149–175, ( 2022 )

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Observation of the variations in the Earth’s rotation at time scales ranging from subdiurnal to multidecadal allows us to learn about its deep interior structure. We review all three types of motion of the Earth’s rotation axis: polar motion (PM), length of day variations ( \(\Delta \text {LOD}\) ) and nutations, with particular attention to how the combination of geodetic, magnetic and gravity observations provides insight into the dynamics of the liquid core, including its interactions with the mantle. Models of the Earth’s PM are able to explain most of the observed signal with the exception of the so-called Markowitz wobble. In addition, whereas the quasi-six year oscillations (SYO) observed in both \(\Delta \text {LOD}\) and PM can be explained as the result of Atmosphere, Oceans and Hydrosphere Forcing (AOH) for PM, this is not true for \(\Delta \text {LOD}\) where the subtraction of the AOH only makes the signal more visible. This points to a missing—possibly common—interpretation related to deep interior dynamics, the latter being also the most likely explanation of other oscillations in \(\Delta \text {LOD}\) on interannual timescales. Deep Earth’s structure and dynamics also have an impact on the nutations reflected in the values of the Basic Earth Parameters (BEP). We give a brief review of recent works aiming to independently evaluate the BEP and their implications for the study of deep interior dynamics.

Avoid common mistakes on your manuscript.

Article Highlights

Review of observations of the three components of Earth's rotation using magnetic, gravimetric and geodetic measurements

Discussion of the implications for the dynamics of the Earth's liquid core

Perspective on future theoretical and experimental development

1 Introduction

Variations in the Earth’s rotation can all be traced back, ultimately, to transfers of angular momentum, either between the planet and its surrounding—primarily due to the torques exerted by the Sun and Moon—or among the internal layers within the planet itself.

1.1 The Three Components of Earth’s Rotation

When talking about variations in rotation, we mean either the changes in the magnitude or direction of the angular velocity vector of the planet or those in the orientation of the planetary figure itself, the two being related by kinematic relations. Changes in the rotation rate result in variations in the length of day ( \(\Delta \text {LOD}\) ). Changes in the rotation axis orientation can be observed as movements of the poles in space. The motion known to astronomers as the Precession of equinoxes is a secular motion of that latter type which takes the Earth’s pole around the normal to the ecliptic in a period of about 25,700 years. Nutations come in addition to this secular motion manifesting themselves as a series of oscillations of shorter periods about the mean equinox. These are, for the most part, caused by the tidal torques exerted by the Sun and Moon. For historical as well as technical reasons, it is customary to further separate these oscillations in two kinds based on their frequencies. Roughly speaking, one then speaks of (precession–) nutations to denote oscillations that have a low frequency as measured from a Celestial Reference Frame (CRF), whereas other kind of oscillations is referred to as the polar motion (PM). The latter consists mainly of oscillations of subdiurnal frequencies as measured in a Terrestrial Reference Frame (TRF) (see Dehant and Mathews ( 2015 ) for details).

1.2 Reference Frames

More specifically, when speaking either of nutations or polar motion, we refer to the motion of the Celestial Intermediate Pole (CIP) with respect to the International Celestial Reference Frame (ICRF) or the International Terrestrial Reference Frame (ITRF), respectively. The definition of the CIP was adopted by the International Astronomical Union (IAU) in 2000 in order to replace the previous Celestial Ephemeris Pole (CEP) and to accommodate significant improvements in the Earth’s observation. This led to the formal classification of nutations and PM according to Fig.  1 , where nutations are identified as oscillations of the CIP with frequencies \(-0.5\le \omega \le 0.5\) (in cycles per sideral day, cpsd) as measured from the ICRF, whereas PM are oscillations of the CIP with frequencies in the intervals \(-\infty <\omega \le -0.5\) and \(0.5\le \omega <\infty\) in the ITRF. As there is a difference of \(+1\) cpsd when going from the ITRF to the ICRF (accounting for the diurnal rotation of the Earth), wobbles of the instantaneous rotation axis with frequencies within the interval \(-1.5\) to \(-0.5\) cpsd (i.e. with periods close to either 2/3 or 2 sideral days corresponding to \(\sim 15.9\) h and \(\sim 47.9\) h, respectively) are equivalent to nutations. Note that one speaks about wobble instead of PM in this case as PM is a term classically used for the CIP motion in the TRF.

Conventional definitions of polar motion (PM) and nutations of the CIP. Nutations have frequencies in the interval \(-0.5\le \omega \le 0.5\) measured in the Celestial Reference Frame (CRF). PM covers all the remaining corresponding frequencies as they appear in the Terrestrial Reference Frame (TRF). The difference of 1 cycle per sideral day (cpsd) between the two frames corresponds to the diurnal rotation rate of the Earth

As the Earth’s rotation is very close to a steady state, it is customary to write the rotation vector as:

where we have chosen the Cartesian basis vector, \(\hat{\varvec{{\mathbf {z}}}}\) , in the direction of the mean spin-axis. \(\varOmega _0=1~\mathrm {cpsd}\) denotes the mean angular rate of rotation, and the vector \(\varvec{{\mathbf {m}}}\) is called the planetary wobble . This last quantity is typically very small with a magnitude of the order of \(10^{-8}\) to \(10^{-6}\) for the Earth. For this reason, the equations of conservation of angular momentum governing the time evolution of \(\varvec{{\mathbf {m}}}\) can be safely restricted to first order. This effectively decouples the axial component of rotation \(m^z\) from the equatorial components \(m^x\) and \(m^y\) where the Cartesian geographical x coordinate points to the Greenwich Meridian and the y coordinate points to the \(90^{\circ }\) East longitude. Classically, one then uses the term ‘wobble’ in relation to the latter two components exclusively. The dynamics of \(m^z\) models the modulations in the LOD while \(m^x\) and \(m^y\) model both PM and nutations expressed in the TRF. Quantitatively, \(\Delta \text {LOD}\) does not exceed a few milliseconds (ms), typical PM is of the order of several hundreds of milliarcseconds (mas) (corresponding to a PM within a square of 20 m side), and the dominant component of nutations (Bradley’s nutation of 18.6-year period) has an amplitude of about \(\pm 19\) arcsec and \(\pm 10\) arcsec in longitude and obliquity, respectively. The wobble corresponding to nutations is of the order of several milliarcseconds (mas).

1.3 Origins of the Earth’s Movement

Broadly speaking, PM is typically associated with variations in the orientation of the rotation axis caused by geophysical processes, i.e. the redistribution of the masses inside the Earth that gets reflected in the inertia tensor. Such redistributions result in both secular and oscillatory variations. On the other hand, nutations are more affected by torques resulting from the tidal forces from external sources (primarily the Moon and Sun). This distinction is, however, a matter of nomenclature. For example, in reality, excitations from outer sources do also cause redistributions of masses inside the Earth (Mathews and Bretagnon 2003 ). For our purpose, the two types of motions can nevertheless be clearly separated on the basis of observation.

Conventionally in geodesy, studies of PM (including wobble) and nutations assume that the flow inside the core has a uniform vorticity, i.e. it resembles a solid-body rotation around an axis forming a finite angle with the mantle’s rotation. It can be shown that the core flow bears little effect on the excitation of PM (see, e.g., Chap. 7 of Bizouard 2020 ). However, as explained in Sect.  2 the mere presence of the fluid core affects the PM by altering the frequency of the free rotational mode known as Chandler Wobble (CW)—and to a lesser extent the Inner Core Wobble (ICW) (see later). Nutations, on the other hand, are directly affected by the core flow, and so their study offers a window on a broad variety of physical processes taking place inside the fluid core as reviewed in Sect.  4 . The fluid core also has an indirect effect on the frequencies of the free rotational modes known as the Free Core Nutation (FCN) and Free Inner Core Nutation (FICN) that manifest themselves as resonances in the forced nutation series. The FCN also appears in the nutation series at its own free frequency excited by the continual forcing attributed mostly to the atmosphere and oceans. Once deconvolved from the excitation signal, the frequency of this mode provides valuable information about the core (Chao and Hsieh 2015 ).

Whereas the functional definitions of PM and nutation allow a clear distinction based on the frequencies as stated above, such a distinction does not exist for LOD. \(\Delta \text {LOD}\) contains a broad variety of timescales. In addition to a secular trend, which can be attributed to the luni-solar tidal frictions (Munk and MacDonald 1960 ) plus contributions from Glacial Isostatic Adjustment (GIA, see later) and tectonic forces (Gross 2015 ), the total \(\Delta \text {LOD}\) signal contains many oscillations with timescales ranging from decadal, interannual, intra-seasonal, diurnal, and up to subdiurnal. Diurnal and subdiurnal oscillations are attributed to tides (Defraigne and Smits 1999 ). Oscillations at the interannual, seasonal, and inter-seasonal timescales have been satisfactorily attributed to the exchange of angular momentum between the solid Earth and its fluid envelope (i.e. atmosphere and oceans) (Viron et al. 1999 , 2001 ) while earthquakes have surely induced relatively small changes (see, e.g., Gross and Chao 2006 ). Although not directly related to rotation, it has been pointed out that the Slichter modes with theoretical frequencies of the same order as the subdiurnal PM (as measured in the TRF) might be detectable in the gravimetric signal which calls for caution in interpreting the data (Rosat and Hinderer 2018 ).

1.4 Implications for the Deep Earth’s Dynamics

Of more interest to this review’s purpose are the decadal and interannual oscillations in the LOD, the amplitudes of which have been proven too large to be attributed to the atmosphere and oceans solely, thus hinting towards the probable implication of deep interior dynamics, where the core motions are the obvious culprit. In fact, the correlation between the variations in the Earth’s magnetic field and the length-of-day variations at decadal timescales (Ball et al. 1969 ) suggests that the core motions inducing the variations in the magnetic field generate a pressure field at the core–mantle boundary (CMB) causing in turn a change in the Earth’s rotation (Hide 1969 ). Jault et al. ( 1988 ) demonstrated that both the frequencies and amplitudes of these \(\Delta \text {LOD}\) fluctuations can be largely explained by the excitation of torsional oscillations, time-variable oscillations in the core flow in the form of coaxial Taylor cylinders around the figure axis which oscillate with a particular period of several years (6 years is typically considered). A more detailed theoretical account of this type of oscillations is given in Triana et al., this issue. The most important implications for our purpose are given in Sect.  3 . The existence of the torsional oscillations warrants the close measurement of the decadal \(\Delta \text {LOD}\) .

We thus see how PM, nutations and \(\Delta \text {LOD}\) observations are complementary when examining the nature and dynamics of the flow inside the Earth’s core at various timescales. In the remainder of this article, we turn to each of these motions and review the techniques used for their modelling and observations before discussing the fluid core dynamics. Section  5 concludes with perspectives on future work.

2 Polar Motion

2.1 modelling and observations.

Polar motion (PM) is the motion of the mean rotation axis (more precisely of the CIP—see Sect.  1 ) around the symmetry axis of the Earth measured in the TRF. The upper panel of Fig.  2 shows the two components, X and Y , of this motion for the past six decades based on the EOP Combined Serie C04 (see IERS ). The signal has two dominant frequency components, as can be observed from the bottom panel representing its simple Fourier Transform. One of the peaks is at the period of one year, the other corresponds to a period of about 432 days in the TRF. The annual wobble is due to the forcing by surface fluid layers. The 432 days resonance is due to the CW which is the Earth’s analog to the Eulerian nutation for a rigid ellipsoidal body (Bizouard 2020 ).

Polar motion coordinates X and Y, where X is defined towards Greenwich and Y towards 90 degree East as determined by Very Long Baseline Interferometry (VLBI) and Global Navigation Satellite Systems (GNSS) techniques (EOP Combined Serie C04, see details at IERS)

The CW is excited mainly by the atmosphere, the oceans and to a lesser extent by hydrological masses through the afore-mentioned exchange of angular momenta. In practice, geophysical excitation functions are provided by services to the geodesy community ( IERS ) based on data assimilation and numerical models of the atmosphere and ocean, considering that the oceans are responding dynamically to the atmospheric pressure changes (see, e.g., Bizouard 2020 , for details).

The CW is a rotational normal mode whose frequency depends directly on the dynamic oblateness of the Earth modified by the planet’s interior structure and feedback from the CW deformation itself (Munk and MacDonald 1960 ). Mathematically, for an elastic ellipsoidal axisymmetric Earth’s model, the CW frequency writes:

where \(\sigma _e=\varOmega _0(C-A)/A\) is the Eulerian nutation of period 304.5 sidereal days (or 303.6 mean solar days) and \(k_2=0.302\) and \(k_s=0.942\) are the degree-2 and secular Love numbers that characterize the Earth’s elastic and anelastic deformability.

If the oblate Earth were rigid, the period of the CW would be equal to the Eulerian period of \(\sim 305\) sideral days. The Earth having an inelastic mantle, a liquid core, an inelastic inner core, and oceans, this period is in reality about 432 days. These different contributions are shown in Fig.  3 , where we see that the presence of a liquid core decreases the CW period by about 40 days. Physically, the CW can be excited by external torques inducing an angle between the principal moment of inertia and the Earth’s figure axis or when a large mass redistribution or surface deformation alters the inertia tensor of the Earth, the latter mechanism being the most prominent. The presence of a liquid core changes the mass involved in the mechanism. Mathematically, the CW period will be proportional to the mantle moment of inertia instead of the whole Earth moment of inertia in Eq. ( 2 ) when the liquid core does not participate in the motion at the timescale under consideration, reducing the CW period accordingly.

Period of the CW is altered by a series of geophysical processes

In addition to the CW, the presence of an ellipsoidal inner core introduces another rotational normal mode which can, in principle, become excited when there is a finite angle between the rotation and figure axes of the inner core. This is the ICW introduced in Sect.  1 and is analogous to the CW for the oblate inner core. The moment of inertia of the inner core is 1400 times smaller than the whole Earth’s, and its dynamical oblateness is also comparatively smaller. Consequently, the frequency of the ICW is presumably much lower than that of the CW. Additionally, if excited, the ICW would have to transmit its angular momentum to the mantle somehow (e.g., via gravitational torque) in order to observe its contribution to polar motion. So far no firm evidence of the ICW within a resolution of a few mas has been found. A spectral search in PM by Guo et al. ( 2005 ) showed that the gravitational perturbation induced by the ICW would be far below the detectability level of current ground (e.g., superconducting gravimeters) and space (e.g., GRACE) gravity measurements. More recently, a 8.7-yr peak detected in PM time-series was suggested to be possibly related to the ICW (Ding et al. 2019 ), although most of the long-period polar motion, except for the Markowitz period (see below), is generally considered to be attributable to climatological signals.

