# Rotation of the Earth about its own axis

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I am wondering how the Earth got its original angular velocity. What was the cause that made the Earth rotate about its own axis?

What about its motion around the Sun? What gave the Earth the push to keep rotating around the Sun?

## Origin of Earth rotation

There are two main phenomena that explain Earth rotation. First one is linked to the history of the solar system: there is angular momentum in the interstellar medium (ISM), so the collapsing cloud that formed our solar system had an initial angular momentum (rotation of the galaxy and turbulence in the ISM are typically a good candidate for that; see for example Mac Low & Klessen 2004). By conservation of the latter, particles in the collapsing cloud accelerate (the norm of the angular momentum $$I$$ is $$mrv$$, with $$m$$ the mass of the system, $$r$$ the radius and $$v$$ the velocity; as the cloud contracts, $$r$$ decreases and then $$v$$ has to increase) and due to centrifugal force, a disk will tend to form. By conservation of the angular momentum in the protoplanetary disk, you will also tend to form bodies that rotates.

The second phenomena is linked to the formation history of the Earth. Simply put, Earth formation history is due to collisions of planetesimals (let say they are big chunks of rock, formed by a collision history of smaller chunks of rock, formed by… ); these planetesimals are orbiting the forming star in the protoplanetary disk, their rotation being linked to the disk formation history (see above). When planetesimals collide, two things can basically happen: either the collision is perfectly central (meaning that the momentum of one of the two colliding objects is pointed toward the center of mass of the second object) and there is no transfer of angular momentum, either the collision is lateral and there is a transfer of angular momentum. Depending on the configuration, you can either transfer momentum that will make the other body rotating on way or the other. That being said, you can then object that there should be a equirepartition of collisions pushing the proto-Earth to rotate in both directions. Which is true, but external planetesimals are rotating faster than internal ones. Therefore, from the proto-Earth perspective (meaning, in a physical sense, in its reference frame), internal planetesimals are seen as coming backward, from front, and external planetesimals are seen as coming forward, from behind. Long story short, there are both transfering momentum in the same "direction", forcing the proto-Earth to rotate in the same direction as the rotation of the protoplanetary disk (see for example Artem'ev & Radzievskii 1965).

## Rotation around the Sun

This one's easy: gravity.

The Earth is constantly falling towards the Sun because of gravity, and the centripetal acceleration counteracts perfectly gravity, mainting the Earth on its orbit.

## How does the Earth rotate on its axis?

The earth rotates about an imaginary line that passes through the North and South Poles of the planet. This line is called the axis of rotation. Earth rotates about this axis once each day (approximately 24 hours). Our clock time is based on the earth's rotation with respect to the sun from solar noon to solar noon.

Secondly, how does the sun rotate around the Earth? As the Earth rotates, it also moves, or revolves, around the Sun. The Earth's path around the Sun is called its orbit. It takes the Earth one year, or 365 1/4 days, to completely orbit the Sun. As the Earth orbits the Sun, the Moon orbits the Earth.

Also Know, what direction does the Earth rotate on its axis?

1). Rotation -Earth's rotation is the rotation of Planet Earth around its axis (Imaginary Axis from North to the South Pole) Earth rotates eastward. As viewed from the north pole star Polaris, Earth turns counterclockwise.

## Variation due to the Rotation of the Earth about its Axis

Since the earth is continuously rotating about its own axis, except the bodies present in two poles, all other bodies on earth are moving along a circular path. The centers of these circular paths are on the axis of the earth. For this reason, all the bodies on the surface of the earth experience centrifugal force. Magnitude of the force on the bodies at the equator is maximum and is zero at the two poles. Since the centrifugal force acts opposite to the gravitational force hence a part of gravitational force is spent to balance the centrifugal force, so the weight of the body apparently decreases.

Suppose a body of mass or is located at point A on the surface of the earth at the lattice γ [Figure]. The earth is rotating with angular velocity about its axis NS, so that body rotates with angular velocity ω along the circular path of radius AB = r. If the radius of the earth is R, then r = R cos γ. Due to this rotation centrifugal force mω 2 r acts on the body along AC. Due to gravitation force F = mg acts on the body along the centre of the earth i.e., along the direction AO. Centrifugal force acting on the body along D i.e., component of opposite to the gravitation is mω 2 r cos γ. So, apparent weight of the body at point A in will be,

mg – mω 2 r cos γ = mg – mω 2 R cos γ

If the magnitude of acceleration due to gravity at point A is g / then the apparent weight of the body is mg / .

