Earth

=== Rotation ===
{{Main|Earth's rotation}}
[[File:EpicEarth-Globespin-tilt-23.4.gif|thumb|upright=1.3|Satellite [[Time-lapse photography|time lapse imagery]] of Earth's rotation showing axis tilt]]
Earth's rotation period relative to the Sun—its mean solar day—is {{nowrap|86,400 seconds}} of mean solar time ({{nowrap|86,400.0025 [[SI]] seconds}}).<ref name="aj136_5_1906" /> Because Earth's solar day is now slightly longer than it was during the 19th century due to [[tidal acceleration|tidal deceleration]], each day varies between {{nowrap|0 and 2 [[millisecond|ms]]}} longer than the mean solar day.<ref name="USNO_TSD" /><ref>{{cite journal |title=Rapid Service/Prediction of Earth Orientation |journal=IERS Bulletin-A |date=9 April 2015 |volume=28 |issue=15 |url=http://maia.usno.navy.mil/ser7/ser7.dat |access-date=12 April 2015 |format=.DAT file (displays as plaintext in browser) |archive-url=https://web.archive.org/web/20150314182157/http://maia.usno.navy.mil/ser7/ser7.dat |archive-date=14 March 2015 |url-status=dead }}</ref>

Earth's rotation period relative to the [[fixed star]]s, called its ''stellar day'' by the [[International Earth Rotation and Reference Systems Service]] (IERS), is {{nowrap|86,164.0989 seconds}} of mean solar time ([[UT1]]), or {{nowrap |23{{smallsup|h}} 56{{smallsup|m}} 4.0989{{smallsup|s}}.}}<ref name="IERS" /><ref group="n" name="Aoki" /> Earth's rotation period relative to the [[precession (astronomy)|precessing]] or moving mean [[March equinox]] (when the Sun is at 90° on the equator)<!-- , misnamed its ''[[sidereal day]]'' [don't know what is this] -->, is {{nowrap|86,164.0905 seconds}} of mean solar time (UT1) {{nowrap|(23{{smallsup|h}} 56{{smallsup|m}} 4.0905{{smallsup|s}})}}.<ref name="IERS" /> Thus the sidereal day is shorter than the stellar day by about 8.4&nbsp;ms.<ref name="seidelmann1992" />

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.<ref name="zeilik1998" /><ref name="angular" />