Beyond the 10-year period, PM is dominated by a 25–35- year pluri-decennal oscillation with a mean amplitude as large as 10 mas. This is the so-called Markowitz wobble (Markowitz 1960 , 1961 ). The coupling between core and mantle is believed to be insufficient to explain that term (Hide et al. 1996 ). As the associated observation errors were too large to definitely conclude about it prior to the use of space geodesy, longer and more precise geodetic time-series should help to further characterize the Markowitz wobble. Centimetre accuracy in the realization of the TRF and of PM is possible thanks to an increasing precision of the space-geodetic observations but necessitates to take into account tectonic plate motions. Considering this and imposing the minimum rotation of the plates as a prepositional condition for the TRF, it is possible to see that the rotation pole has shifted by over 10 meters (about 300 mas) since first defined in 1900. This secular PM is mostly attributed to the Glacial Isostatic Adjustment (GIA) modified by present-day ice melting (Adhikari and Ivins 2016 ) and slightly by large earthquakes (Xu and Chao 2019 ) in recent years (see below).

In addition to the above-mentioned components, PM signal also contains shorter timescales that correlate with changes in the angular momenta of the atmosphere, ocean and hydrosphere and earthquake. By virtue of the definition of the TRF and CRF evoked in the introduction to this work, the retrograde diurnal motions are equivalent to the wobble associated with nutation. This includes a Nearly Diurnal Free Wobble (NDFW) which is perfectly equivalent to the FCN but expressed in the TRF. However, the retrograde diurnal wobble motion is observed as a long-term CIP motion in the CRF including an excitation of the FCN (Bizouard et al. 2019 ).

2.2 Interpretation

The observed PM is the convolution of the Chandler Wobble resonance with the geophysical excitation function representing Atmospheric, Oceans and Hydrosphere Forcing (AOH) (Munk and MacDonald 1960 ). The PM excitation function can hence be obtained through a deconvolution procedure (see, e.g., Chao 1985 ). Its two equatorial components, designated as PM- X and PM- Y , are both characterized by a significant long-term variability on top of strong seasonal and intraseasonal variations (see comparisons in Fig.  4 ). The seasonal variability in PM- Y is notably larger than that of PM- X . This is attributed to the geographical distribution of major continents which are more aligned in the Y axis and produce relatively larger excitation of PM- Y from atmospheric surface pressure loading and terrestrial water storage changes, whereas atmospheric loading effect over the oceans is largely compensated by the inverted barometer response of the ocean surface. The long-term variability of PM- X and - Y is presumably mainly excited by solid Earth geophysical effects, e.g., GIA (Peltier 2004 ). At decadal and longer time scales, mass loss in polar ice sheets and glaciers and corresponding sea level changes are found to play a fairly important role in exciting PM as well (Chen et al. 2013 ; Adhikari and Ivins 2016 ).

Deconvolved monthly excitations of the observed polar motion: ( a ) PM- X and ( b ) PM- Y , from the IERS EOP C04 series and compared geophysical excitations from AOH (the sum of atmospheric, oceanic, and hydrological contributions) over the period 1976–2019. The AOH series are from the GFZ EAM products. EOP-AOH residuals are shown in green curves for comparisons

While broad-band decadal variability, other than the Markowitz wobble, is not so evident in PM excitation (as compared to \(\Delta \text {LOD}\) , see Sect.  3 ), PM- X and - Y do exhibit clear interannual oscillations. Comprehensive analysis of interannual oscillations in PM excitation and AOH geophysical excitations has been carried out to identify periodic interannual oscillations in both components (Chen et al. 2019 ). The AOH excitations are from the effective angular momentum (EAM) products provided by the German GeoforschungsZentrum (GFZ) (Dobslaw and Dill 2018 ). The daily EOP C04 and 3-hourly EAM series have been averaged into monthly intervals. Large uncertainties in EAM series are expected, especially in the hydrological components due to the immaturity of hydrological models. PM- X shows a clear oscillation near 5.9 years, plus smaller variations at shorter periods. Atmospheric, Ocean and Hydrosphere (AOH) sources largely account for the 2.5–and 3.65–year components found in PM- Y . It is interesting to note that AOH excitations also show notable quasi six-year oscillation (SYO) signals in both PM- X and - Y that are mostly in phase with Earth’s Orientation Parameters (EOP) Combined Serie C04 (see IERS for details). After AOH excitations are removed, the SYO in PM becomes even less notable so that it is not possible at present to conclude that this SYO in PM, if present, is caused by core processes (Rosat et al. 2020 ). This is contrary to the case with the SYO in \(\Delta \text {LOD}\) where the SYOs in \(\Delta \text {LOD}\) and AOH are out of phase, and removing AOH makes the SYO in \(\Delta \text {LOD}\) appear more clearly (see Sect.  3 ).

The quantification of the SYO in PM is more difficult due to the small magnitudes of the signal (compared to seasonal variability) and existence of other interannual oscillations more clearly related to AOH sources. The main challenges in quantifying the SYO in PM, especially the variation related to the solid Earth, include how to improve the quantification of SYO amplitudes and phases in both PM observations and the atmospheric, oceanic and hydrological AOH model predictions. The latter may play a more important role, as the uncertainties of atmospheric, oceanic and hydrological model predictions (including mass and motion terms) appear to be the major limitations affecting the appropriate separation of any SYO signals that might be related to core-mantle interactions (Chen et al. 2019 ). The magnitudes of the SYO in PM appear to also depend strongly on time spans of the EOP time series. Nevertheless, further investigation of interannual oscillations in PM from both EOP observations and AOH sources is needed.

At multi-decadal time scales, AOH excitations of PM can only be determined from atmospheric, oceanic and hydrological model predictions due to inadequate in situ observations. Among the three major components of the climate system, the largest uncertainty appears to come from model-based estimates of terrestrial water storage (TWS) change (Chen and Wilson 2005 ; Nastula et al. 2019 ), although it has long been recognized that global TWS change plays an important role in exciting PM at seasonal and interannual time scales (Chen et al. 2000 ; Nastula et al. 2007 ).

Another important feature of PM is its attenuation with time due to dissipative processes. Dissipation occurs through anelastic deformation in the mantle, viscomagnetic coupling at the core–mantle boundary, friction at the bottom of the oceans, etc. Dissipation is quantified by a quality factor, Q , that may be related to the rheological parameters. The determination of Q at the CW frequency, which again requires a Chandler resonance deconvolution, thus provides additional constraints on the Earth rheology at frequencies intermediate between seismic and tidal frequencies (Benjamin et al. 2006 ).

The movement of the Earth’s rotation axis induces a perturbation of the surface gravity field through variation in the centrifugal potential, surface deformation and mass redistribution. These changes have been analysed from superconducting gravimeter (SG) measurements longer than a decade by Ziegler et al. ( 2016a ). In particular, assuming some given rheological models for the Earth’s mantle the link between the gravimetric factor phase and the CW quality factor could be made (Ziegler et al. 2016a , b ). However, one would need data sets spanning at least 31 years to obtain stable estimates of the CW damping (Gross 2015 ).

At decadal time-scales, the exchange of angular momentum between the fluid outer core and solid mantle was long thought to be responsible for the observed decadal PM. However, the electro-magnetic (EM) torque computed from geomagnetic observations at the Earth’s surface is too weak to explain the observed decadal PM. Notwithstanding, Kuang et al. ( 2019 ) have recently simulated the toroidal magnetic field in the D”-layer from the induction and convection of the toroidal field in the outer core showing that it could be much stronger than that from the advection of the poloidal field in the outer core. This reassessment of EM coupling using dynamo simulations shows that it could still contribute largely to decadal PM to magnitude of approximately 10 mas.

Besides CW, measurements of the ICW period would provide a valuable fundamental constraint on the deep Earth’s properties. Theoretical estimates based on the conservation of angular momentum and the PREM model of interior predict a period of about 7.5 years prograde. The indirect (resonance) effect of the ICW on nutations hints to a somewhat smaller period of about 2400 days (Mathews et al. 2002b ). Ding et al. ( 2019 ) have recently showed how the ICW detection could provide a new independent constraint on the density contrast at the ICB. If the detected signal in PM data at 8.7-yr is the signature of the ICW, then it would imply a density contrast of \(\sim 507 ~\mathrm {kg/m}^3\) (Ding et al. 2019 ). Such a value is smaller than the \(600~\mathrm {kg/m}^3\) for PREM model (see Dehant et al., this issue).

3 \(\Delta \text {LOD}\)

3.1 modelling and observations.

The best way to measure \(\Delta \text {LOD}\) precisely is by using Very Long Baseline Interferometry (VLBI). This technique consists of observing light emitted by radio-sources in S- or X-bands using large antennas on Earth of typically \(25~\mathrm {m}\) diameter. The arrival time of the signal at two different stations is then used to compute the (time varying) time delays and infer the Earth orientation relative to the celestial radio-sources taken as stationary (or with a small proper motion characterized a priori ). After correcting for the time delays induced by atmospheric and other miscellaneous environmental effects, it is possible to derive UT1-UTC, a quantity directly related to the Earth rotation angle with respect to the mean Earth rotation by using a large network of stations and radio-sources (Fey et al. 2015 ). Information on \(\Delta \text {LOD}\) may then be recovered from:

where \(\Delta \text {LOD}\) (t) is called the excess of length of day , \(\varOmega _0=86400s\) is the mean rotation rate of Earth, UT1 is the Universal Time and TAI is the reference International Atomic Time (related to UTC—Coordinated Universal Time—by a set of leap seconds). VLBI is the only method that can access the absolute UT1 information. It is classically determined weekly by the International VLBI Service (IVS) R1 and R4 sessions with an error of the order of \(2-3~\mu s\) (microsecond), as well as via daily one-hour intensive sessions at the level of \(\sim 20\)   \(\mu s\) accuracy (Malkin 2020 ).

\(\Delta \text {LOD}\) as measured with VLBI and Global Navigation Satellite Systems (GNSS) techniques (Combined Serie C04, see IERS for details) with uncertainties in blue. \(\Delta \text {LOD}\) contains a rich spectrum of information covering periods from days to multidecadal. It can be partly explained by the mass redistribution or the motion of the atmosphere, the hydrosphere, and the core

\(\Delta \text {LOD}\) takes place on all observable time scales, from subdaily to decadal and beyond (see Fig.  5 ). It is due to angular momentum changes of the solid Earth in two forms: mass redistribution and the relative motion of the atmosphere, ocean, and the liquid outer core. Tidal forces, primarily from the Sun and Moon, cause \(\Delta \text {LOD}\) through the deformation that they induce on the Earth’s mass distribution. These effects are observed using high accuracy space geodetic measurements system (Schuh and Behrend 2012 ). The periods of the principal zonal tidal components are mostly at the intra-annual timescale with a few notable exception such as the ‘regression of the lunar nodes’ ( a.k.a. lunar precession) which has a period of 18.6 years. Table  1 reproduces the 20 tidal components that have the largest effect on \(\Delta \text {LOD}\) (Ray and Erofeeva 2014 ).

The redistribution of masses due to atmospheric, oceanic and hydrological excitations also alter Earth’s angular momentum causing \(\Delta \text {LOD}\) at seasonal and interannual timescales. Numerical models computing the angular momentum attributed to meteorological processes have improved significantly in recent years reaching a time resolution of three hours in the evaluation of atmospheric (AAM), oceanic (OAM) and hydrologic (HAM) angular momentum (Dobslaw and Dill 2018 ). Gross et al. ( 2004 ) showed that AAM explains \(85.8\%\) of \(\Delta \text {LOD}\) variations at the interannual time scale, but that taking into account OAM and HAM contributions only offers a marginal improvement. In addition, climatic oscillations most likely play a role in \(\Delta \text {LOD}\) . A high correlation was found, for example, between \(\Delta \text {LOD}\) , the El Niño–Southern Oscillation (ENSO) and the quasi-biennial oscillation (Chao 1989 ).

In addition to the oscillatory components mentioned above, a much longer secular variation is partly attributed to the deceleration caused by tidal frictions in the Earth–Moon system (Munk 1997 ). The linear regression fitting of ancient \(\Delta \text {LOD}\) observations going back to 4000 years ago based on lunar and solar eclipses records by Babylonian, Chinese, Greek and Arab astronomers predicts a +1.8 ms/cy per century increase in \(\Delta \text {LOD}\) . This is, however, contrasting with the +2.3 ms/cy predicted by tidal friction alone (see Fig.  6 ) leaving an unexplained gap of about +0.7 ms/cy. Part of this gap is at least partly explicable as the result of the GIA inside the Earth (Mitrovica et al. 2015 ), a process through which the figure (and the inertia tensor) of the Earth is altered by deglaciation. Models suggest that GIA-induced \(\Delta \text {LOD}\) is very sensitive to the assumed value of lower mantle viscosity (Wu and Peltier 1984 ; Peltier and Jiang 1996 ; Peltier 2007 ) as well as to changes in the mass of glaciers and ice sheets. The Antarctic ice sheet melting-induced mass changes could lead to a −0.72 ms/cy to +0.31 ms/cy in \(\Delta \text {LOD}\) according to Ivins and James ( 2005 ). In addition, the subsurface tectonic activities (Van der Wal et al. 2004 ), earthquakes (Chao and Gross 1987 ), plate subduction, deformation of the mantle and core–mantle interactions, etc. could also alter \(\Delta \text {LOD}\) (Dumberry and Bloxham 2006 ; Holme 1998 ; Jault and Le Mouël 1990 ; Mound and Buffett 2003 ).

Secular change in the LOD from 2000 B.C. to present day based on Stephenson et al. ( 2016 ). The historical \(\Delta \text {LOD}\) change is estimated from past lunar and solar eclipses documented by Babylonian, Chinese, Greek and Arab astronomers. The linear regression shows a +1.6 ms/cy in \(\Delta \text {LOD}\) with time, which is 0.7 ms/cy lower than the decrease in Earth’s rotation rate (+2.3 ms/cy) predicted by tidal friction alone

3.2 Interpretation

AAM changes have long been recognized as dominantly driving \(\Delta \text {LOD}\) , accounting for \(\sim 90\%\) of LOD variations at seasonal and shorter time scales (Rosen and Salstein 1983 ; Barnes et al. 1983 ). This is clearly illustrated in the comparisons (shown in Fig.  7 ) of observed \(\Delta \text {LOD}\) excitations derived from the IERS EOP C04 series and total geophysical excitations from the atmospheric, oceanic, and hydrological contributions (noted as AOH). The strong decadal and long-term variability in LOD is likely driven by mass movement in the interior of Earth and angular momentum exchange between the core and mantle (Jault et al. 1988 ; Hide et al. 1993 ; Buffett 1996 ; Mound and Buffett 2006 ).

In addition, a quasi-SYO at period of \(\sim 5.9\) -year has also been observed in \(\Delta \text {LOD}\) , which could not be attributed to either AAM excitation because of different amplitude and phase (see Fig.  8 and Chen et al. 2019 ) or to other sources in the surface geophysical fluids system (Abarca del Rio et al. 2000 ; Chao and Hsieh 2015 ). Indeed, after removing AOH excitations, the amplitude of the SYO becomes more prominent in the LOD residuals and can be better isolated, as can be seen by comparing the black and red curves in Fig.  8 .