So, g / = g [1 – (ω 2 R cos 2 γ /g)]

at the equator, γ = 0 0 so cos γ = 1

then, g / = g (1 – ω 2 R/g) = g – ω 2 R

Again, at the poles γ = 90 0 so, cos γ = 0

So, value of g changes due to the rotation of the earth about its axis. The value of g becomes minimum at the equator and maximum at the poles. At other places the value of g is in between these two terminal values. It is apparent that the change of the value of g is identical due to the shape and rotation of the earth.

## Answers and Replies

1) At the time of Copernicus, actually Ptolemy's model more closely matched experimental data than Copernicus's model (due in large part to the idea that the planets were still in circular orbits). But through the careful observation of Tycho Brahe and the subsequent analysis by his apprentice Kepler, a more accurate model was derived, the Keplarian model of the solar system wherein the planets orbit the Sun in elliptical orbits with the Sun at one focus. In addition, Galileo found aspects of the solar system in contradiction with Ptolemy's model. Namely, he found the phases of Venus, and the Galilean moons of Jupiter Io, Europa, Ganymede, and Callisto which orbited Jupiter and not the Earth.

So, we can reject the Ptolemaic model on grounds of experimental accuracy.

2) Rotation is an absolute measure. Practically speaking, we can measure the Earth's rotation with respect to the distance stars. Stars very far away are roughly fixed in the sky, because any proper motion becomes a tiny angular motion due to the distance. We can use these fixed stars to figure out Earth's rotation as well as the Precession and Nutation of the Earth's axis.

3) As above, Earth's rotation is an absolute motion. Why do you think it's not?

Well I thought we measure rotation with respect to a reference frame. So rotation of an object should seem different from different frames.
Would you please explain why we consider rotation to be absolute? Doesn't it depend on reference frame of the observer?

2. The Earth rotates on its own axis. This rotation is measured relative to some reference frame, right? Now which reference frame did we use to measure earth's rotation speed?

One can use the sun as a measure of rotation and obtain solar time. This would be in keeping with measuring the shadow of a stick coming to the same location each day.
One can use the distant stars and obtain sidereal time. One measure the angle between a star and some reference, say the earth's horizon, to be the same from one day to the next.
http://en.wikipedia.org/wiki/Sidereal_time

If you are just freely floating out in space, and you detect no centrifugal, Coriolis, or Euler forces, then your frame is non-rotating.

For such purposes, a gyroscope is perhaps the most practical: http://en.wikipedia.org/wiki/Gyroscope

Although velocity is always relative, acceleration is absolute ("frame-invariant" might be a better term than "absolute"). Some of the confusion in this thread comes from people not clearly distinguishing the two when they say "motion".

Yes, velocity is relative because it depends on the frame of the observer. When I say that I'm at rest while you're moving north, and you say that you're at rest while I'm moving south, your description is just as good as mine no experiment will give different results either way.

Acceleration, which is change in velocity, is not relative. I can take a small box, suspend a weight inside it with six springs (one to each face of the box), and then measure acceleration by observing the motion of the weight relative to the box and the stretching of the springs. All observers, no matter what frame they choose to use, will agree about whether the springs are stretching or not and hence whether the box is being accelerated or not. (A device like this that measures acceleration is called an "accelerometer").

Every point on a rotating body is undergoing acceleration - its direction of motion and hence its velocity is constantly changing, and an accelerometer will detect that acceleration.

## Doing the Wobble

Earth's spin has a bit of a wobble to it, as the axis drifts at the poles. The spin has been drifting faster than normal since 2000, NASA has measured, moving 7 inches (17 cm) per year to the east. Scientists determined that it continued east instead of going back and forth because of the combined effects of the melting of Greenland and Antarctica and a loss of water in Eurasia the axis drift appears to be especially sensitive to changes happening at 45 degrees north and south. That discovery led scientists to finally be able to answer the long-held question of why there was drift in the first place. Having dry or wet years in Eurasia has caused the wobble to the east or west.

## Designbenedito

Rotation Definition Astronomy. Blue marble © 2002 nasa earth observatory. Earth rotates eastward, in prograde motion.

Rotation of a planar body is the movement when points of the body travel in circular trajectories around a fixed point called the center of rotation. Rotation axis definition, an imaginary line through a crystal about which the crystal may be rotated a specified number of degrees and be brought back to its original position. Movement in a circle around a fixed point: As viewed from the north pole star polaris, earth turns counterclockwise. A rotation is the movement of something through one.

