An animation showing the rotation of the Earth.
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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.
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 86164.098 903 691 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 86164.090 530 832 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 86400 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 1674.4 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.
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 Earth's shape (an oblate spheroid), climate, ocean depth, and tectonic forces.[8]
Origin of rotation
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).[9] 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.
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.
See also
References
- ^ Leap seconds by USNO
- ^ a b c IERS EOP Useful constants
- ^ 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.
- ^ Explanatory Supplement to the Astronomical Almanac, ed. P. Kenneth Seidelmann, Mill Valley, Cal., University Science Books, 1992, p.48, ISBN 0-935702-68-7.
- ^ IERS Excess of the duration of the day to 86400s … since 1623 Graph at end.
- ^ IERS Variations in the duration of the day 1962–2005
- ^ Permanent monitoring
- ^ Physical effects
- ^ "Why does the earth spin?". Ask a Scientist Astronomy Archice.
External links
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