Comparisons of observed monthly LOD excitations from the IERS EOP C04 series and geophysical excitations from AOH (the sum of atmospheric, oceanic, and hydrological contributions) over the period 1976-2019. The AOH series are from the GFZ EAM products (covering the period 1976 onward)

Observed \(\Delta \text {LOD}\) (only zonal tides were removed), AOH series from the GFZ EAM products and \(\Delta \text {LOD}\) residuals after AOH products were removed. (left) Time-series that were band-pass filtered between 4 and 8 years; (right) amplitude Fourier spectra in \(\mathrm {ms}\) with respect to period, T , in years

This is shown in Fig.  9 showing the time evolution of the LOD power spectrum in the past 4 decades. The SYO indicated by the lower dashed horizontal line is clearly visible.

Continuous Wavelet Transform (CWT) of the LOD signal (upper-left) and the same thing with the contribution from AAM removed (upper-right). Removal of the Oceanic (OAM) and Hydrosphere (HAM) contributions have comparatively small effect on the remaining signal (bottom panels). The horizontal dashed lines correspond to 5.9, 7, and 8.5 years

The remarkable stability of the observed SYO in \(\Delta \text {LOD}\) hints towards a dynamical deep Earth origin (Chao et al. 2014 ). It is difficult to explain the nearly out of phase SYO in AOH and its dynamical connection with the SYO in observed LOD. It is unlikely to be attributable to errors in the AOH LOD excitations, as similar SYO is also captured in AOH PM excitations, indicating the existence of SYO in the climate system. Further investigations are needed. The quantification of the SYO is also affected by co-existences of other interannual oscillations, in particular a reported 4.9-year (Chen et al. 2019 ), 8.3-year (Duan and Huang 2020 ) and 7.3-year (Hsu et al. 2021 ) oscillation in \(\Delta \text {LOD}\) observations. While the 4.9-year oscillation is largely accounted for by AAM (Duan et al. 2015 ), a possible core origin was suggested for the 8.3 and 7.3-year signals. An extended long record of LOD series is necessary in order to successfully separate these interannual oscillations with close–by periods in \(\Delta \text {LOD}\) (see Fig.  8 ).

At least two hypotheses have been proposed treating SYO as a rotational normal mode: the Mantle–Inner-Core Gravitational coupling (MICG) (Buffett 1996 ; Mound and Buffett 2006 ; Chao 2017 ; Ding and Chao 2018a ; Chao and Yu 2020 ) and the torsional mode in the fluid outer core (Buffett et al. 2009 ; Gillet et al. 2010 ), while certain mechanisms have been postulated for the excitation of the mode (see, e.g., Gillet et al. 2010 ; Silva et al. 2012 ; Holme and De Viron 2013 ). Indeed, the SYO was reported to be correlated with the observed geomagnetic jerks (Bloxham and Jackson 1991 ) although their remain some open questions and doubt regarding the exact physical nature of this correlation (Ding et al. 2021 ). Jerks appear at several locations on a time-scale of a few months as sharp V-shaped features in graphs of magnetic field changes (Mandea et al. 2010 ). A definite physical model is still lacking at the moment to explain their appearance on the global scale. These might be the result of magneto-hydrodynamic waves causing sharp changes in the flow (Aubert and Finlay 2019 ). Jerks might also be related to the presence of torsional oscillations in the liquid core.

The MICG mechanism, on the other hand, has been the subject of debate because of the amplitude of the strength required to transfer angular momentum between core and mantle through gravitational coupling associated with the inner-core superrotation (Davies et al. 2014 ). Chao ( 2017 ) has further developed the dynamics of the former MICG mechanism in terms of gravitational multipole formulation, in particular for the sectorial quadrupoles of the MICG system that gives rise to the Axial Torsional Libration (ATL) of the inner core (see also Chao and Shih 2021 ). Based on equating the SYO with ATL, postulating that the shape of the inner core conforms to the equipotential surface under the MICG influence, Shih and Chao ( 2021 ) deduced separately the equatorial ellipticity of the inner core and the corresponding sectorial quadrupole of the lower mantle plus CMB. The latter has important implications to the density anomaly associated with the constructs of LLSVP (large low shear velocity provinces; see, e.g., McNamara ( 2019 ), for a review) residing in the lower mantle above the core-mantle boundary. This constitutes a profoundly interesting case where space-geodesy observed Earth rotation variations serve as key independent information for the inversion of deep Earth structures found in seismological tomography observations. A more detailed discussion of the LLSVP can be found in Dehant et al., this issue.

4.1 Modelling and Observation

In practice, the conventional initial precession and nutation model is constructed based on a simplified solid Earth model (Kinoshita 1977 ). The IAU1980 nutation series (Seidelmann 1982 ) are developed from an elastic rotational oceanless Earth model (Wahr 1981 ). In those products, the contribution from planetary gravitational attraction was originally neglected. It was introduced around the same time by subsequent works (Roosbeek and Dehant 1998 ; Bretagnon et al. 1998 ; Hartmann et al. 1999 ). All those new series for the rigid Earth were truncated at the level of a tenth of a \(\mu as\) and compared with each others.

Concerning the non-rigid Earth nutations, with the accumulated VLBI observations (see Sect.  3 ), the discrepancy between observations and the nutation series IAU1980, which were originally quite significant, has been the object of investigation by several authors (Herring et al. 1991 ). Zhu et al. ( 1990 ) developed a method of covariance analysis fitting 106 nutation terms, thereby providing corrections to the IAU1980 series. Herring et al. ( 2002 ) later estimated the corrections for 21 major nutation terms with respect to the IAU1980 nutation model and the secular trends in longitude and obliquity with respect to the IAU1977 precession rate from a combined series covering the period from 1980 to 2000. These authors then used the resulting VLBI observation series as input to systematically estimate a series of physical effects such as indirect loading, deformations (estimated through compliances) and the core–mantle coupling constants. They used these parameters (named Basic Earth Parameters , see below) to build a non-rigid Earth nutation series named MHB2000. This series was adopted by IAU as the IAU2000 nutation model (Soffel et al. 2003 ). The correction to the precession rate on the IAU2000 values was discussed by Capitaine et al. ( 2003 ), which led to updated IAU2006 precession model, using improved polynomial expressions for the precession. These joint initiatives have led to the new IAU2006/2000A precession-nutation model composed of 678 lunisolar and 687 planetary terms induced by the gravitational attraction from the Moon, the Sun, and the other planets.

Updates and corrections to the major IAU2006/2000A nutation component can be found in Gattano et al. ( 2017 ) and Zhu et al. ( 2021 ). These authors report an average root-mean-square (RMS) errors after correcting some of the nutation harmonic components of about \(130~\mu \mathrm {as}\) and \(110~\mu \mathrm {as}\) , respectively. Both studies are based on IERS 14C04 EOP observations. Figure  10 (based on Zhu et al. 2021 ) shows the signal residuals which are mostly dominated by the signature of the FCN.

Nutation residuals from IERS 14C04, which is a combined solution about the IAU2006/IAU2000A precession nutation model. The free core nutation (thick black line) is estimated using an eight years sliding window

As discussed in the introduction to this work, the FCN is a free rotational mode with a retrograde frequency as measured from the inertial reference frame. Its excitation results from a misalignment between the rotation axis of the Earth’s (spheroidal) liquid outer core and the planet’s figure axis (see, e.g., Rekier et al. 2020 ; Dehant and Mathews 2015 ). The FCN resonantly amplifies the Earth’s response to tidal forcing, as observed in the VLBI data (in the celestial frame) as well as in the retrograde diurnal tidal waves in the records of geophysical sensors in the terrestrial frame (e.g., gravimeters, tiltmeters, strainmeters, etc.); in that latter case, the resonance is designated as the NDFW (see Sect.  2 ). These resonances have been widely studied by means of VLBI network measurements (Zhu et al. 2021 ; Herring et al. 1986 ; Lambert and Dehant 2007 ; Koot et al. 2008 ; Rosat and Lambert 2009 ; Krásná et al. 2013 ), superconducting gravity records (Florsch and Hinderer 2000 ; Ducarme et al. 2007 ; Rosat et al. 2009 ) or a combination of both (Rosat and Lambert 2009 ; Ziegler et al. 2020 ). It is also worth mentioning other experiments performed with strainmeter records (Sato 1991 ; Zaske et al. 2000 ; Amoruso et al. 2012 ; Amoruso and Crescentini 2020 ) tiltmeters (Riccardi et al. 2018 ) providing independent, yet somewhat poorer, constraints on the FCN/NDFW parameters. Traces of the FCN are also visible in hydrographic data, demonstrating the observability of the phenomenon long before it was actually first observed (Agnew 2018 ).

The International VLBI Service (IVS) is currently working to improve its data through intensive campaigns as well as by developing strategies to balance the current geometry of the VLBI station network. Efforts are also underway in order to incorporate information on the proper motion of distant radio-sources (Lambert 2014 ). Another source of improvement is provided by the recent reassessment of the influence of atmospheric-oceanic effects on the amplitude of nutations. These have been shown to be particularly important for the prograde annual term (Nurul Huda et al. 2019 ).

As already mentioned in introduction, in addition to the FCN and FICN resonance effects in the forced nutations, VLBI data feature a contribution from the ‘free’ FCN mode at the level of a few tenths of mass. The amplitude of this free mode varies with time, which could indicate either a convolution of the free and forced FCN or an interaction between the FCN rotational mode and an inertial mode inside the liquid core (Zhu et al. 2021 ). Although the existence of this kind of interaction is still largely speculative, it has been demonstrated to be theoretically possible by means of numerical simulations (Triana et al. 2019 ).

Similar to the FCN, the FICN could, in principle, be observed via its resonance in diurnal tides and nutations, but the effect being small, it has never been clearly detected yet (Rosat et al. 2016 ). The only constraints obtained on the FICN are via its influence on the long period 18.6-yr nutation term (Mathews et al. 2002a ; Koot et al. 2008 ; Nurul Huda et al. 2019 ).

4.2 Interpretations

The nutation series determined from VLBI (see Sect.  4.1 ) provides the 2-D Earth nutation motion of the CIP relative to the inertial space in terms of the celestial pole offsets \(d\psi\) and \(d\epsilon\) , i.e. the deviations of the longitude \(\psi\) and the obliquity \(\epsilon\) of the equator (relative to \(\epsilon _0=23.439^{\circ }\) the mean obliquity of Earth) in the ecliptic coordinates, or often expressed in the complex form \(\sin \epsilon _0~d\psi (t)+ i~d\epsilon (t)\) . Or, alternatively, it is simply in terms of the X - and Y -components of the CIP in space.

The free FCN motion is, in principle, the single major signal once all the astronomical nutation terms are removed according to the state-of-the-art nutation model (see, e.g., Zhu et al. 2021 ) or the reference model values of IAU2000A (Mathews et al. 2002b ). The present reference model has a precision of a few mas in the time domain. Only a few nutation components are not properly determined (see Fig.  10 ). Fitting the amplitudes of these ‘new’ components allows us to improve the accuracy of the FCN (and FICN) amplification parameters in order to obtain new information about the core.

Figure  11 shows the power spectrum of \(\sin \epsilon _0~d\psi (t)+ i~d\epsilon (t)\) , where the positive and negative frequencies correspond to the prograde and retrograde components of the nutational motion in the inertia frame, respectively. Major spectral peaks can be seen in the Fourier spectrum and can be even more distinctively identified in the stabilized AR-z spectrum (Ding and Chao 2018a ). At longer periods, Fourier and AR-z nutation spectra show very distinct characteristics (see left portion of the inset of Fig.  11 ). The Fourier spectrum shows hardly any identifiable peaks other than that corresponding to the FCN, whereas the AR-z spectrum exhibits multiple distinctive peaks, among which only two are readily identifiable: besides the retrograde annual (Sa) nutation as expected, the peak at \(-449\pm 5\) days belongs to the FCN frequency band. It is important to note that the quoted period here is merely FCN’s apparent value during the timespan covered by the data. It is close to but generally not coinciding with the ‘true’ natural FCN period because it is a result of convolution of the latter with certain excitation function (Chao and Hsieh 2015 ). Previous estimates based on a reconstruction from the tide amplified signal give a value within the range of \(-425\) to \(-435\) days for the ‘true’ FCN. A recent estimate by Zhu et al. ( 2017 ) places this value at \(T=-429.5 \pm 0.7\) days. Next to the main FCN signal, the presence of secondary spectral peaks of astronomical nutations at tidal periods indicates the imperfection of the reference model IAU2000A. Conversely, these findings provide clues for the reference nutation models.

For the Earth’s nutation (1984–2017) in the complex form \(\sin \epsilon _0~d\psi (t)+ i~d\epsilon (t)\) from VLBI data after all the astronomical nutation terms are removed according to model IAU2000A. ( a ) Fourier spectrum (logarithmic scale in dB) and ( b ) stabilized AR-z spectrum. Panels ( c ), ( d ), ( e ), ( f ) and the inset give the zoom-ins of the frequency bands of interest. (taken from Ding and Chao 2018b )

From what precedes, we see that the FCN is the main window to the structure of the Earth’s liquid core as far as nutations are concerned. Comparison between the values of its frequency and quality factor (i.e. its damping) inferred from observation to their theoretical values provides constraints on a number of physical properties. In their simplest form, these properties are represented by a series of parameters describing the strength of the coupling between the solid parts of the Earth (mantle and inner core) and its liquid core. These parameters enter explicitly in the expression of the analytical frequency of the FCN which reads (Mathews et al. 2002b ):

where \(A_m\) , \(A_s\) and \(A_f\) denote the principal axis of inertia of the mantle, solid inner core and fluid outer core, respectively, and \(e_f\) is the dynamical oblateness of the CMB. Equation ( 4 ) is based on the formalism developed by Sasao et al. ( 1980 ) in which the values of the two (complex) coupling constants, \(K_\mathrm {CMB}\) , and \(K_\mathrm {ICB}\) , parametrize the total torque that comes in addition to the dynamical pressure torque on the CMB and ICB, respectively (see Dehant et al. 2017 ). The imaginary parts of \(K_\mathrm {CMB}\) and \(K_\mathrm {ICB}\) represent the damping of the FCN and FICN, related to their quality factor Q , and manifesting itself as a phase lag between the tidal forcing and the induced nutation response. Finally, \(\beta\) is the mantle compliance which accounts for the elastic response of the mantle. This last parameter is typically estimated from interior models, as are \(A_m\) , \(A_s\) , and \(A_f\) . Based on this formalism, Gwinn et al. ( 1986 ), and Herring et al. ( 1986 ) estimated that the dynamical oblateness of the CMB, \(e_f\) , must be \(\sim 5\%\) larger than predicted by hydrostatic interior models. This increase, however, cannot account on its own for the whole discrepancy between the observed and derived amplitudes of nutations, nor can it explain the observed phase lag between the tidal forcing and the nutation response (as \(e_f\) is a real number, see above). The complete set of so-called Basic Earth Parameters (BEP), \(e_f\) , \(K_\mathrm {CMB}\) and \(K_\mathrm {ICB}\) (to which one must add the dynamical flattening of the whole Earth, e ), can be estimated from data inversion giving the values presented in Table  2 updated from Zhu et al. ( 2017 ) (see also Koot et al. 2010 ).