Astronomy 1001 > Humphreys > Flashcards > Midterm 1 . from classconnection.s3.amazonaws.com Movement in a circle around a fixed point: The strict definition of rotation is the circular movement of an object about a point in space. now, since astronomy often deals with multiple objects in motion, things can get complex. An operation that rotates a geometric figure about a fixed. This definition applies to rotations within both two and three dimensions (in a plane and in space in astronomy, rotation is a commonly observed phenomenon. A complete circular movement (definition of rotation from the cambridge academic content dictionary © cambridge university.

### Stars, planets and similar bodies all spin.

The amount of time a planet requires to make one complete spin about its axis. Stars, planets and similar bodies all spin. Rotation definition, the act of rotating Rotation axis definition, an imaginary line through a crystal about which the crystal may be rotated a specified number of degrees and be brought back to its original position. The strict definition of rotation is the circular movement of an object about a point in space. now, since astronomy often deals with multiple objects in motion, things can get complex. Rotation synonyms, rotation pronunciation, rotation translation, english dictionary definition of rotation. If you spin the ball once, so the same spot is facing you. The north pole, also known as the geographic north pole or terrestrial north pole. Let there be a planet for which the duration of a solar day is equal to a year around the. Preferably confined to the motion of a spherical body upon an axis, in contradistinction to its orbital revolution about another body. Movement in a circle around a fixed point: The sun rotates around an axis which is roughly perpendicular to the plane of the ecliptic As astronomy 330 and 320 cover details of various derivations (but do not go into details of how you do if we know the galactic rotation curve, v (r), we can use this equation to estimate the radial.

The amount of time a planet requires to make one complete spin about its axis. Rotational period rotational period is the term describing the length of time necessary for a space object to make one complete. A rotation is the movement of something through one. The study of the rotation curve of the andromeda galaxy, m 31, the nearest spiral galaxy to ours in order to place this result in context, it is necessary to describe the process whereby the rotation. Astronomical and technical terms used frequently in astronomy and space exploration can be below is a list of definitions and explanations to help you navigate astronomical texts and services.

Why 1000mph Earth rotation is deceptive and the wrong way . from i.ytimg.com The strict definition of rotation is the circular movement of an object about a point in space. now, since astronomy often deals with multiple objects in motion, things can get complex. Rotation axis definition, an imaginary line through a crystal about which the crystal may be rotated a specified number of degrees and be brought back to its original position. We now discuss one of the most important diagrams in astronomy, a way of organizing and presenting information about all of the various kinds of stars, with which we can instill order from the cosmos and. Also w = v/r where v is the measured velocity and r is the radius to the axis of rotation (therefore v. The study of the rotation curve of the andromeda galaxy, m 31, the nearest spiral galaxy to ours in order to place this result in context, it is necessary to describe the process whereby the rotation.

### 7 rotation acronyms and abbreviations related to astronomy share rotation abbreviations in astronomy page.

A rotation is the movement of something through one. The north pole, also known as the geographic north pole or terrestrial north pole. Stars, planets and similar bodies all spin. As viewed from the north pole star polaris, earth turns counterclockwise. The study of the rotation curve of the andromeda galaxy, m 31, the nearest spiral galaxy to ours in order to place this result in context, it is necessary to describe the process whereby the rotation. Rotation axis definition, an imaginary line through a crystal about which the crystal may be rotated a specified number of degrees and be brought back to its original position. Rotation synonyms, rotation pronunciation, rotation translation, english dictionary definition of rotation. Let there be a planet for which the duration of a solar day is equal to a year around the. Blue marble © 2002 nasa earth observatory. A more precise terminology would. Rotation does not involve movement of the center of others have covered the mechanical definition of revolution, but perhaps you might mean, what. English language learners definition of rotation. Definition of rotation versus revolution.

A rotation is the movement of something through one. Earth rotates eastward, in prograde motion. 7 rotation acronyms and abbreviations related to astronomy share rotation abbreviations in astronomy page. Rotation synonyms, rotation pronunciation, rotation translation, english dictionary definition of rotation. see related link imagine a ball with a spike through it.

Skeptic's Play: The science of Ophiuchus from 2.bp.blogspot.com Alternatively search google for rotation. Learn vocabulary, terms and more with flashcards, games and other study tools. Earth's rotation or spin is the rotation of planet earth around its own axis. Rotation synonyms, rotation pronunciation, rotation translation, english dictionary definition of rotation. Also w = v/r where v is the measured velocity and r is the radius to the axis of rotation (therefore v.

