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Earth's rotation

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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 rotation is the rotation of the solid Earth around its own axis. The Earth rotates towards the east. As viewed from the North Star Polaris, the Earth turns counter-clockwise.

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).

Earth's rotation period relative to the Sun (its mean solar day) is 86,400 seconds of mean solar time. Each of these seconds is slightly longer than an SI second because Earth's solar day is now slightly longer than it was during the 19th century due to tidal acceleration. The mean solar second between 1750 and 1892 was chosen in 1895 by Simon Newcomb as the independent unit of time in his Tables of the Sun. These tables were used to calculate the world's ephemerides between 1900 and 1983, so this second became known as the ephemeris second. The SI second was made equal to the ephemeris second in 1967.[1]

Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86,164.098 903 691 seconds of mean solar time (UT1) (23h 56m 4.098 903 691s).[2][3] Earth's rotation period relative to the precessing or moving mean vernal equinox, misnamed its sidereal day, is 86,164.090 530 832 88 seconds of mean solar time (UT1) (23h 56m 4.090 530 832 88s).[2] Thus the sidereal day is shorter than the stellar day by about 8.4 ms.[4] The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005[5] and 1962–2005.[6]

Recently (1999–2005) the average annual length of the mean solar day in excess of 86,400 SI seconds has varied between 0.3 ms and 1 ms, which must be added to both the stellar and sidereal days given in mean solar time above to obtain their lengths in SI seconds.

The mean angular velocity of Earth is 7.2921150 ×10−5 radians per second.[2] The equatorial radius of Earth in the WGS84 ellipsoid is 6378137.0 m. Multiplying Earth's equatorial radius by its angular velocity yields Earth's rotational speed at the equator, 465.1 m/s or 1,674.4 km/h (factors of 2π needed to convert equatorial radius to circumference and angular velocity to rotational period cancel each other).

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.[7]

Over millions of years, the rotation is significantly slowed by gravitational interactions with the Moon: see tidal acceleration. However some large scale events, such as the 2004 Indian Ocean earthquake, have caused the rotation to speed up by around 3 microseconds.[8]

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.

Physical effects

The velocity of the rotation of Earth has had various effects over time, including the Earth's shape (an oblate spheroid), climate, ocean depth and currents, and tectonic forces.[9]

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 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).[10] 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.

Manifestations and Evidence of Earth's Rotation

In the Earth's rotating frame of reference, a freely moving body follows an apparent path that deviates from the one it would follow in a fixed frame of reference. Because of this Coriolis effect, falling bodies veer eastward from the vertical plumb line below their point of release, and projectiles veer right in the northern hemisphere (and left in the southern) from the direction in which they are shot. The Coriolis effect has many other manifestations, especially in meteorology, where it is responsible for the differing rotation direction of cyclones in the northern and southern hemispheres. Hooke, following a 1679 suggestion from Newton, tried unsuccessfully to verify the predicted half millimeter eastward deviation of a body dropped from a height of 8.2 meters, but definitive results were only obtained later, in the late 18'th and early 19'th century, by Giovanni Battista Guglielmini in Bologna, Friedrich Benzenberg in Hamburg and Ferdinand Reich in Freiberg, using taller towers and carefully released weights [11].

Foucault pendulum

The most celebrated test of Earth's rotation is the Foucault pendulum first built by physicist Léon Foucault in 1851, which consisted of an iron sphere suspended 67m from the top of the Panthéon in Paris. Because of the earth's rotation under the swinging pendulum (see explanation at Foucault pendulum) the pendulum's plane of oscillation appears to rotate at a rate depending on latitude. At the latitude of Paris the predicted and observed shift was about 11 degrees clockwise per hour. Foucault pendulums now swing in museums around the world.

See also

References

  1. ^ Leap seconds by USNO
  2. ^ a b c IERS EOP Useful constants
  3. ^ Aoki, the ultimate source of these figures, uses the term "seconds of UT1" instead of "seconds of mean solar time". Aoki, et al., "The new definition of Universal Time", Astronomy and Astrophysics 105 (1982) 359–361.
  4. ^ Explanatory Supplement to the Astronomical Almanac, ed. P. Kenneth Seidelmann, Mill Valley, Cal., University Science Books, 1992, p.48, ISBN 0-935702-68-7.
  5. ^ IERS Excess of the duration of the day to 86,400s … since 1623 Graph at end.
  6. ^ IERS Variations in the duration of the day 1962–2005
  7. ^ Permanent monitoring
  8. ^ Sumatran earthquake sped up Earth's rotation, Nature, December 30, 2004.
  9. ^ Physical effects
  10. ^ "Why does the earth spin?". Ask a Scientist Astronomy Archice.
  11. ^ http://de.wikipedia.org/wiki/Fallexperimente_zum_Nachweis_der_Erdrotation German Wikipedia article
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