Three types of contributions to the value of \(K_\mathrm {CMB}\) are generally considered, namely that attributed to the fluid core viscosity, the magnetic field and the topography of the CMB (Buffett et al. 2002 ; Greff-Lefftz and Legros 1995 ; Koot and Dumberry 2013 ). The relative contributions of these torques cannot be easily disentangled, primarily because of the large uncertainty that characterizes the shape and electrical conductivity of the lower mantle. Palmer and Smylie ( 2005 ) estimated that the viscous torque caused by the molecular viscosity of the liquid core on the CMB is negligible, being 5 orders of magnitude smaller than the estimates for the electromagnetic (EM) torque. The latter is, however, difficult to model due to our poor knowledge of both the electrical conductivity of the lower part of the mantle as well as the intensity of the non-dipolar part of the magnetic field at this location (whereas the non-dipolar part can be inferred from surface measurements, Langel and Estes 1982 ; Mathews and Guo 2005 ). Estimates based on the spectral analysis of the harmonics components of large-scale fields place the rms value of the non-dipolar field at about \(\sim 0.28\mathrm {mT}\) , the same order of magnitude as the dipolar field. Computations of the EM torque based on this value concluded that the EM torque alone could not explain the values of \(K_\mathrm {CMB}\) and \(K_\mathrm {ICB}\) without assuming a magnetic field amplitude not supported by observations even in the very high conductivity limit (Buffett et al. 2002 ; Mathews and Guo 2005 ) and regardless of the shape of the non-dipolar field (Koot and Dumberry 2013 ). This may point towards the need to reassess the contribution from viscosity upwards by invoking the importance of a larger effective viscosity at the CMB (Lumb and Aldridge 1991 ). This possibility was recently suggested again by Triana et al. ( 2021 ) based on their computational estimate of the FCN decay rate due to ohmic and viscous dissipation.

Another ingredient that could potentially complement the effects of the viscous and EM torques is the topographic torque produced by the pressure forces within the fluid on a ‘rough’ CMB. Formally, any deviation of the CMB from an elliptical surface should alter the shape of the flow inside the core, causing it to deviate from the simple solid-body rotation traditionally assumed when modelling the FCN (see Sect.  5 ). When the amplitude of the CMB topography is small, its effect can be treated as a perturbation. This is the view taken by Wu and Wahr ( 1997 ) who predicted a deviation of about \(0.2\mathrm {mas}\) on the amplitude of the retrograde annual nutation caused by the small shift in the FCN frequency. However, their results depend strongly on the model of CMB topography chosen. More work is needed to link together the results obtained on the topography with those mentioned above (see also Dehant et al. 2012 , on the subject).

Finally, there is the possibility that the FCN might be influenced by the presence of other free modes with nearby frequencies, the natural candidates being Inertial Modes of which the FCN is the simplest representative (Rekier et al. 2020 ; Triana et al. 2021 ; Rekier et al. 2019 ). Triana et al. ( 2019 ) have shown how these modes can interact in a non-trivial way when the viscosity and ratio of moments of inertia of the core and mantle are such that the FCN frequency and damping are close to that of some other mode thereby causing a shift in the values of the former. Although the regime of parameters at which such interactions can take place is far from that relevant to the Earth, it might theoretically be reachable when the magnetic field is taken into account (Triana et al. 2019 ).

5 Conclusions and Prospects

Earth’s rotation signal covers a broad range of time-scales and contains fundamental information related to deep Earth’s processes. The study of these processes is complicated by the influence of surficial processes related to geophysical fluids (i.e. the oceans and atmosphere) that also affect PM, LOD and nutations thus masking smaller core signals. Combined geodetic, magnetic and gravimetric observations have already provided significant constraints on the physics of the outer core boundary. On the other hand, our current knowledge of the inner core is still limited.

Regarding PM and \(\Delta \text {LOD}\) , satellite gravity measurements from GRACE and GRACE Follow-On (GFO) offer a revolutionary means of measuring large-scale mass changes in the climate system, especially those associated with the global hydrological cycle (Tapley et al. 2019 ), and can help improve the quantification and interpretation of geophysical excitations of \(\Delta \text {LOD}\) and PM (Chen et al. 2016 ; Göttl et al. 2018 ; Nastula et al. 2019 ). GRACE/GFO have collected nearly two decades of time-variable satellite gravity measurements so far, and the series are expected to be extended to well over 20 years (possibly 30). With the future generations of GRACE missions that are under planning, satellite gravimetry will bring a new era of studying geophysical excitations of \(\Delta \text {LOD}\) and PM with unprecedented accuracy. The existence of a 5.9–year variation (SYO) in both PM and \(\Delta \text {LOD}\) signals is intriguing. Although the SYO is almost completely accounted for by surface processes for PM, this is not the case for \(\Delta \text {LOD}\) for which the removal of the AOH contribution only makes the SYO appear more clearly. This hints towards a possible deep interior origin, likely interactions between the core and mantle. In this respect, development of more accurate surficial models (especially in hydrology) and continuous, ever more precise, space observations cannot miss to provide new insights on core processes in the future. As regards the theoretical understanding that this will provide, progress in the near future will most likely come from a better characterization of the interannual oscillations in LOD and PM driven by interactions between the core and mantle. A unified model of these oscillations and how they might relate to core processes that also affect long period nutations (e.g., geomagnetic jerks) is still lacking at the moment and would prove very valuable.

Regarding nutations, continued VLBI observations of nutations will secure the determination of the forced nutation amplitudes in general and the 18.6-year nutation in particular. These nutations are essential to derive the values of the BEP (see Table  2 ). To be more exact, the BEP allows to determine the imaginary part of the coupling constants, \(K_\mathrm {CMB}\) and \(K_\mathrm {ICB}\) , as well as a combination of the core dynamical flattening, \(e_f\) , and the real part of the coupling constant (see Eq.  4 ). As we already discussed in Sect.  4.2 , the relative importance of these parameters cannot be disentangled without making additional hypotheses as regard the nature and dynamics of the flow inside the Earth’s core. Therefore, improvement in the current nutation model will necessarily come from a combination of improved measurements and theoretical exploration. The present model currently relies on the following assumptions:

The angular momentum of the liquid core is equal to that of an inviscid non-magnetic and neutrally buoyant flow with uniform vorticity ( a.k.a. Poincaré flow)

The outer core exchanges angular momentum with the inner core and the mantle via the (dynamic) pressure and electromagnetic torques acting on the ICB and the CMB, both of which approximated as oblate spheroidal surfaces

The damping of the FCN is attributed to the ohmic dissipation inside a thin electrically conducting layer at the base of the mantle

As we already discussed in Sect.  4.2 , (a) is well supported by the theoretical study of inertial modes which may be seen as the basis of the fluid motion inside the core (Ivers 2017 ). However, this simple picture is currently challenged by recent studies that show how angular momentum can be transported through the core when the effects of viscosity, magnetic field and density stratification are taken into account, thereby also questioning both (b) and (c) (see Triana et al., this issue). In the nearest future, progress will most likely come from a detailed reassessment of the viscous and electromagnetic couplings at the CMB. Efforts in this direction are currently undertaken within the GRACEFUL project.

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Acknowledgements

We would like to extend our gratitude to the anonymous reviewers whose comments helped to significantly improve the quality of our manuscript. The International Space Science Institute (ISSI) is gratefully acknowledged for the support in organizing the workshop that has led to the writing of this paper. The research leading to the results provided by VD, JR and PZ for this paper has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (RotaNut Advanced Grant 670874 + GRACEFUL Synergy Grant agreement No 855677).

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Rekier, J., Chao, B.F., Chen, J. et al. Earth’s Rotation: Observations and Relation to Deep Interior. Surv Geophys 43 , 149–175 (2022). https://doi.org/10.1007/s10712-021-09669-x

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The Effect of the Earth’s Rotation & Revolution

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The Effect of the Earth’s Rotation & Revolution

When watching the stars at night, they do appear to move very slowly.  This is because the Earth is constantly moving. The Earth completes one “rotation” every twenty-four hours.  A rotation is when the planet spins around once. The Earth rotates counterclockwise; this is why the Sun “rises” in the East and “sets” in the West.  It is not the Sun’s movement that causes days, but rather the Earth turning around in front of the Sun.

The Earth’s axis (the point at which it rotates around, for example, if you were to spin around while standing in one spot, your axis would be an imaginary line running through your head straight down to your feet) is in line with a star named “Polaris”. Polaris is also known as the “North Star” since it is directly above the Earth’s axis. 

Since this star is directly above the Earth’s axis, it does not appear to move, however, the rest of the stars in the sky move around Polaris (for example: when you spin around, the object directly above your head does not appear to move but everything else seems to spin around that object). Polaris is only seen in the Northern hemisphere and it belongs to the Little Dipper constellation (it’s the last star at the end of the “handle”).

The Effect of The Earth’s Rotation

Another type of motion is known as “revolution”.  Revolution is when one object completes a circular path around another object. The Earth takes 365.24 days to revolve around the Sun. This is why a year is 365 days long. During the year the Earth is angled differently towards the Sun.  These changing angles provide us with different Sun intensities and therefore we get four different seasons. Since the Earth is at different positions in space over the year, we see different constellations throughout the year.

Coriolis Effect: Defection of wind due to rotation of Earth

UP [NORTH]: West DOWN [SOUTH]: East (On Surface)

Northern Hemisphere: Deflected to the right (clockwise)

Southern Hemisphere: Deflected to the left (counter-clockwise)

Trade Winds: high pressure wind blown to the west from 30N

Westerlies: deflected to the east

Earth is currently in a cool phase characterized by formation of glaciers ( glacial maxima ), followed by warm periods with glacial melting ( interglacial periods ). These glacial–interglacial cycles occur at frequencies of about 100,000 years. We are currently in an interglacial period; these have lasted about 23,000 years in the past. The last glacial maximum was about 18,000 years ago.

The glacial–interglacial cycles have been explained by regular changes in the shape of Earth’s orbit and the tilt of its axis— Milankovitch cycles .

Circular rotation causes glaciers to melt; more solar radiation; Elliptical= less radiation. The intensity of solar radiation reaching Earth changes, resulting in climatic change. The shape of Earth’s orbit changes in 100,000-year cycles. The angle of axis tilt changes in cycles of about 41,000 years. Earth’s orientation relative to other celestial objects changes in cycles of about 22,000 years.

The Effect of Planet’s Motions

Thousands of years ago, people were able to clearly see the night sky (no “light pollution”).  The one thing they noticed is that five “stars” seemed to wander faster through the night sky than other stars.  These “stars” were actually the planets Mercury, Venus, Mars, Jupiter, and Saturn. People called these objects “wandering stars”. 

Their names were then changed to planets which is after the Greek word “planetes” which means “wanderers”. All planets rotate on their axes and revolve around the Sun, however these times are different for each planet. Planets move through constellations as well.  This motion usually takes a few weeks. Many constellations are named after animals. 

The Greek word for “animal sign” is “zodion”.  This is why we have star groups called the zodiac constellations. Depending on which zodiac constellation was visible when you were born is the “sign” you have been assigned.  For example: Aquarius, Leo, Gemini, Sagittarius, etc. Many people believe that zodiac signs determine certain traits and characteristics of people.  This is known as “astrology” and is not a legitimate science based on truth or facts.  Astrology is simply for entertainment.

Revolution Around the Sun vs. Rotation upon Axis

Revolve, as in orbiting the Sun? Yes, all the planets in our solar system orbit the Sun in the same direction Earth does. Some comets and asteroids orbit backwards, and some (more so comets than asteroids) orbit virtually perpendicular to the plane of Earth’s orbit.

Rotate, as to spin on ones axis (the thing that causes day and night on Earth)? Earth rotates counter-clockwise, as seen from above Earth’s north pole, the same direction it revolves around the Sun. But two planets (used to be 3, when Pluto was a planet) rotate clockwise – Venus and Uranus. Some might quibble about Uranus, as it spins on its side, but technically it rotates clockwise.

Why do they all revolve in the same direction, and most rotate in the same direction? Because of the way the solar system formed. It formed out of a nebula – a giant cloud of gas and dust in space. This cloud had a slight rotation to it. Gravity caused the dust and gas to come together, but since the nebula was spinning, it collapsed into a disk instead of a sphere.

The center of the disk, that’s where the Sun formed. The rest of the disk (now rotating quite nicely) is where the planets formed. So all the planets revolve in the same direction because that’s the direction the original nebula was rotating.

Why do some planets now rotate backward? They got clobbered by one or more large asteroids while they were forming, which caused their rotation rate/direction to change. Earth got clobbered, too, at least once – that’s how we got our Moon!

Effects of Earth’s revolution and tilt

The Earth’s revolution has several effects including the seasons and the variable duration of days/ nights. Also, the Earth’s tilt and axis relative to its orbital plane have a significant effect as well. This results in one hemisphere tilting toward the sun and the contralateral hemisphere tilting away. The hemisphere tilted towards the sun will experience warmer weather and longer daytime hours.

Whereas the hemisphere titled away from the sun will experience cooler temperatures and shorter daytime hours. This variation in daytime hours and average temperature cases by revolution and tilt results in the different seasons of the year. If the Earth were exactly perpendicular to its orbital plane, the seasons would not occur. It would also cause both hemispheres to experience approximately 12 hours of daylight and darkness during a 24-hour period. The Earth’s current axis is 23.5 degrees, if it were to be tilted more, this would result in warmer summers and colder winters. respectively.

For example, the summer solstice occurs when the Northern Hemisphere is at its maximum tilt toward the sun. During this period the sun will be directly overhead long the latitude of 23.5 degree N; otherwise known as the Tropic of Cancer. During the first day of summer, location along the latitude of 23.5 degree of the North pole experience 24hrs of daylight.

Altitude & Latitude

First, altitude describes how high a certain point is located above sea level. It mainly affects the climate in regions situated at high altitudes by making them cooler as the air pressure and temperature decreases. An example of a high-altitude region is the Himalayas, with an altitude of nearly 9000 meters, and fall in temperature from 0.2 to 1.2 degree Celsius every 100 meters.

These regions are typically characterized by high amounts of precipitation, strong winds and low levels of oxygen due to the lower air pressure. Altitude does affect climate, but primarily the local climate of a specific location; it does not contribute to affecting the entire planet’s climate. This thus downplays its significance in contributing to the Earth’s climate.

The further away a location is from the equator, the less sunlight it receives to heat the atmosphere because the sun’s rays are dispersed over a larger area of land as you move away from the equator due to the curvature of the Earth. As a result, places nearer to the equator such as the Sahara Desert tend to be hotter with a mean temperature of 30-40 degrees Celsius, as opposed to the polar regions with an average temperature of 0 to -40 degrees Celsius.