### Rotation does not involve movement of the center of others have covered the mechanical definition of revolution, but perhaps you might mean, what.

Rotation does not involve movement of the center of others have covered the mechanical definition of revolution, but perhaps you might mean, what. Click on the first link on a line below to go directly to a page where rotation is defined. If you spin the ball once, so the same spot is facing you. This glossary of astronomy terms contains definitions for some of the most common words used in axis also known as the poles, this is an imaginary line through the center of rotation of an object. Rotation / rotational on thesaurus.com. Rotational period rotational period is the term describing the length of time necessary for a space object to make one complete. Astronomical and technical terms used frequently in astronomy and space exploration can be below is a list of definitions and explanations to help you navigate astronomical texts and services. A rotation is the movement of something through one. The sun's the sun's rotation period varies with latitude on the sun since it is made of gas. see related link imagine a ball with a spike through it. An operation that rotates a geometric figure about a fixed. The strict definition of rotation is the circular movement of an object about a point in space. now, since astronomy often deals with multiple objects in motion, things can get complex. English language learners definition of rotation.

7 rotation acronyms and abbreviations related to astronomy share rotation abbreviations in astronomy page. Also w = v/r where v is the measured velocity and r is the radius to the axis of rotation (therefore v. Preferably confined to the motion of a spherical body upon an axis, in contradistinction to its orbital revolution about another body. Definition of rotation versus revolution. An object rotates about its center of mass.

As astronomy 330 and 320 cover details of various derivations (but do not go into details of how you do if we know the galactic rotation curve, v (r), we can use this equation to estimate the radial. A complete circular movement (definition of rotation from the cambridge academic content dictionary © cambridge university. As viewed from the north pole star polaris, earth turns counterclockwise. The act or process of moving or turning around a central. 7 rotation acronyms and abbreviations related to astronomy share rotation abbreviations in astronomy page.

Rotation definition, the act of rotating This definition applies to rotations within both two and three dimensions (in a plane and in space in astronomy, rotation is a commonly observed phenomenon. 7 rotation acronyms and abbreviations related to astronomy share rotation abbreviations in astronomy page. Rotation of a planar body is the movement when points of the body travel in circular trajectories around a fixed point called the center of rotation. The study of the rotation curve of the andromeda galaxy, m 31, the nearest spiral galaxy to ours in order to place this result in context, it is necessary to describe the process whereby the rotation.

The study of the rotation curve of the andromeda galaxy, m 31, the nearest spiral galaxy to ours in order to place this result in context, it is necessary to describe the process whereby the rotation. The amount of time a planet requires to make one complete spin about its axis. Rotation of a planar body is the movement when points of the body travel in circular trajectories around a fixed point called the center of rotation. The sun's the sun's rotation period varies with latitude on the sun since it is made of gas. Rotation does not involve movement of the center of others have covered the mechanical definition of revolution, but perhaps you might mean, what.

Preferably confined to the motion of a spherical body upon an axis, in contradistinction to its orbital revolution about another body. Click on the first link on a line below to go directly to a page where rotation is defined. The act or process of moving or turning around a central. Rotation / rotational on thesaurus.com. Earth's rotation or spin is the rotation of planet earth around its own axis.

Source: www.creationresearches.com

A more precise terminology would.

Source: www.learn4yourlife.com

The act or process of turning around a center or an axis.

A more precise terminology would.

Earth's rotation or spin is the rotation of planet earth around its own axis.

Rotational period rotational period is the term describing the length of time necessary for a space object to make one complete.

## Earth

Earth is an ocean planet. Our home world’s abundance of water — and life — makes it unique in our solar system. Other planets, plus a few moons, have ice, atmospheres, seasons and even weather, but only on Earth does the whole complicated mix come together in a way that encourages life — and lots of it.

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System’s four terrestrial planets. It is sometimes referred to as the world or the Blue Planet.

Earth formed approximately 4.54 billion years ago, and life appeared on its surface within its first billion years. Earth’s biosphere then significantly altered the atmospheric and other basic physical conditions, which enabled the proliferation of organisms as well as the formation of the ozone layer, which together with Earth’s magnetic field blocked harmful solar radiation, and permitted formerly ocean-confined life to move safely to land. The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist. Estimates on how much longer the planet will be able to continue to support life range from 500 million years (myr), to as long as 2.3 billion years (byr).

