Earth::Rotation period


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Earth {{#invoke:main|main}} Earth's rotation period relative to the Sun (its mean solar day) consists of 86,400 seconds of mean solar time, by definition. 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 deceleration. 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.<ref>Leap seconds by USNO</ref>

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) Earth Earth's rotation period relative to the precessing or moving mean vernal equinox, its sidereal day, is 86164.090 530 832 88 seconds of mean solar time (UT1) Earth Thus the sidereal day is shorter than the stellar day by about 8.4 ms.<ref>Explanatory Supplement to the Astronomical Almanac, ed. P. Kenneth Seidelmann, Mill Valley, Cal., University Science Books, 1992, p.48, ISBN 0-935702-68-7.</ref> The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005<ref>IERS Excess of the duration of the day to 86400s … since 1623 Graph at end.</ref> and 1962–2005.<ref>IERS Variations in the duration of the day 1962–2005</ref> 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.

Rotation period sections
Intro  Measuring rotation  Earth  Rotation period of selected objects  See also  References  External links  

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