E vidence of the Earth’s Rotation and Revolution

Computing the speed of Earth’s revolution around the Sun:

•Circumference of Earth’s orbit = 940,000,000 kilometers

•Time for one revolution = 365 1/4 days = 8766 hours

•Speed of revolution = Distance/Time = 940,000,000 km / 8766 hr = 107,000 km/hr = 30 km/sec

•Uniform motion is difficult to detect. Although it is possible to detect the Earth’s circular motions, the effects are subtle, and were not detected until the 18th and 19th centuries, long after Copernicus proposed that the Earth was in motion.

The Coriolis effect was first described in 1835 by a French scientist by the name of Gustave Coriolis. If you are located at the equator, and fire a cannonball north or south, you find that the cannonball swerves to the east.

The net result of the Coriolis Effect: In the Northern Hemisphere , projectiles swerve to the right. In addition, air rushing inward to a low pressure area will swerve to the right, and set up a COUNTERCLOCKWISE hurricane. 

In the Southern Hemisphere , projectiles swerve to the left, and air rushing inward to a low pressure area will set up a CLOCKWISE hurricane.

The Foucault pendulum was first demonstrated in 1851 by yet another French scientist; Jean Foucault. The Foucault pendulum is nothing more than a very long pendulum suspended from a well-oiled ball-and-socket joint overhead, so it is free to swing in any direction. 

Foucault set up such a pendulum in the Pantheon in Paris, and set it swinging north to south. As hours passed, however, the direction in which the pendulum was swinging moved around in a clockwise direction. After a while, the pendulum was swinging northeast-southwest; after a while longer, it was swinging east-west, then southeast-northwest, then north-south again.

What causes this change in the pendulum’s direction of swing? The rotation of the Earth, of course.

The important fact (independent of where you’re standing) is that the Earth and the pendulum’s swing are rotating relative to each other. If the Earth did not rotate on its axis, the direction of swing of a Foucault pendulum would remain fixed relative to the surface of the Earth.

When Copernicus proposed his heliocentric theory, his critics pointed out that if the Earth orbits the Sun once per year, then the Earth’s location in October (for instance) should be 2 astronomical units away from its location in April, half a year later. This change in the Earth’s location must cause the nearby stars to shift in apparent location relative to more distant stars.

Stellar parallax was searched for by astronomers from antiquity onward. However, prior to the invention of the telescope, stellar parallax was not observed.

There are two possible hypotheses:

(1) There is no stellar parallax because the Earth is stationary. This is the hypothesis put forward by the supporters of the geocentric universe.

(2) Stellar parallax exists, but it is too small to be detected, because the stars are too far away. This is the hypothesis put forward by Copernicus and other supporters of the heliocentric universe.

In fact, stellar parallax was first detected by Bessel (using a telescope) in the year 1837, nearly three centuries after the death of Copernicus. In general terms, parallax can be defined as the shift in the observed position of an object, resulting from a change in the observer’s location.

So, Copernicus was right:

•The Earth does rotate about its axis.

•The Earth does revolve around the Sun.

•The stars are very distant from the Sun.

However, Copernicus wasn’t vindicated by direct observational evidence until centuries after his death!

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51 Comments

What about effects of rotation of the earth on its own axis

not enough information to complete my report

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It doesn’t mention some important facts; such as when it is summer, winter, spring and autumn

The phenomena of revolution gives us?

This is actually amazing. The difference of Time from place to place is caused by the earth rotation.

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I have a “Question”so i tried to look why does it affect oír view of the might sky

I need more explanation on solar system

I want further distinctions and details

Can I get the major effects of the earth movement

There has to be more information on the earth’s rotation, there was only one THING about it.

I need information on how earth’s rotation around the sun shapes the Earth. I also am needing information on how different forces shape the earth; Volcanoes, earthquakes, erosion, Earth’s rotation around the sun, ocean currents, tides, weather, and the water cycle. I am currently researching how these shape the earth and i will need more info.

In the picture with the earth rotating around the sun… if the earth rotates exactly 360 degrees, or 1 revolution every 24 hours (i.e. one revolution every day), should not the same identical picture of the earth be in each of the four positions for each of the four months indicated (jan, march, sep, dec)… oh, but that might might mess up the shading from the sun on each of the earth pictures… but, maybe some food for thought. That will really get those brain juices flowing. Just something to think about. So if it were noon in southern California on January 1, the sun would be directly overhead. Now fast forward exactly 180 days. I will have made exactly 180 revolutions around the earth. I will also have made a trip exactly 180 degrees around the sun. The problem is the sun is exactly 180 degrees behind me, which is now behind the other side of the earth. No one has ever explained that this model of the sun and earth don’t work, and you just figured it out. OOPs!

ok man i got you.

there are 2 things you aren’t taking into consideration. 1) after one day the earth only moves 1/365 of a way around the sun, not 1/360 2) sidereal days exist. a sidereal day is how long the earth takes to rotate once. the whole 360 degrees. a solar day is the day we use, and it’s the amount of time it takes for the sun to be in the exact same position. a sidereal day is only 4 minutes shorter than a solar day, so from day to day you can hardly tell, but if you take 4 minutes every day for the next 180 days you get 720 minutes. 720 minutes divided by 60 (60 minutes in an hour) gets you 12 hours, or half a rotation. so you’d still be facing the sun at solar noon 180 days later.

I only want about earths rotation

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What about the spin of the Earth?

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guys can you tell me how revolution and rotation affect the seasons on earth?

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I THOUGHT IT WAS VERY GOOD!

INFORMATION AND REASONS SHOULD BE EXTENDED.

Not full think but good. I need a big one on effects of revolution

Question: Is the total orbital distance traveled in one year the same distance in the “more elliptical” and the “less elliptical” cycle?… is the time it takes to complete 1 revolution around the sun the same in each maximum / minimum orbits? ( Just wonder if one year has always been the length of the current year)

I thought this was good to! I hope to read more of these articles

yap. wow. now i know every thing about earth

I need more effects of earth’s revolution

not enough info to complete homework

Completing summer holiday homework 🙁

There are other sources of information. I was taught to not get all my research from one source anyway.

Yes you are right

thank you helped me so much on my exam studies!

Our Solar System formed out of a gaseous dust cloud. When it condensed by gravity, it began to spin. That spin caused the system to flatten out into a pancake form. The Sun and planets formed but the spin never stopped. So all objects revolve around the Sun in the same direction.

DO ALL THE PLANETS REVOLVE AROUND THE SUN IN THE SAME DIRECTION?

yeah. thanks for the info. but i really need more explicit information. 🙂

Thank you so much!

@mirz claire

On what specifically?

I need more info!

I need more spesific effects of the revolution and rotation

i would like 2 learn more.

now,iknow why there is a night and day

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• Africa: Countries
• Americas: Countries
• Asia and Oceania: Countries
• Europe: Countries
• Africa: Regions
• Americas: Regions
• Asia and Oceania: Regions
• Europe: Regions
• Hydrosphere
• Lithosphere
• Prehistoric Life
• Solar System

Rotation of the Earth: Day and Night

The rotation the spinning of the Earth around its axis. " >rotation of the Earth is the cause of our days and nights. The Earth rotates around its axis the straight line around which an object rotates. " >axis . The axis is an imaginary line that runs from the North to South Poles through the Earth’s center. One rotation takes 24 hours to complete. A solar day is the amount of time needed for the Earth to make one rotation.

Elements of the Earth’s Rotation

The earth’s axis and solar day.

The line from the North Pole to the South Pole (through the center of the earth) is the Earth’s axis. The earth rotates around this axis. The planet a celestial body that orbits a star and has enough mass to take on a round shape. " >planet makes one complete rotation every 24 hours. This is one solar day. However, due to the effects of the moon, the length of one solar day is slowly increasing by a few milliseconds each century.

Daylight and Darkness

As Earth makes one complete rotation, half of the planet is exposed to the sun. This half of the planet experiences daylight. The other half of the planet facing away from the sun experiences darkness. In this manner, the rotation of the Earth around its axis is responsible for days and nights.

Direction of Rotation

The Earth rotates from the west to the east. It rotates in a counter-clockwise direction when viewed from space (above the North Pole). As a result, the sun rises in the east and sets in the west.

Cause of the Rotation

The Earth was formed from a cloud of gas and dust. As the gas and dust collapsed the momentum of these particles caused the cloud to spin. Momentum is the energy of a moving object. The collision of the Earth and Theia also contributed to this effect.

Reflections

• The axis is the line running through the Earth from the North Pole to the South pole.
• The earth rotates around its axis every 24 hours.
• The earth’s rotation is responsible for days and nights.

Bibliography

• A Lithosphere Study Guide
• About the Earth’s Surface
• Explore Earth, NASA.
• lithosphere

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Kant's cosmogony as in his Essay on the retardation of the rotation of the earth and his Natural history and theory of the heavens, with introd., appendices, and a portrait of Thomas Wright of Durham. Edited and translated by W. Hastie

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Short Essay on Our Planet Earth [100, 200, 400 words] With PDF

Earth is the only planet that sustains life and ecosystems. In this lesson, you will learn to write essays in three different sets on the planet earth to help you in preparing for your upcoming examinations.

Short Essay on Our Planet Earth in 100 Words

Earth is a rare planet since it is the only one that can support life. On Earth, life is possible for various reasons, the most essential of which are the availability of water and the presence of oxygen. Earth is a member of the Solar System. The Earth, along with the other seven planets, orbits the Sun.

One spin takes approximately twenty-four hours, and one revolution takes 365 days and four hours. Day and night, as well as the changing of seasons, occurs due to rotation and revolution. However, we have jeopardized our planet by our sheer ignorance and negligence. We must practise conservation of resources and look after mother earth while we have time.

Short Essay on Our Planet Earth in 200 Words

Earth is a blue planet that is special from the rest of the planets because it is the only one to sustain life. The availability of water and oxygen are two of the most crucial factors that make life possible on Earth. The Earth rotates around the Sun, along with seven other planets in the solar system. It takes 24 hours to complete one rotation, and approximately 365 days and 4 hours to complete one revolution. Day and night, as well as changing seasons, are all conceivable due to these two movements. 

However, we are wasting and taking advantage of the natural resources that have been bestowed upon us. Overuse and exploitation of all-natural resources produce pollution to such an alarming degree that life on Earth is on the verge of extinction. The depletion of the ozone layer has resulted in global warming. The melting of glaciers has resulted in rising temperatures.

Many animals have become extinct or are endangered. To protect the environment, we must work together. Conversation, resource reduction, reuse, and recycling will take us a long way toward restoring the natural ecosystem. We are as unique as our home planet. We have superior intelligence, which we must employ for the benefit of all living beings. The Earth is our natural home, and we must create a place that is as good as, if not better than, paradise.

Short Essay on Our Planet Earth in 400 Words

Earth is a unique planet as it is the only planet that sustains life. Life is possible on Earth because of many reasons, and the most important among them is the availability of water and oxygen. Earth is a part of the family of the Sun. It belongs to the Solar System.

Earth, along with seven other planets, revolves around the Sun. It takes roughly twenty-four hours to complete one rotation and 365 days and 4 hours to complete one revolution. Rotation and revolution make day and night and change of seasons simultaneously possible. The five seasons we experience in one revolution are Spring, Summer, Monsoon, Autumn, and Winter.

However, we are misusing resources and exploiting the natural gifts that have been so heavily endowed upon us. Overuse and misuse of all the natural resources are causing pollution to such an extent that it has become alarming to the point of destruction. The most common form of pollution caused upon the earth by us is Air Pollution, Land Pollution, Water Pollution, and Noise Pollution.

This, in turn, had resulted in Ozone Layer Depletion and Global Warming. Due to ozone layer depletion, there harmful ultraviolet rays of the sun are reaching the earth. It, in turn, is melting glaciers and causing a rise in temperature every year. Many animals have either extinct or are endangered due to human activities.

Some extinct animals worldwide are Sabre-toothed Cat, Woolly Mammoth, Dodo, Great Auk, Stellers Sea Cow, Tasmanian Tiger, Passenger Pigeon, Pyrenean Ibex. The extinct animals in the Indian subcontinent are the Indian Cheetah, pink-headed duck, northern Sumatran rhinoceros, and Sunderban dwarf rhinoceros.

The endangered animals that are in need of our immediate attention in India are Royal Bengal Tiger, Snow leopard, Red panda, Indian rhinoceros, Nilgiri tahr, Asiatic lion, Ganges river dolphin, Gharial and Hangul, among others. We have exploited fossil fuels to such an extent that now we run the risk of using them completely. We must switch to alternative sources of energy that are nature friendly. Solar power, windmills, hydra power should be used more often, and deforestation must be made illegal worldwide.

We must come together to preserve the natural environment. Conversation, reduction, reuse and recycling of the resources will take us a long way in rebuilding the natural habitat. We are as unique as our planet earth. We have higher intelligence, and we must use it for the well-being of all living organisms. The Earth is our natural abode, and we must make a place as close to Paradise, if not better.

Hopefully, after going through this lesson, you have a holistic idea about our planet Earth. I have tried to cover every aspect that makes it unique and the reasons to practise conversation of natural resources. If you still have any doubts regarding this session, kindly let me know through the comment section below. To read more such essays on many important topics, keep browsing our website. 

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Essay on Earth

500 words essay on earth.

The earth is the planet that we live on and it is the fifth-largest planet. It is positioned in third place from the Sun. This essay on earth will help you learn all about it in detail. Our earth is the only planet that can sustain humans and other living species. The vital substances such as air, water, and land make it possible.

All About Essay on Earth

The rocks make up the earth that has been around for billions of years. Similarly, water also makes up the earth. In fact, water covers 70% of the surface. It includes the oceans that you see, the rivers, the sea and more.

Thus, the remaining 30% is covered with land. The earth moves around the sun in an orbit and takes around 364 days plus 6 hours to complete one round around it. Thus, we refer to it as a year.

Just like revolution, the earth also rotates on its axis within 24 hours that we refer to as a solar day. When rotation is happening, some of the places on the planet face the sun while the others hide from it.

As a result, we get day and night. There are three layers on the earth which we know as the core, mantle and crust. The core is the centre of the earth that is usually very hot. Further, we have the crust that is the outer layer. Finally, between the core and crust, we have the mantle i.e. the middle part.

The layer that we live on is the outer one with the rocks. Earth is home to not just humans but millions of other plants and species. The water and air on the earth make it possible for life to sustain. As the earth is the only livable planet, we must protect it at all costs.

Get the huge list of more than 500 Essay Topics and Ideas

There is No Planet B

The human impact on the planet earth is very dangerous. Through this essay on earth, we wish to make people aware of protecting the earth. There is no balance with nature as human activities are hampering the earth.

Needless to say, we are responsible for the climate crisis that is happening right now. Climate change is getting worse and we need to start getting serious about it. It has a direct impact on our food, air, education, water, and more.

The rising temperature and natural disasters are clear warning signs. Therefore, we need to come together to save the earth and leave a better planet for our future generations.

Being ignorant is not an option anymore. We must spread awareness about the crisis and take preventive measures to protect the earth. We must all plant more trees and avoid using non-biodegradable products.