Earth’s lithosphere is divided into several rigid segments, or tectonic plates, that migrate across the surface over periods of many millions of years. About 71% of the surface is covered by salt water oceans, with the remainder consisting of continents and islands which together have many lakes and other sources of water that contribute to the hydrosphere. Earth’s poles are mostly covered with ice that is the solid ice of the Antarctic ice sheet and the sea ice that is the polar ice packs. The planet’s interior remains active, with a solid iron inner core, a liquid outer core that generates the magnetic field, and a thick layer of relatively solid mantle.

Earth gravitationally interacts with other objects in space, especially the Sun and the Moon. During one orbit around the Sun, the Earth rotates about its own axis 366.26 times, creating 365.26 solar days, or one sidereal year. The Earth’s axis of rotation is tilted 23.4° away from the perpendicular of its orbital plane, producing seasonal variations on the planet’s surface with a period of one tropical year (365.24 solar days).

The Moon is Earth’s only natural satellite. It began orbiting the Earth about 4.53 billion years ago (bya). The Moon’s gravitational interaction with Earth stimulates ocean tides, stabilizes the axial tilt, and gradually slows the planet’s rotation. The planet is home to millions of species of life, including humans. Both the mineral resources of the planet and the products of the biosphere contribute resources that are used to support a global human population. These inhabitants are grouped into about 200 independent sovereign states, which interact through diplomacy, travel, trade, and military action. Human cultures have developed many views of the planet, including its personification as a planetary deity, its shape as flat, its position as the center of the universe, and in the modern Gaia Principle, as a single, self-regulating organism in its own right.

Formation of Earth

The earliest material found in the Solar System is dated to 4.5672±0.0006 bya therefore, it is inferred that the Earth must have been formed by accretion around this time. By 4.54±0.04 bya the primordial Earth had formed. The formation and evolution of the Solar System bodies occurred in tandem with the Sun. In theory a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disk, and then the planets grow out of that in tandem with the star. A nebula contains gas, ice grains and dust (including primordial nuclides). In nebular theory planetesimals commence forming as particulate accrues by cohesive clumping and then by gravity. The assembly of the primordial Earth proceeded for 10–20 myr. The Moon formed shortly thereafter, about 4.53 bya.

The Moon’s formation remains debated. The working hypothesis is that it formed by accretion from material loosed from the Earth after a Mars-sized object dubbed Theia impacted with Earth. The model, however, is not self-consistent. In this scenario, the mass of Theia is 10% of the Earth’s mass, it impacts with the Earth in a glancing blow, and some of its mass merges with the Earth. Between approximately 3.8 and 4.1 bya, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon, and by inference, to the Earth. Earth’s atmosphere and oceans formed by volcanic activity and outgassing that included water vapor. The origin of the world’s oceans was condensation augmented by water and ice delivered by asteroids, proto-planets, and comets. In this model, atmospheric “greenhouse gases” kept the oceans from freezing while the newly forming Sun was only at 70% luminosity. By 3.5 bya, the Earth’s magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.

A crust formed when the molten outer layer of the planet Earth cooled to form a solid as the accumulated water vapor began to act in the atmosphere. The two models that explain land mass propose either a steady growth to the present-day forms or, more likely, a rapid growth early in Earth history followed by a long-term steady continental area. Continents formed by plate tectonics, a process ultimately driven by the continuous loss of heat from the earth’s interior. On time scales lasting hundreds of millions of years, the supercontinents have formed and broken up three times. Roughly 750 mya (million years ago), one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540 mya, then finally Pangaea, which also broke apart 180 mya.

The shape of the Earth approximates an oblate spheroid, a sphere flattened along the axis from pole to pole such that there is a bulge around the equator. This bulge results from the rotation of the Earth, and causes the diameter at the equator to be 43 km (kilometer) larger than the pole-to-pole diameter. For this reason the furthest point on the surface from the Earth’s center of mass is the Chimborazo volcano in Ecuador. The average diameter of the reference spheroid is about 12,742 km, which is approximately 40,000 km/π, as the meter was originally defined as 1/10,000,000 of the distance from the equator to the North Pole through Paris, France.

Local topography deviates from this idealized spheroid, although on a global scale, these deviations are small: Earth has a tolerance of about one part in about 584, or 0.17%, from the reference spheroid, which is less than the 0.22% tolerance allowed in billiard balls. The largest local deviations in the rocky surface of the Earth are Mount Everest (8,848 m above local sea level) and the Mariana Trench (10,911 m below local sea level). Due to the equatorial bulge, the surface locations farthest from the center of the Earth are the summits of Mount Chimborazo in Ecuador and Huascarán in Peru.

