Earth's rotation

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An animation showing the rotation of the Earth.

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 Earth's rotation is the rotation of the solid earth around its own axis, which is called Earth's axis or rotation 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.

1 Rotation period
2 Precession
3 Physical effects
4 Origin of rotation
5 Foucault pendulum
6 See also
7 References
8 External links

[edit] Rotation period

The average rotation period of the Earth is called a "stellar day". Its value is 23 hours 56 minutes and 4.099 seconds, and describes the rotation with respect to the cosmic background of the stars. The resulting rotation rate of the earth is 7.29211510 × 10^-5 radians/second. The period with respect to the vernal point (the equinoxe of Spring) is slightly smaller : 23 hours 56 minutes and 4.091 seconds (the sidereal day). In contrast to this the rotation with respect to the Sun is 24 hours (the mean solar day), differing because the Earth revolves around the sun once per year. 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 24,900 miles or about 25,000 miles and the Earth rotates about once in 24 hours. Therefore, if we divide 25000 by 24 we get that the earth rotates at a speed of just over 1000 miles per hour.

Also, the Earth is orbiting around the Sun at about 67,000 miles 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.

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 cloud began to accelerate its rotation, gravity and inertia flattened it into a protoplanetary disk oriented perpendicularly to its axis of rotation. Most of the mass concentrated in the middle and began to heat up, but small perturbations due to collisions and the angular momentum of other large debris created the means by which protoplanets began to form. The infall of material, increase in rotational speed and the crush of gravity created an enormous amount of kinetic heat at the center. Its inability to transfer that energy away through any other process at a rate capable of relieving the build-up resulted in the disk's center heating up. Ultimately, nuclear fusion of hydrogen into helium began, and eventually, after contraction, a T Tauri star, ignited into our early Sun. Meanwhile, as gravity caused matter to condense around the previously perturbed objects outside of the new sun's gravity grasp, dust particles and the rest of the protoplanetary disk began separating into rings. Successively larger fragments collided with one another and became larger objects, ultimately destined to become protoplanets.^[4]

[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 20 stories high. 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.