Further, it is vital to choose sustainable options and use reusable alternatives. We must save the earth to save our future. There is no Planet B and we must start acting like it accordingly.

Conclusion of Essay on Earth

All in all, we must work together to plant more trees and avoid using plastic. It is also important to limit the use of non-renewable resources to give our future generations a better planet.

FAQ on Essay on Earth

Question 1: What is the earth for kids?

Answer 1: Earth is the third farthest planet from the sun. It is bright and bluish in appearance when we see it from outer space. Water covers 70% of the earth while land covers 30%. Moreover, the earth is the only planet that can sustain life.

Question 2: How can we protect the earth?

Answer 2: We can protect the earth by limiting the use of non-renewable resources. Further, we must not waste water and avoid using plastic.

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Geography Notes

Essay on the earth: top 8 essays on earth.

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Here is a compilation of essays on ‘Earth’ for class 7, 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Earth’ especially written for school and college students.

Essay on Earth

Essay Contents:

• Essay on the Energy Intercepted by the Earth

Essay # 1. Origin of the Earth :

The earth came into existence between 5000 to 6000 million years ago from condensed form of a cloud of gases. By studying the hot, luminous gases of the sun, we find that sun is made of the same basic elements that are found by chemical analysis of earth’s material. In fact all the stars which have been studied seem to have the same elements.

Origin of Continents and Basins :

At present the crust of the earth is largely made up of two different kinds of materials of rocks called granite and basalt. The average specific gravity of the earth is 5.5 while that of granite and basalt is 2.7 and 3.0, respectively. Granite is typically found in continental areas and the ocean floors are made of basalt.

If rafts of basalt and granite could be imagined as floating on a very heavy plastic material, the elevation of continents could be imagined as being due to the lower specific gravity of granite in the continents and the greater specific gravity in the basins.

We do not know just how the surface of the crust became separate into granite and basaltic sectors. One theory is that while the earth was still liquid, masses of granite, like flocks of foam, floated in a still liquid basaltic sea.

When the crust was solidified, the granite masses projected to form the continents. Another hypothesis holds that the continents have been growing throughout earth’s history by building of successive thick mountain ranges.

Essay # 2. Composition of the Earth :

The outer envelopes of the gaseous material surrounding the earth are called atmosphere. Under the atmosphere is our earth on which we live. That part of the earth, which is in the form of a land, is known as the earth’s crust. It also includes the highest peaks of mountains and floors of the oceans. Part of the land, which is visible on the Globe, is called the Lithosphere (Greek, Litho = Stone).

We know that nearly 75 per cent of the whole surface of the earth is covered with natural waters like oceans, seas, lakes, rivers etc. Which is in the form of more or less, a continuous envelope around the earth.

This envelope of water is called Hydrosphere (Greek, Hudous = Water). Thus, Lithosphere and Hydrosphere in a combined form is known as the Earth’s crust. Under the Earth’s crust is the interior of the Earth. It is further sub-divided into three shells. Depending upon the nature, the material is made up as shown in the Fig. 1.1.

The earth is composed of different rocks. In an ordinary sense the term rock means something hard and resistant but the meaning of the word has been extended so as to include all natural substances of the Earth’s crust, which may be hard like granite or soft like clay and sand.

It has been estimated that 95 per cent of the Earth’s crust is made up of primary i.e., first formed (Igneous) rocks which is mostly composed of Granite having Quartz, Feldspar, Biotite mica and Hornblende in varying proportions the remaining 5 per cent of the crust is made up of Secondary (Sedimentary or Metamorphic) rocks (as shown in Fig. 1.2). The Earth’s crust is in the form of a very thin layer of solidified rocks and is heterogeneous in nature.

These rocks may be classified on the basis of their density into the following two groups:

1. Sial (Si = Silicon and A1 = Aluminium) having density 2.75 to 2.90.

2. Sima (Si = Silicon and Ma= Magnesium) having density 2.90 to 4.75.

It has been estimated that the Sial rocks are about 70 per cent of the Earth’s crust, which include chiefly Granite and Silica. These rocks are generally on the upper regions of the crust.

Sima rocks include heavy and dark coloured rocks like Basalts. In these rocks, the percentage of Silica is reduced and Magnesium attains the next importance in place of Aluminium of Sial rocks. These rocks are generally found on the floors of the Oceans and beneath Sial rocks.

It is the part of the earth below the crust and surrounding the core. The imaginary line that separates the lithosphere from the mantle is known as ‘Moho’ (Mohorovicic discontinuity). Because of high temperature and great pressure, the mineral matter in this part is the molten condition.

It is the innermost layer of the earth; it extends from below the mantle (Gutenberg discontinuity) to the central part of the earth.

On the basis of earthquake waves, the core has been further divided into two cores:

(a) Outer core.

(b) Inner core.

The outer core is 2,250 km thick and surrounds the core. It is believed that it is still in molten condition.

The inner core is also called ‘Nife’ because it consists of Nickel and iron. Its thickness is about 1,228 km. It is very hard in nature.

Essay # 3. Motions of the Earth :

The earth is held in space by combined gravitational attraction of sun and other heavenly bodies and has motions that are controlled by them.

The two principal motions of earth are:

1. Rotation of earth about its axis

2. Revolution of earth around sun

(i) Rotation :

The earth rotates upon an imaginary axis, which owing to the polar flattening is the shortest diameter. The earth rotates from west to east (in anti-clockwise direction) and it takes 24 hours to complete one rotation. During this period most of the places on the sphere are turned alternately towards and away from the sun, have experienced a period of light and darkness.

This causes day and night. This unit of time is called solar day. The direction of rotation (west to east) not only determines the direction in which the sun and stars rise but is also responsible for the direction of prevailing winds and ocean currents.

Importance of earth’s rotation :

The effects of the earth’s rotation are of great importance to the environment. The rotation indirectly accounts for the diurnal changes in weather such as warming up during daytime and cooling down at night. Thus the rotation affects diurnal rhythm, day light, air temperature, air humidity and air motion. Plants and animals respond to this diurnal rhythm. Green plants store energy during day and consume some part of heat during night.

Rotation of earth turns both air and water in one direction. The flow of air and water are turned towards right in the northern hemisphere towards left in the southern hemisphere. This phenomenon is called the coriolis effect. It is of great importance in studying the earth’s systems of winds and ocean currents.

(ii) Revolution :

The path of the earth around the sun is called orbit and the rotating earth revolves in a slightly elliptical orbit about the sun from which it keeps an average distance of 150 million km.

The time required for the earth to pass one complete orbit fixes the length of the year and this journey takes a few minutes less than 365 1/4 days (365.242 solar days). Earth revolves around the sun in anticlockwise direction. The rate of earth’s revolution is more than 1,06,260 km/hour.

Importance of revolution :

The rotation and revolution of earth are of great significance in meteorology. The rotation indirectly accounts for the diurnal changes in weather such as warming up during daytime and cooling down at night. Seasonal changes are dependent on the revolution of earth. When the earth is at perihelion (January 3), the sun is close to the earth, as a result greater intensity of solar radiation is received at the earth surface.

This position occurs during winter season. When the earth is at aphelion, the earth is farthest from sun, as a result the heat received at the earth surface is less. This occurs during summer season (July 4). However, the distance between sun and earth varies only about 3 per cent during one revolution.

As the earth moves forward in its orbit, its axis remains inclined at 23 1/2° from the perpendicular to the plane of the earth’s orbit. This tilt of 23 1/2° does not change throughout the year as the earth revolves around the sun.

It causes the change in seasons regularly through spring, summer, autumn and winter because of the inclination of the earth’s axis, constant direction of tilt of that axis and revolution of the earth around the sun.

Sidereal day :

The true rotation time is called sidereal day. It is denoted by ‘S’.

The number of hours, minutes and seconds in a sidereal day are given below:

Lengths of the day :

Earth receives solar radiation from the sun during day time. Day length can be defined as the total time between sunrise and sunset. Length of the day is partly controlled by the latitude of the earth and partly by the season of the year. The day length at the equator is about. 12 hours throughout the year, whereas at the poles it varies between 0 and 21 hours from winter to summer.

Solar radiation received at any location of the earth depends upon the day length. Maximum amount of solar radiation is received in the higher latitude during summer solstice because it is period of continuous day.

The amount of solar radiation received during the December solstice in southern hemisphere is theoretically greater than that received in the northern hemisphere during the June solstice. The equator has two radiation maxima at the equinoxes and two minima at the solstices.

Length of the day plays an important role in the life cycle of the crop plants. In fact, day length indicates the photoperiod available for the growth of the crop plants. Every plant requires different photoperiod for the initiation of flowering. On the basis of day length, plants can be divided into different categories. The plants which require less than 10 hours day length, are called short day plants.

If the requirement of the plants is greater than 14 hours day length, then these are called long day plants. In between these two types, there are intermediate plants, which require photoperiod of 12 – 14 hours. However, the plants which are not affected by day length, are called day neutral plants.

Essay # 4. Movement of the Earth:

Many changes on the crust of the earth can be seen as a result of the works of internal forces in the earth’s interior. The works of internal forces are generally called earth movement. Sometimes, the earth movement may be very very slow and sometimes it may be sudden.

It is believed that originally the landmasses were united together in the form of a great landmass known as Pangaea. In course of time the Pangaea had broken into several pieces and drifted into different directions. The drifting is called Continental Drift and the theory was propounded by Alfred Wagner. The northern part of the landmass was known as Laurasia.

Eventually it had broken down to form North America, Europe and Asia. About 120 million years ago, the southern part of India, East Africa, Madagascar, Australia, South Africa, South America and the Antarctica were together and formed the single landmass known as ‘Gondwana land’. The ‘Gondwana land’ started breaking into several pieces and India took its present shape about 60 million years ago.

During the last million years, the Himalayas had risen to its present height due to earth movements. Similarly, it has been proved that the Aravallies and the Vindhyas in the middle of India were once at the bottom of the sea.

The forested areas near Bombay harbour, the Mahabalipuram temple in the sea, submergence of a vast area of nearly 5000 sq km in the Runn of Kutch during 1819 and a land of about 1500 sq km raised to a height of several metres, are some of the results of the earth movement.

The earth movements which bring about vast changes are called Tectonic movements. It has been already mentioned that earth movements may be very slow and sudden.

Slow Movements :

The slow movements of the earth’s crust are due to various chemical and physical reactions that take place at the earth’s interior. The movement may be so slow that its result may not be seen on the surface during 100 to 200 years.

The raising of the eastern coastal plain up to a height of 15-30 metres, the existence of coal beds below the sea level in the Sundarban Delta, the existence of a forest near the Bombay harbour and submergence of a vast area in the Runn of Kutch are some of the Indian examples of slow earth movement.

A change in sea level in its advance or retreat with respect to adjacent land is relative to each other. When the sea advances to land, it is generally called a Positive Movement and the land advancing against the sea is known as Negative Movement.

On the basis of the structural changes that are caused by the tectonic movement, the earth movements may be grouped into two classes:

(i) Vertical or Epeirogenic Movement

(ii) Horizontal or Orogenic Movement.

Vertical Movement :

Due to earth movement some parts of earth surface may be raised or sunk with respect to the surrounding areas. This type of movement is known as Vertical Movement. When a part of the earth’s crust is raised in relation to its surrounding area, it is known as uplift. In the same way when a portion is sunk in relation to its surrounding areas, it is called subsidence.

Earth movement of this type, when takes place over extensive area generally leads to the building up of continents and Plateaus. That is why, this type of movement is also known as Epeirogenic movement or Continent building movement. As a result of their movement, the horizontal arrangement of the earth’s crust remains almost undisturbed.

Millions of years ago there used to be a continent where we find Atlantic Ocean today. In the beginning of the earth’s history such movements had been more frequent and the present-day arrangement of the continents might have been the result of this movement.

Horizontal Movement :

The forces of horizontal movement affect tangentially. It involves both the forces of compression and tension. These two types of movements are related to each other. Compression in one part of the crust is bound to produce tension at another place. The compression leads to the bending of horizontal layers into a shape known as fold.

The tension is responsible for breaking of rock layers with subsequent sliding or displacement. It is known as fault. The processes of making folds and faults are known as folding and faulting.

When two horizontal forces act towards a common point from opposite directions folding takes place. Deep within the earth, this force tends to cause bending of rock strata. Like seawaves, rocks are thrown into upfolds and downfolds. The upfolds are called anticlines and the downfolds are known as synclines.

When two forces act horizontally in opposite directions from a common point, it generates tension and the process is known as faulting. As a result, the rocks break along a line which is known as fault line. The faulted rocks may be thrown upwards or slided downwards.

The mountains over the surface of the earth owe their origin to the process of folding and faulting. That is why, the horizontal movement is also known as orogenic or mountain building movement. The Himalayas on the northern border of India, the Alps of Europe, the Rockies of the North America and the Andes of South America are some of the newly folded mountain ranges of the world.

Aravallies, Ural, Tiensan and Appalachia are some of the old folded mountains of the world. Similarly, the Black Forest of Germany, the Voges of France, the Vindhyas and the Satpura of India are some of the examples of fault or block mountains of the world.

Plate Tectonics :

Plate tectonics is the most modern theory about the formation of folded mountains. According to this theory, the world has been divided into six major plates and several smaller plates. Each of the plates is composed of crust up to a depth of 100 km from the surface of the earth.

Due to the forces at the earth’s interior, these plates are moving in different directions. As a result of rubbing of the two plates, the folded mountains have been formed at the edges of the plates.

The six major plates are:

(1) Pacific Plate

(2) North American Plate

(3) South American Plate

(4) African Plate

(5) Eurasian Plate

(6) Indo-Australian Plate.

The smaller plates are:

(a) China Plate

(d) Nazca Plate

(c) Cocos Plate

(b) Antarctic Plate

(e) Caribbean Plate

Where plates separate and new ocean floor is created, mid-ocean ridges are the boundaries. The plates are rigid. Their boundaries are marked by earthquakes and often by volcanoes. Where plates collide and overlap, young mountains, arcs and trenches are the boundaries.

Sudden Movement :

Sudden movement of the earth crust can be noticed during earthquake. Some parts of the land surface of New Zealand were raised by about 3 metres during the earthquake of 1885. Similarly, some areas of Japan sank by about 6 metres during 1891 earthquake. Recently, during the earthquake of 1950, the bed of the Brahmaputra River had been raised leading to various changes in the valley.

Essay # 5. Interior of the Earth :

To know exactly about the interior of the earth is more difficult than that of taking photograph of other planets with the help of satellites or by walking on the surface of the moon. It has not been possible till today to collect direct evidences about the structure of the earth.

However, geographers and geologists have collected indirect evidences about the structures and composition. On the average, the radius from the centre to the surface of the earth is 6320 km but, the deepest mine in the world in South Africa is about only 4 km deep and man could dig up to a maximum depth of 6 km in search of oil.

In other words, man has been able to get direct evidences about the structure and composition of the earth’s interior up-to a depth not more than 5 to 6 km from the surface.