Chemical Composition

The mass of the Earth is approximately 5.98×1024 kg. It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%) with the remaining 1.2% consisting of trace amounts of other elements. Due to mass segregation, the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.

The geochemist F. W. Clarke calculated that a little more than 47% of the Earth’s crust consists of oxygen. The more common rock constituents of the Earth’s crust are nearly all oxides chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. The principal oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table at right), with the other constituents occurring in minute quantities.

Internal Structure

The interior of the Earth, like that of the other terrestrial planets, is divided into layers by their chemical or physical (rheological) properties, but unlike the other terrestrial planets, it has a distinct outer and inner core. The outer layer of the Earth is a chemically distinct silicate solid crust, which is underlain by a highly viscous solid mantle. The crust is separated from the mantle by the Mohorovičić discontinuity, and the thickness of the crust varies: averaging 6 km (kilometers) under the oceans and 30-50 km on the continents. The crust and the cold, rigid, top of the upper mantle are collectively known as the lithosphere, and it is of the lithosphere that the tectonic plates are comprised. Beneath the lithosphere is the asthenosphere, a relatively low-viscosity layer on which the lithosphere rides. Important changes in crystal structure within the mantle occur at 410 and 660 km below the surface, spanning a transition zone that separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquid outer core lies above a solid inner core. The inner core may rotate at a slightly higher angular velocity than the remainder of the planet, advancing by 0.1–0.5° per year.

Earth cutaway from core to exosphere. Not to scale. Credit: Wikipedia

Orbit and Rotation

Earth’s rotation period relative to the Sun—its mean solar day—is 86,400 seconds of mean solar time (86,400.0025 SI seconds). As the Earth’s solar day is now slightly longer than it was during the 19th century due to tidal acceleration, each day varies between 0 and 2 SI ms longer.

Earth’s rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86,164.098903691 seconds of mean solar time (UT1), or 23h 56m 4.098903691s. Earth’s rotation period relative to the precessing or moving mean vernal equinox, misnamed its sidereal day, is 86,164.09053083288 seconds of mean solar time (UT1) (23h 56m 4.09053083288s) as of 1982. Thus the sidereal day is shorter than the stellar day by about 8.4 ms. The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005 and 1962–2005.

Apart from meteors within the atmosphere and low-orbiting satellites, the main apparent motion of celestial bodies in the Earth’s sky is to the west at a rate of 15°/h = 15’/min. For bodies near the celestial equator, this is equivalent to an apparent diameter of the Sun or Moon every two minutes from the planet’s surface, the apparent sizes of the Sun and the Moon are approximately the same.

Earth orbits the Sun at an average distance of about 150 million kilometers every 365.2564 mean solar days, or one sidereal year. From Earth, this gives an apparent movement of the Sun eastward with respect to the stars at a rate of about 1°/day, which is one apparent Sun or Moon diameter every 12 hours. Due to this motion, on average it takes 24 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian. The orbital speed of the Earth averages about 29.8 km/s (107,000 km/h), which is fast enough to travel a distance equal to the planet’s diameter, about 12,742 km, in seven minutes, and the distance to the Moon, 384,000 km, in about 3.5 hours.

The Moon revolves with the Earth around a common barycenter every 27.32 days relative to the background stars. When combined with the Earth–Moon system’s common revolution around the Sun, the period of the synodic month, from new moon to new moon, is 29.53 days. Viewed from the celestial north pole, the motion of Earth, the Moon and their axial rotations are all counterclockwise. Viewed from a vantage point above the north poles of both the Sun and the Earth, the Earth revolves in a counterclockwise direction about the Sun. The orbital and axial planes are not precisely aligned: Earth’s axis is tilted some 23.4 degrees from the perpendicular to the Earth–Sun plane (the ecliptic), and the Earth–Moon plane is tilted up to ±5.1 degrees against the Earth–Sun plane. Without this tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses.

The Hill sphere, or gravitational sphere of influence, of the Earth is about 1.5 Gm or 1,500,000 km in radius. This is the maximum distance at which the Earth’s gravitational influence is stronger than the more distant Sun and planets. Objects must orbit the Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.

Earth, along with the Solar System, is situated in the Milky Way galaxy and orbits about 28,000 light years from the center of the galaxy. It is currently about 20 light years above the galactic plane in the Orion spiral arm.

