Stellar and Sidereal Day
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) (23 56 4.098 903 691, 0.997 269 663 237 16 mean solar days). 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) (23 56 4.090 530 832 88, 0.997 269 566 329 08 mean solar days). Thus the sidereal day is shorter than the stellar day by about 8.4 ms.
Both the stellar day and the sidereal day are shorter than the mean solar day by about 3 minutes 56 seconds. The mean solar day in SI seconds is available from the IERS for the periods 1623–2005 and 1962–2005.
Recently (1999–2010) the average annual length of the mean solar day in excess of 86,400 SI seconds has varied between 0.25 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 (see Fluctuations in the length of day).
Read more about this topic: Earth's Rotation, Rotation Period
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