The knowledge beyond this limit is based primarily on indirect scientific evidences. The indirect evidences are based on temperature and pressure inside the earth, density of materials and behaviour of earthquake waves. Still uncertainty persists. On the basis of the different scientific observations it has been concluded that there exists different layers inside the earth.

Temperature and Pressure of the Earth’s Interior :

The hot and molten lava, ash, smoke that come out at the time of volcanic eruption as well as the hot water springs are some of the evidences which confirm that interior of the earth is having a very high temperature. It has been found from mining operations also that the temperature increases at the average rate of 1°C per 32 metres depth.

At this rate of increase of temperature, the rocks at great depth of the earth’s interior should be in molten state. Actually, this was the view earlier that the crust of the earth is floating on a massive molten materials.

But, the study of earthquake waves has indicated that the temperature does not increase uniformly from the surface to the centre of the earth. The rate of increase in temperature is not uniform. Scientists also have proved that the main reasons of increase in temperature are the fusion of radio-active materials and other chemical reactions. The tremendous pressure from the overlaying materials makes the melting point higher.

On the basis of this, in upper 100 km, increase of temperature is estimated to be 12°C per km; in the next 300 km it is 2°C per km and below it 1°C per km. At this rate the temperature at the core of the earth is estimated to be 6000°C.

At this temperature, the materials in the central part of the earth’s interior should have been at gaseous state but due to tremendous pressure from the outer layers, the materials assume liquid properties and acquire properties of solid or plastic state. Therefore, the earth behaves mostly as solid down to a depth of 2900 km because of tremendous pressure.

Density and Composition of the Earth’s Interior :

By studying the speed and path of earthquake waves, temperature and pressure conditions inside the earth, scientists are of the opinion that the physical properties, density and composition of the materials are different at varying depths. The structure of the earth is therefore layered. The earth consists of three layers, one inside the other like an onion. They are the crust, the mantle and the core.

The topmost layer of the earth is solid, the thinnest, and the lightest and is known as Lithosphere. The lithosphere has again two layers-outer part immediately below the newer sedimentary formation, popularly known as crust and the inner part of greater strength. The crust of the earth is composed of sedimentary and granitic rocks.

The inner layer of lithosphere has basaltic and ultra-basic rocks. While the outer layer of lithosphere is found mainly under continents, the inner layer is found partly under oceans. The average density of lithosphere is 2.65 to 2.90. ‘Silica’ and ‘Aluminium’ are abundant. Therefore, it is popularly known as SIAL (Silica + Aluminium). The average thickness is 8 to 100 km.

Below this top layer is the layer of basalt rock which is heavier than the topmost layer. The density varies from 3.1 to 5.00. It assumes the properties of solid and partly plastic materials. The average thickness of this layer is 100 to 2900 km. In this layer, Silica and Magnesium elements predominate and it is popularly known as SIMA (Silica + Magnesium).

The SIMA also has two layers—Inner silicate layer at the top with average thickness of 100 to 1700 km and Transitional zone of mixed metals and silicates with an average thickness of 1700 to 2900 km. These two SIMA layers are also known as MANTLE. The surface that separates crust and mantle is known as Mohorovicic Discontinuity or simply MOHO.

Finally, the innermost layer exists at the central core of the earth with density 5.1 to 13.00. It is composed of the heaviest mineral materials. This central mass is mainly made of ‘nickel’ and ‘iron’ therefore known as NIFE (Nickel + Ferrous). The core contains 1/3 of the entire mass of the earth.

The materials of this part may be in liquid, plastic or even solid state due to tremendous pressure from above. It has also two layers-outer metallic core with average thickness of 2900 to 4980 km and inner metallic core between 4980 to 6400 km. The inner metallic core is also known as Barysphere (Fig. 2.4).

The three layers of the earth have been called by different geologists in different manners. German scientist Gracht called them SIAL, SIMA and NIFE. Jeffrey called them as Top, Middle and Lower layers while Professor Holmes called them the Crust, the Substratum and the Core. The relationship has been mentioned in Table 2.2.

From Table 2.2, it can be estimated that crust of the earth forms less than 1 per cent; mantle 16 per cent and 83 per cent makes the core. The earth being a spherical body has materials of varying densities at varying depths.

Materials of the Earth’s Crust :

The word ‘Lithosphere’ means a sphere of rocks. The upper portion of lithosphere is referred to as the crust of the earth. Down to depth of nearly 16 km from the surface under the continents, 95 per cent of the materials that form the crust consist of rocks and the rest 5 per cent minerals.

The term rock refers to hard masses of earth’s crust as well as loose and soft particles like sand and clay. The rocks are formed of the mixture of various minerals. All rocks do not have same chemical composition and structure.

But, every mineral has its own chemical composition and physical properties. The minerals generally occur in the form of crystals. The rocks and minerals are generally composed of certain chemical elements like oxygen, silica, aluminium, iron and calcium, etc.

Each mineral usually contains two or more simple substances called elements. There are about 2000 minerals but only 12 are common all over the earth. These 12 minerals are basically responsible for the formation of rocks.

Mineral may be defined as a naturally occurring non-living solid substance possessing certain physical properties and definite chemical composition. The minerals may be either elements or compounds and also metallic or non-metallic.

The most abundant elements in nature are silicates, carbonates, chlorides, sulphates and oxides. As much as 87 per cent of the minerals of the earth’s crust are silicates and 59 per cent of the rocks are formed of the minerals of silica group.

The distribution of minerals in the earth’s crust is as follows (Table 2.3):

Minerals are of two types -Rock forming and Ore forming:

It is one of the most abundantly available minerals in earth’s crust. It has two elements— Silicon and Oxygen. They unite together to form a compound, known as carbonate of lime. It is transparent in its pure state. However, quartz may be of different colours when it is mixed up with other elements. Its hardness is 7, specific gravity 2.65. Quartz is hexagonal and its structure is SiO 2 (Silicon dioxide).

Feldspar is one of the important elements of rock and nearly 50 per cent of the earth’s crust is composed of feldspar. It is made of silicates of aluminium, potassium, sodium, calcium and oxygen. Feldspar is of two types—Orthoclase and Plagioclase.

Orthoclase has specific gravity of 2.57, its hardness is 6 and structure is Ca 2 SiO 4 (Calcium Silicate). The specific gravity of plagioclase feldspar is 2.60 to 2.74, hardness is 6 to 6.5 and the structural formula is Na 2 OAl 2 O 6 SiO 2 (Sodium Aluminium Silicate) and CaOAl 2 O 3 2SiO 2 (Calcium Aluminium Silicate).

Mica is formed of the elements of hydrogen, potassium, aluminium, magnesium, iron and silicon. Mica is of two types—Black Biotite and White Muscovite. Mica is found in thin sheets.

Its hardness is 2.5 to 3, specific gravity is 2.70 to 3 and the structural formulae are:

Black Biotite – (AlFe) 2 (MgFe) (HK) 2 (SiO 4 ) 3

White Muscovite – K 2 O 3 Al 2 O 3 6SiO 2 2H 2 O. (Potassium Aluminium Silicate)

Calcite is formed of the chemical composition of calcium, magnesium, carbon dioxide and oxygen. It is white in colour, it may take other colour also. Its hardness is 3, specific gravity 2.70 and structural formula is CaCO 3 (Calcium Carbonate).

Magnetite :

It is composed of Silicon, Iron and Oxygen. Its hardness is from 5.5 to 6.5 and specific gravity is 5.19. Magnetite is not transparent and chemical formula is Fe 3 O 4 (Ferros Ferric Oxide).

Haematite :

Haematite is also made up of Iron and Oxygen. Its hardness is 5.5 to 6.5, specific gravity is 4.9 to 5.3 and structural formula is Fe 2 O 3 (Ferric Oxide).

Graphite is another mineral made of carbon. Its hardness is 1.5 to 2, specific gravity is 2.15 and structural formula is C (Carbon).

In addition to the above, there are many other rock and ore forming minerals. The minerals that form with oxygen are called oxides. Quartz, Magnetite, Limonite (2Fe 2 O 3 ) 3 , (Ferric Oxide), Cromite (FeOCr 2 O 3 ), Alumina (Al 2 O 3 ) belong to oxide group.

The minerals that form of Calcium, Carbon and Oxygen are called carbonates. Calcite (CaCO 3 ) (Calcium Carbonate), Dolomite (CaMg) (Calcium Magnesium), Cidarite (FeCO 3 ) (Ferrous Carbonate) ‘etc.’ belong to this group. Mineral salt (NaCl) belong to chloride group and Gypsum (CaSO 4 2H 2 O) (Hydrated Calcium Sulphate) are of sulphate group.

The minerals that have only one element are known as Native Minerals, Gold, Silver, Lead, Copper, etc., belong to this group.

The Rocks :

In terms of origin, the rocks can be classified into two main varieties, namely, Igneous rocks and Sedimentary rocks. But when these rocks are subjected to prolonged fluctuations of temperature and pressure, they are transformed to a new variety which is termed as metamorphic rocks.

Essay # 6. Theories of the Earth:

There are several hypotheses about the origin of the universe and the earth. In 1755, German philosopher Immanuel Kant put forward a theory that a spherical mass of gas called Nebula was rotating and its size was like that of the sun.

Due to rotation and cooling through radiation, the outer portion became denser and rings were thrown out. In course of time, these rings condensed into planets while the remnant continues as the sun (Table 2.1).

In 1796, Laplace- a great French Mathematician supported the Nebular hypothesis. However, there are several criticisms against Laplace’s theory and the most important one is that rings cannot condense into planets.

Kelvin’s Nuclear clots Hypothesis and Chamberlin and Moulton’s Planetesimal Hypothesis are other two hypotheses relating to the origin of the earth. However, they also lack in certain aspects of acceptability.

Tidal Hypothesis :

Looking from different angles, the Tidal Hypothesis of Jean and Jeffreys – well-known scientist of England has been found to be more acceptable. According to this hypothesis, the sun was a big and extensive mass of gas moving in space. Once, another much larger star happened to come closer to the sun and due to gravitational pull of this star a tide occurred on the surface of the sun.

As a result, protuberances of material from the sun came out towards the approaching star and in course of time it gave birth to earth and other planets of the solar system (Fig. 2.1).

There are several points in favour of Jean and Jeffreys (Tidal Hypothesis):

(1) If the density of the sun increases from its surface towards the interior, it is quite natural that the protuberances come out from the surface like a filament having lesser density. Naturally, the protuberances produced by the passing star should have been thicker in the middle and thinner at both the ends. The arrangement of the planets in the solar system is also like a cigar-middle portion bigger and tapering towards the two ends.

When the passing star had gone far away, the filament has been broken into pieces and due to the gravitational force of the sun started rotating around the sun. The planets of bigger size in the middle and smaller size towards the two ends can be seen in Fig. 2.2. Mercury is the smallest and nearest to the sun, Jupiter, the largest is in the middle.

(2) The arrangement of the satellites of respective planet also confirms the validity of this hypothesis. Saturn, the second largest planet has the largest number of 18 satellites and Jupiter, the largest planet has 16 satellites. Uranus and Neptune have 17 and 8 satellites respectively.

(3) Lastly, on the basis of this hypothesis it can be seen that bigger planets remained in the gaseous state for a longer time and have helped in formation of more number of satellites than that of the smaller planets which condensed quickly and did not have scope for formation of satellites.

This hypothesis is also known as Hit and Run Hypothesis or Catastrophic Hypothesis or Tidal Action Hypothesis.

Tidal Disruption Theory :

The earth and planets and their satellites were all part of the sun or another sun like star at that time. There are many theories regarding the formation of the solar system and our earth. One is the tidal disruption theory by Jaans and Jeffereys. This theory states that in the beginning sun was hot and in a gaseous state.

A big star moved across, which caused the tidal disruption of hot gas and it left the sun with a revolving arm or a filament of hot gas, like a spiral nebula .The cooling of filament broke up the sun into masses which began to contract toward nuclei forming planets. The gaseous planets and their satellites continued to revolve as they did after the star passed away. Gradual cooling formed liquids and final solids.

The more volatile material of the earth remained in the gaseous state and formed our atmosphere, originally much deeper and of a higher temperature than now. As the atmosphere cooled, the water vapour condensed and formed clouds. As cooling continued, rain fell and oceans formed.

Steady State Theory:

Another theory which can be considered as alternative to the Big Bang Theory is the Steady State Theory. It is propounded by Hoyle. Here, Hoyle propounded that the Universe remained of the same size at any given point of time. However, this theory has been discarded after evidences of expanding universe.

Big Bang Theory :

Another recent and most convincing proposition regarding the origin of the universe including the planet earth is the Big Bang Theory or Expanding Universe Theory. Eduin Hubble provided evidence of expanding universe in 1920. The theory assumes that the universe began from huge mass of atoms called primeval atoms or cosmic eggs having a state of infinite density.

The universe initiated its origin 15 million years ago when a dense mass of material exploded in the so-called big bang. The explosion sent all of the materials of the universe outward in a cosmos that is still expanding. All of the galaxies, planets, asteroids, and other bodies in the universe were formed from the gas and dust of this extraordinary explosion.

This theory was put forward by an astronomer cum priest named George Lamaitre in 1927. It is felt that the expansion of the universe will continue for a long time. After that perhaps the expansion will slow down. At any point of time, if similar condition prevails the stimeval atoms may become active again and another explosion (big bang) may take place.

Although many scientists contributed to the development of this theory, it was George Gamow who coined the term Big Bang in 1946. Gamow with RA Alpher envisaged a high temperature state in the beginning of the universe.

Evolution of the Earth and Life forms :

The evolution of the earth can be described as follows in terms of various stages. In the initial stages the earth was barren, rocky and hot. It had thin layers of hydrogen and helium. During the period of 4000 million years till now the earth had been through several processes and life evolved.

From the top of the atmosphere to the centre of the earth different density materials have formed different layers. The process of separating denser materials from the lighter materials is called differentiation. The present atmosphere contains water vapour, nitrogen, carbon dioxide, methane and small quantity of oxygen. Plants are the major sources of oxygen on earth.

The oceans formed within 500 million years from the formation of the earth. Life began approximately 3000 million years ago. The process of photosynthesis began between 2500 and 3000 million years ago. The first life of the earth was confined to oceans in the form of small bacteria. The evolution of life from bacteria to modern man is shown by Geological Time Scale expressed in terms of Eons, Era, Period and Epoch.

Essay # 7. Numerical Facts about the Earth :

The earth is spherical in shape with a bulge at the middle and a slight flattening at the poles. The equatorial radius is 6374 km and the polar radius is 6357 km. The mean distance from the sun is 150 million km.

Continental Drift Theory :

The making of the continents began 200 million years ago (during the Persian period) with the split of gigantic landmass known as PANGEA. Two continents laurasia to the north and Gondwana to the south were formed. Later on, these were sub divided into smaller parts approximately the shapes of Africa, Eurasia North and South America, Australia and Antarctica as we know today. This theory is known as continental drift theory.