Earth’s axial tilt (or obliquity) and its relation to the rotation axis and plane of orbit. Credit: NASA

Axis Tilt and Seasons

Due to the axial tilt of the Earth, the amount of sunlight reaching any given point on the surface varies over the course of the year. This causes seasonal change in climate, with summer in the northern hemisphere occurring when the North Pole is pointing toward the Sun, and winter taking place when the pole is pointed away. During the summer, the day lasts longer and the Sun climbs higher in the sky. In winter, the climate becomes generally cooler and the days shorter. Above the Arctic Circle, an extreme case is reached where there is no daylight at all for part of the year—a polar night. In the southern hemisphere the situation is exactly reversed, with the South Pole oriented opposite the direction of the North Pole.

By astronomical convention, the four seasons are determined by the solstices—the point in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when the direction of the tilt and the direction to the Sun are perpendicular. In the northern hemisphere, Winter Solstice occurs on about December 21, Summer Solstice is near June 21, Spring Equinox is around March 20 and Autumnal Equinox is about September 23. In the Southern hemisphere, the situation is reversed, with the Summer and Winter Solstices exchanged and the Spring and Autumnal Equinox dates switched.

The angle of the Earth’s tilt is relatively stable over long periods of time. The tilt does undergo nutation a slight, irregular motion with a main period of 18.6 years. The orientation (rather than the angle) of the Earth’s axis also changes over time, precessing around in a complete circle over each 25,800 year cycle this precession is the reason for the difference between a sidereal year and a tropical year. Both of these motions are caused by the varying attraction of the Sun and Moon on the Earth’s equatorial bulge. From the perspective of the Earth, the poles also migrate a few meters across the surface. This polar motion has multiple, cyclical components, which collectively are termed quasiperiodic motion. In addition to an annual component to this motion, there is a 14-month cycle called the Chandler wobble. The rotational velocity of the Earth also varies in a phenomenon known as length of day variation.

In modern times, Earth’s perihelion occurs around January 3, and the aphelion around July 4. These dates change over time due to precession and other orbital factors, which follow cyclical patterns known as Milankovitch cycles. The changing Earth–Sun distance causes an increase of about 6.9%[note 15] in solar energy reaching the Earth at perihelion relative to aphelion. Since the southern hemisphere is tilted toward the Sun at about the same time that the Earth reaches the closest approach to the Sun, the southern hemisphere receives slightly more energy from the Sun than does the northern over the course of a year. This effect is much less significant than the total energy change due to the axial tilt, and most of the excess energy is absorbed by the higher proportion of water in the southern hemisphere.

## Rotation of the Earth about its own axis - Astronomy

As a father and a science lover I was dumbfounded when my 7yr old son asked me "What makes our earth turn (rotate) ? He then asked "Did it start a long time ago and it just keeps turning or is something pushing it to turn"? How does the Earth continue to rotate around its axis? Where does the energy to keep it moving come from?

Our everyday experience teaches us that an object must be "pushed" by a force in order to keep it moving. Otherwise, it will slow down and eventually stop. But this intuition is absolutely wrong. If an object is moving, then a force is required *to slow it down or stop it*, not to keep it moving. (Hence, "Objects in motion tend to stay in motion. Objects at rest tend to stay at rest.") In our everyday experience, it's the force of friction that tends to stop Earth-bound objects from moving forever. But for the Earth rotating on its axis, there is no force working to counteract the rotation (except the tidal effect of the Moon, but that's working very slowly), so you don't need to have any input energy to keep it spinning.

What started the earth rotating in the first place?

The shortest answer is angular momentum. Angular momentum is simply the name we give for the fact that things tend to rotate. (Just like regular momentum is the tendency for things to move.) The Earth formed out of a nebula that collapsed. As the nebula collapsed it began rotating, which may seem odd, but actually not rotating is far stranger than rotating. The Earth's rotation comes from the initial tendency to rotate that was imparted on it when it formed, only the relatively weak tidal forces from the Moon act to slow it down.

This page was last updated on June 27, 2015.

### About the Author

#### Laura Spitler

Laura Spitler was a graduate student working with Prof. Jim Cordes. After graduating in 2013, she went on to a postdoctoral fellowship at the Max Planck Institute in Bonn, Germany. She works on a range of projects involving the time variability of radio sources, including pulsars, binary white dwarfs and ETI. In particular she is interested in building digital instruments and developing signal processing techniques that allow one to more easily identify and classify transient sources.

## Aristarchus of Samos

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Aristarchus of Samos, (born c. 310 bce —died c. 230 bce ), Greek astronomer who maintained that Earth rotates on its axis and revolves around the Sun. On this ground, the Greek philosopher Cleanthes the Stoic declared in his Against Aristarchus that Aristarchus ought to be indicted for impiety “for putting into motion the hearth of the universe.”

