Rotation

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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.^[105]

Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86164.098903691 seconds of mean solar time (UT1), or 23^h 56^m 4.098903691^s. ^{[106][note 13]} Earth's rotation period relative to the precessing or moving mean vernal equinox, misnamed its sidereal day, is 86164.09053083288 seconds of mean solar time (UT1) (23^h 56^m 4.09053083288^s).^[106] Thus the sidereal day is shorter than the stellar day by about 8.4 ms.^[107] The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005^[108] and 1962–2005.^[109]

Apart from meteors within the atmosphere and low-orbiting satellites, the main apparent motion of celestial bodies in the Earth's sky is to the west at a rate of 15°/h = 15'/min. This is equivalent to an apparent diameter of the Sun or Moon every two minutes; the apparent sizes of the Sun and the Moon are approximately the same.^[110][111]