Continents and Oceans of the World :

During Persian period, outer surface of the earth was broken into 10 major and a number of minor sections called plates. It is on these plates that continents rest. The rifts between the plates were filled with molten material from the mantle pushing the plates to either side and farther and farther as the material continued to seep through. Since this material was heavier, it levelled off below sea level forming ocean floors as water from the pacific flowed in.

A continent is a large, continuous area of land on the earth. All continents together constitute less than one-third of the earth’s surface, more than two-third of the earth’s surface are covered with water. Two third of the continental land mass is located in the northern hemisphere.

There is no standard definition for the number of continents in the world. By most standards, there are a maximum of seven continents Africa, Antarctica, Asia, Australia/Oceania, Europe, north America and south America. In Europe, many students are taught about six continents, where north and south America are considered to form one America.

Many geographers and scientists now refer to six continents, where Europe and Asia are combined, called Eurasia, because they are one solid land mass. By the definition of a continent as a large continuous area of land, the pacific islands of Oceania are not a continent, but one could say, they belong to a continent e.g. Oceania is sometimes associated with the continent of Australia.

Essay # 8. Energy Intercepted by the Earth :

Radius of the earth = r

Area of the earth = π r 2

Solar constant = S

Total energy intercepted by the earth in unit time = π r 2 S

= 6.37 x 10 21 cal day -1

Surface area of the earth = 4 π r 2

If this energy is spread uniformly over the full surface of earth, then the energy received per unit area per unit time (Q S ) can be given as follows:

But the distribution of solar radiation over the earth surface is not uniform, annual value at the equator is 2.4 times that at the poles. Incident solar energy at the surface depends upon geographic location, orientation of the surface, time of the day, time of the year and atmospheric conditions i.e. clear, cloudy, foggy etc..

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Essay On Earth – 10 Lines, Short And Long Essay For Kids

• Key Points To Remember When Writing An Essay On Earth For Lower Primary Classes
• 5 Lines On The Earth For Children
• 10 Lines On The Earth For Kids
• A Paragraph On Earth For Children
• Short Essay On Earth In English For Kids
• Long Essay On Earth In English For Children

Amazing Facts About Earth For Kids

• What Will Your Child Learn From The Essay On Earth?

The Earth plays a vital role in our lives. It provides us with habitat, water, food, etc. The Earth came into existence millions of years ago, and there have been billions of animals and humans that have walked the same Earth as we do now. The Earth is home to over 5 million species of plants and animals, most of which have still not been identified or recorded. Essay on Earth in English is a common subject in schools as it is an important topic for children to think about and discuss. The Earth can be studied and written about in many different ways; you can write about it in terms of climate change, species, land formation, water composition and even the formation of the solar system and Earth’s position in it. The possibilities are endless! Here, we will discuss essay on Earth for class 1, 2 & 3 for kids.

Key Points To Remember When Writing An Essay On Earth For Lower Primary Classes 

Essay writing is an important skill that children must excel at as it helps them in life. Making their foundations strong helps to develop their skills and focus on improving the content of their essays. While writing essays, there are a few key points that one must remember –

• The language must be simple and comprehensible. 
• Teach words and sentences that your child understands and will be able to write when not assisted. 
• Focus on the very basics, as this is for a lower primary class; children aren’t expected to write in detail.  
• A good place to start would be what the Earth means to us as humans. What the Earth provides us with.  
• Take care of the format of the essay. If in paragraph form, ensure that each paragraph is neither too short nor too long. 
• Be clear about the direction of the essay in the beginning to ensure consistency. 
• Keeping track of the word limit is key. 

5 Lines On The Earth For Children 

For young children, we will stick to the very basics in this form of the essay. We will note down five basic points about the Earth we live on. We will progressively increase the intensity.  

• The Earth orbits the Sun, which is the centre of our Solar System. 
• Earth is the 3rd planet in our Solar System out of eight in total. 
• It is the only planet which supports life. 
• It has both land and water bodies. 
• It has rivers, valleys, mountains, hills, forests, oceans, plains and beaches.  

10 Lines On The Earth For Kids 

Now that we have some basics laid down, we can start adding more details. If we closely observe the essay lines above, we can see the flow of information. We start off with the position of the Earth in the Solar System and then come down to the geographical features of the Earth. Now, we can go into more detail for an essay for class 1 and 2.

Here’s how to describe Earth in a few lines –

• Our Earth is located in the Milky Way galaxy. 
• The Sun is the centre of the Solar System, with eight planets revolving around it. 
• Earth is the 3rd planet from the Sun, and it has one Moon. 
• It is the only planet in our Solar System which is suitable for sustaining life. 
• The composition of the Earth’s surface is 70% water and only 30% land. 
• Water bodies such as oceans, rivers, lakes, glaciers and seas make up 70% of the water content on Earth. 
• Landforms such as mountains, hills, plateaus and plains are the four major types of land we see on Earth. 
• The water bodies are home to aquatic animals such as fishes of different species and mammals, crustaceans, reptiles and more.
• Landforms are home to plants, vertebrates, and invertebrates such as lizards, elephants, eagles, sunflowers, and of course, us humans! 
• The Earth provides land and aquatic animals with food, water and shelter. We would not have existed without the Earth! 

A Paragraph On Earth For Children 

Now that we are comfortable with writing essays (information) in a numeric form, we can move along to writing essays in paragraph form. Below is a short paragraph on Earth.

Millions of years ago, the Earth was formed in one small corner of the galaxy named the Milky Way. The Big Bang caused the formation of the Sun, eight planets, their moons, and other bodies, such as dwarf planets (Pluto!). The Earth is the only planet in our Solar System which could sustain life. This is due to its strategic position; it is not too close to the Sun, nor is it too far away from the Sun. This, coupled with the right elements, allowed landforms and water bodies to form. This, in turn, supported the evolution of life on Earth. Indeed, the Earth is one of a kind! 

Short Essay On Earth In English For Kids 

Moving on to a slightly longer form of essay, we can start adding in more information and maybe even add paragraphs. Since you have limited words, be choosy about what you wish to write and what you wish to omit. Below is an essay for class 1, 2 and 3 on Earth.

The Milky Way galaxy is home to many stars, planets and planetary systems. One such planetary system is our Solar System. Our Solar System has eight planets, of which Earth is the fourth. The Earth rotates on its axis, which causes days and nights. It also revolves around the Sun in a fixed orbit, which causes the change in seasons.

The strategic position and movement of the Earth support the millions of species of plants and animals that inhabit it. The right elements and external forces allowed the formation of land and water bodies which provide homes and nutrients to the millions of species on the planet.

Water bodies such as oceans, rivers and lakes are homes to aquatic animals like fishes, whales and sea horses. Landforms are home to plants, animals and insects. However, in more recent times, we humans have been overusing our resources as well as polluting the environment, which is negatively affecting the planet and our co-habitants. We must strive to save the planet now before it is too late. 

Long Essay On Earth In English For Children 

Lastly, we will discuss and look at long-form essay for class 3. Since we have more words to play with, we can start going in-depth and look at specific topics. We can also add sub-heads and paragraph breaks. We will first start with an introduction, followed by the subheads.

The Earth is unique. It is indeed one of a kind. When the Big Bang occurred, the right elements, temperature and pressure (among other factors) created the Earth. Subsequently, the topography and the organisms emerged. Years of evolution have brought us to today, where we can study and understand not only the Earth but also other planets and galaxies.  

What Is Earth? 

The word ‘Earth’ is a Germanic word which simply means “the ground.” Earth is the only planet known that homes and nurtures living organisms such as ourselves. It is the fifth largest planet in our Solar System. It is also the only planet which has water on its surface. About 71% of the Earth’s surface is water, while the remaining 29% is land. It has one natural satellite, the Moon. 

Origin Of The Planet Earth 

The beginning of our Universe was the Big Bang. It was too hot, but it slowly cooled down. Different particles started bumping into each other, eventually forming common elements. Our solar system was formed roughly 8.7 years after the Big Bang. All solar systems begin in the same way – from Nebulas. Collapsing of dust and gas molecules within the nebulae causes the formation of planets and stars. The gravitational pull comes into action here and pulls the gas molecules and dust particles together. As these particles increase in size, the attraction between the molecules increases. This eventually forms a planet. However, the planet was still too hot to sustain life. Eventually, the planet began cooling down. The oceans are where the origin of life occurred. Slowly, evolution caused organisms to move onto land too. Over millions of years, the Earth has gone through many cycles of heating up and cooling down. This has resulted in mass extinctions and the wipeout of civilizations and organisms. However, planet Earth has managed to give birth to new organisms and help them evolve every single time. 

Different Layers Of The Earth 

The Earth is made up of three layers – The Crust, The Mantle and The Core 

1. The Crust  

This is the outer-most part of the Earth. It is mostly made up of solid rock and minerals. It is about 40km in thickness and is only 1% of the Earth’s mass. However, this part of the Earth harbours all known life in the Universe.  

2. The Mantle 

This is the middle part of the Earth. It is about 2900kms in thickness, and it consists of hot, dense, iron and magnesium-rich solid rock. The Crust and the Mantle make up the lithosphere, which is broken into plates, both large and small.  

3. The Core 

The core is the innermost part of the Earth. It is further divided into two parts – the liquid outer core and the solid inner core. The temperatures here can rise upwards of 50,000 C.  

Motion Of The Earth 

The Earth has mainly two motions – Rotation and Revolution.  

1. Rotation

The Earth rotates on its axis in a clockwise motion. It takes the Earth 23.9 hours to complete one rotation on its axis. The rotation of the Earth causes the change in day and night. 

2. Revolution

The Earth revolves around the Sun in a fixed orbit in an anticlockwise direction. It takes the Earth 365 days, 6 hours, and 9 minutes to complete one rotation around the Sun. The revolution of the Earth causes the change in seasons.  

How Can We Protect The Mother Earth? 

There are many ways we can protect our Earth. Some ways are: 

• Be conscious about overusing and overexploiting resources. 
• Conserve energy, both fuel and electricity. 
• Do not pollute your surroundings, especially with plastic. 
• Remember the 3Rs – Reuse, Recycle and Reduce. 
• Strive to conserve your local flora and fauna.  

Below are something amazing facts about our Earth for kids:

• The name ‘Earth’ comes from the old English and Germanic words that mean ‘the ground’.  
• The Earth orbits around the sun at a whopping speed of 30 kilometres per second!  
• The Earth’s diameter is 12,800 kilometres, making it the 5 th largest planet in our solar system.  
• The Earth is the only planet known to support life. The availability of abundant oxygen and water makes this possible.  
• Due to the Moon slowing down Earth’s rotation, the days on Earth are, in fact, getting longer!  

What Will Your Child Learn From The Essay On Earth? 

Your child will learn a lot about our planet, its origins, its movements, etc. The essay on planet earth will also help your child learn how to write a good composition with perfect techniques. This article takes you through essay writing in a step-by-step manner.

1. Why Planet Earth Is Called A Blue Planet? 

Planet Earth is called the Blue Planet because 71% of its surface is covered with water. 

2. When Is World Earth Day Celebrated? 

World Earth Day is celebrated on 22nd April every year.  

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Manicouagan Crater

Asteroids were not only important in Earth's early formation, but have continued to shape our planet. A five-kilometer (three-mile) diameter asteroid is theorized to have formed the Manicouagan Crater about 215.5 million years ago.

We live on Earth’s hard, rocky surface, breathe the air that surrounds the planet , drink the water that falls from the sky, and eat the food that grows in the soil. But Earth did not always exist within this expansive universe, and it was not always a hospitable haven for life. Billions of years ago, Earth, along with the rest of our solar system, was entirely unrecognizable, existing only as an enormous cloud of dust and gas. Eventually, a mysterious occurrence—one that even the world’s foremost scientists have yet been unable to determine—created a disturbance in that dust cloud, setting forth a string of events that would lead to the formation of life as we know it. One common belief among scientists is that a distant star collapsed, creating a supernova explosion, which disrupted the dust cloud and caused it to pull together. This formed a spinning disc of gas and dust, known as a solar nebula . The faster the cloud spun, the more the dust and gas became concentrated at the center, further fueling the speed of the nebula . Over time, the gravity at the center of the cloud became so intense that hydrogen atoms began to move more rapidly and violently. The hydrogen protons began fusing, forming helium and releasing massive amounts of energy. This led to the formation of the star that is the center point of our solar system—the sun—roughly 4.6 billion years ago. Planet Formation The formation of the sun consumed more than 99 percent of the matter in the nebula . The remaining material began to coalesce into various masses. The cloud was still spinning, and clumps of matter continued to collide with others. Eventually, some of those clusters of matter grew large enough to maintain their own gravitational pull, which shaped them into the planets and dwarf planets that make up our solar system today. Earth is one of the four inner, terrestrial planets in our solar system. Just like the other inner planets —Mercury, Venus, and Mars—it is relatively small and rocky. Early in the history of the solar system, rocky material was the only substance that could exist so close to the Sun and withstand its heat. In Earth's Beginning At its beginning, Earth was unrecognizable from its modern form. At first, it was extremely hot, to the point that the planet likely consisted almost entirely of molten magma . Over the course of a few hundred million years, the planet began to cool and oceans of liquid water formed. Heavy elements began sinking past the oceans and magma toward the center of the planet . As this occurred, Earth became differentiated into layers, with the outermost layer being a solid covering of relatively lighter material while the denser, molten material sunk to the center. Scientists believe that Earth, like the other inner planets , came to its current state in three different stages. The first stage, described above, is known as accretion, or the formation of a planet from the existing particles within the solar system as they collided with each other to form larger and larger bodies. Scientists believe the next stage involved the collision of a proto planet with a very young planet Earth. This is thought to have occurred more than 4.5 billion years ago and may have resulted in the formation of Earth’s moon. The final stage of development saw the bombardment of the planet with asteroids . Earth’s early atmosphere was most likely composed of hydrogen and helium . As the planet changed, and the crust began to form, volcanic eruptions occurred frequently. These volcanoes pumped water vapor, ammonia, and carbon dioxide into the atmosphere around Earth. Slowly, the oceans began to take shape, and eventually, primitive life evolved in those oceans. Contributions from Asteroids Other events were occurring on our young planet at this time as well. It is believed that during the early formation of Earth, asteroids were continuously bombarding the planet , and could have been carrying with them an important source of water. Scientists believe the asteroids that slammed into Earth, the moon, and other inner planets contained a significant amount of water in their minerals, needed for the creation of life. It seems the asteroids , when they hit the surface of Earth at a great speed, shattered, leaving behind fragments of rock. Some suggest that nearly 30 percent of the water contained initially in the asteroids would have remained in the fragmented sections of rock on Earth, even after impact. A few hundred million years after this process—around 2.2 billion to 2.7 billion years ago—photosynthesizing bacteria evolved . They released oxygen into the atmosphere via photosynthesis and, in a few hundred million years, were able to change the composition of the atmosphere into what we have today. Our modern atmosphere is comprised of 78 percent nitrogen and 21 percent oxygen, among other gases, which enables it to support the many lives residing within it.

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October 19, 2023

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