Aristarchus’s work on the motion of Earth has not survived, but his ideas are known from references by the Greek mathematician Archimedes, the Greek biographer Plutarch, and the Greek philosopher Sextus Empiricus. Archimedes said in his Sand-Reckoner that Aristarchus had proposed a new theory which, if true, would make the universe vastly larger than was then believed. (This is because a moving Earth should produce a parallax, or annual shift, in the apparent positions of the fixed stars, unless the stars are very far away indeed.)

In the 16th century Aristarchus was an inspiration for Polish astronomer Nicolaus Copernicus’s work. In his manuscript of Six Books Concerning the Revolutions of the Heavenly Orbs (1543), Copernicus cited Aristarchus as an ancient authority who had espoused the motion of Earth. However, Copernicus later crossed out this reference, and Aristarchus’s theory was not mentioned in the published book.

Aristarchus’s only extant work is On the Sizes and Distances of the Sun and Moon, the oldest surviving geometric treatment of this problem. Aristarchus takes as premises that

Using premise 3, Aristarchus showed that the Sun is between 18 and 20 times farther away from Earth than the Moon is. (The actual ratio is about 390.) Using this result and premises 1 and 2 in a clever geometric construction based on lunar eclipses, he obtained values for the sizes of the Sun and Moon. He found the Moon’s diameter to be between 0.32 and 0.40 times the diameter of Earth and the Sun’s diameter to be between 6.3 and 7.2 times the diameter of Earth. (The diameters of the Moon and the Sun compared with that of Earth are actually 0.27 and 109, respectively.)

In Aristarchus’s day the geometric method was considered more important than numerical measurements. His premise 1 is reasonably accurate. Premise 2 overestimates the Moon’s angular diameter by a factor of four, which is puzzling, since this is an easy measurement to make. (In a later publication, Aristarchus gave the angular size of the Moon as half a degree, which is about right, but he apparently did not modify his earlier work.) Premise 3 was probably not based on measurement but rather on an estimate it is equivalent to assuming that the time from first quarter Moon to third quarter Moon is one day longer than the time from third quarter to first quarter. The true angle between Sun and Moon at the time of quarter Moon is less than 90 degrees by only 9 minutes of arc—a quantity impossible to measure in antiquity.

Later Greek astronomers, especially Hipparchus and Ptolemy, refined Aristarchus’s methods and arrived at very accurate values for the size and distance of the Moon. However, because of the influence of premise 3, all ancient results grossly underestimated the size and distance of the Sun. Aristarchus’s 19:1 ratio nevertheless remained more or less standard until the 17th century.

## Rotation of the Earth about its own axis - Astronomy

As a father and a science lover I was dumbfounded when my 7yr old son asked me "What makes our earth turn (rotate) ? He then asked "Did it start a long time ago and it just keeps turning or is something pushing it to turn"? How does the Earth continue to rotate around its axis? Where does the energy to keep it moving come from?

Our everyday experience teaches us that an object must be "pushed" by a force in order to keep it moving. Otherwise, it will slow down and eventually stop. But this intuition is absolutely wrong. If an object is moving, then a force is required *to slow it down or stop it*, not to keep it moving. (Hence, "Objects in motion tend to stay in motion. Objects at rest tend to stay at rest.") In our everyday experience, it's the force of friction that tends to stop Earth-bound objects from moving forever. But for the Earth rotating on its axis, there is no force working to counteract the rotation (except the tidal effect of the Moon, but that's working very slowly), so you don't need to have any input energy to keep it spinning.

What started the earth rotating in the first place?

The shortest answer is angular momentum. Angular momentum is simply the name we give for the fact that things tend to rotate. (Just like regular momentum is the tendency for things to move.) The Earth formed out of a nebula that collapsed. As the nebula collapsed it began rotating, which may seem odd, but actually not rotating is far stranger than rotating. The Earth's rotation comes from the initial tendency to rotate that was imparted on it when it formed, only the relatively weak tidal forces from the Moon act to slow it down.

This page was last updated on June 27, 2015.

### About the Author

#### Laura Spitler

Laura Spitler was a graduate student working with Prof. Jim Cordes. After graduating in 2013, she went on to a postdoctoral fellowship at the Max Planck Institute in Bonn, Germany. She works on a range of projects involving the time variability of radio sources, including pulsars, binary white dwarfs and ETI. In particular she is interested in building digital instruments and developing signal processing techniques that allow one to more easily identify and classify transient sources.