Earth's rotation

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An animation showing the rotation of the Earth.
An animation showing the rotation of the Earth.
Earth's axial tilt (or obliquity) and its relation to the rotation axis and plane of orbit.
Earth's axial tilt (or obliquity) and its relation to the rotation axis and plane of orbit.

Earth's rotation is the rotation of the solid Earth around its own axis. The Earth rotates towards the east, which can be observed by orientation with a magnetic compass at sunrise. As viewed from the star Polaris, the Earth turns counter-clockwise.

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[edit] Rotation period

On a prograde planet like the Earth, the sidereal day is shorter than the solar day. At time 1, the Sun and a certain distant star are both overhead. At time 2, the planet has rotated 360° and the distant star is overhead again but the Sun is not (1→2 = one sidereal day). It is not until a little later, at time 3, that the Sun is overhead again (1→3 = one solar day).
On a prograde planet like the Earth, the sidereal day is shorter than the solar day. At time 1, the Sun and a certain distant star are both overhead. At time 2, the planet has rotated 360° and the distant star is overhead again but the Sun is not (1→2 = one sidereal day). It is not until a little later, at time 3, that the Sun is overhead again (1→3 = one solar day).

The average rotation period of the Earth relative to the stars is 23 hours 56 minutes and 4.096 seconds. The resulting rotation rate of the Earth is 7.29211538×10-5 radians/second. The period with respect to the vernal point (the equinox of Spring) is slightly smaller: 23 hours 56 minutes and 4.088 seconds (the sidereal day). In contrast to this the rotation with respect to the Sun is 24 hours (the mean solar day), slightly larger because of the extra fractional rotation due to Earth's orbit around the Sun. Per day the difference is: 86,400 s / 365.25 = 236 seconds = 3 m 56 s (as can be seen above).

The circumference of the Earth at the equator is 40,000 km. If we divide the circumference by the number of hours it takes to rotate (40,000 by 24) we get the rate of the Earth's rotation at the equator, which is about 1,700 kilometers per hour.

Also, the Earth is orbiting around the Sun at about 110,000 kilometers per hour.

The IERS defines the "excess of length of day (LOD)" as the difference between the duration of the mean solar day and 86,400 SI seconds [1]. Excess of Length of Day since 2000

The permanent monitoring of the Earth's rotation requires the use of Very Long Baseline Interferometry coordinated with the Global Positioning System, Satellite laser ranging, and other satellite techniques. This provides the absolute reference for the determination of universal time, precession, and nutation.[2]

Over millions of years, the rotation is significantly slowed by gravitational interactions with the Moon: see tidal acceleration.

[edit] Precession

The axis of the Earth's rotation tends, like the axis of a gyroscope, to maintain its orientation with respect to inertial space. External forces acting on Earth from the Sun, Moon, and planets cause deviations from the fixed orientation. The large, periodic shift of the Earth's axis is called precession, while the smaller corrections are nutation and polar motion.

[edit] Physical effects

The velocity of the rotation of Earth has had various effects over time, including Earth's shape (an oblate spheroid), climate, ocean depth, and tectonic forces.[3]

[edit] Origin of rotation

An artist's impression of protoplanetary disk.
An artist's impression of protoplanetary disk.

It is theorized that Earth formed as part of the birth of the Solar System: what eventually became the solar system initially existed as a large, rotating cloud of dust, rocks, and gas. It was composed of hydrogen and helium produced in the Big Bang, as well as heavier elements ejected by supernovas. Then, as one theory suggests, about 4.6 billion years ago a nearby star was destroyed in a supernova and the explosion sent a shock wave through the solar nebula, causing it to gain angular momentum. As the rotating cloud flattened out, some of the gas and dust clustered together due to gravity (eventually becoming planets). Because the initial angular momentum needed to be conserved, the clustered mass started rotating faster (much in the same way an ice skater rotates quicker with his/her arms "clustered" closely to his/her body).[4] The current rotation period of the Earth is the result of this initial rotation and other factors, including tidal friction and possibly the giant impact hypothesis.

[edit] Foucault pendulum

One remarkable piece of evidence for Earth's rotation was built by physicist Léon Foucault in 1851. By attaching an iron sphere to a very long wire, Foucault constructed a pendulum that was 67m long. Physicists of the time knew that once a pendulum is set in motion, its direction of swing would not change. Foucault, however, observed that the direction of swing of his pendulum seemed to change. Each hour it shifted about 11 degrees in a clockwise direction. After eight hours the pendulum was swinging at a right angle to its starting direction. Because the pendulum itself could not have changed its direction of swing, Foucault concluded that the shift he saw was caused by Earth's turning beneath his pendulum. The Foucault pendulum is now a famous demonstration of Earth's rotation.

[edit] See also

[edit] References

  1. ^ Universal Time (UT1) and Length of Day (LOD)
  2. ^ Permanent monitoring
  3. ^ Physical effects
  4. ^ Why does the earth spin?. Ask a Scientist Astronomy Archice.

[edit] External links