Why does the Earth spin? Earth's rotation period relative to the Sun-its mean solar day-is 86,400 seconds of mean solar time (86,400.0025 SI seconds). Because Earth's solar day is now slightly longer than it was during the 19th century due to tidal deceleration, each day varies between 0 and 2 ms longer than the mean solar 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 86,164.0989 seconds of mean solar time (UT1), or 23h 56m 4.0989s. Earth's rotation period relative to the precessing or moving mean March equinox (when the Sun is at 90° on the equator), is 86,164.0905 seconds of mean solar time (UT1) (23h 56m 4.0905s). Thus the sidereal day is shorter than the stellar day by about 8.4 ms.
Apart from meteors within the atmosphere and low-orbiting satellites, the main apparent motion of celestial bodies in Earth's sky is to the west at a rate of 15°/h = 15'/min. For bodies near the celestial equator, this is equivalent to an apparent diameter of the Sun or the Moon every two minutes; from Earth's surface, the apparent sizes of the Sun and the Moon are approximately the same.
This video used the following article under Creative Commons - CC BY-ND 4.0 DEED: "Why does the Earth spin?" (theconversation.com/why-does-...) Silas Laycock, Professor of Astronomy, UMass Lowell (Published: February 13, 2023); slight stylistic modifications to original text.
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