Editing Synodic day (section)

==Earth==
{{Main|Earth's rotation|Solar time|Day}}

[[Earth]]'s synodic day is the time it takes for the [[Sun]] to pass over the same [[meridian (astronomy)|meridian]] (a line of [[longitude]]) on consecutive days, whereas a sidereal day is the time it takes for a given distant star to pass over a meridian on consecutive days.<ref>{{cite web|title=Sidereal vs. Synodic Motions|url=https://astro.unl.edu/naap/motion3/sidereal_synodic.html|last=|first=|date=|website=Astronomy Education at the University of Nebraska-Lincoln|publisher=The University of Nebraska-Lincoln|accessdate=22 September 2020}}</ref>  For example, in the [[Northern Hemisphere]], a synodic day could be measured as the time taken for the Sun to move from exactly true south (i.e. its highest [[Declination of the Sun|declination]]) on one day to exactly south again on the next day (or exactly true north in the [[Southern Hemisphere]]).

[[File:Length of solar day.png|thumb|450px|Derivative of −Δt. The axis on the right shows the length of the solar day.]]

For Earth, the synodic day is not constant, and changes over the course of the year due to the [[orbital eccentricity|eccentricity]] of Earth's orbit around the Sun and the [[axial tilt]] of the Earth.<ref>{{cite journal |title=The Equation of Time |author1=David W. Hughes |author2=B.D. Yallop |author3=C.Y. Hohenkerk |journal=Monthly Notices of the Royal Astronomical Society |publisher=[[Royal Astronomical Society]] |issn=0035-8711 |volume=238 |date=15 June 1989 |issue=4 |pages=1529–35 |doi= 10.1093/mnras/238.4.1529|bibcode=1989MNRAS.238.1529H|doi-access=free }}</ref> The longest and shortest synodic days' durations differ by about 51 seconds.<ref>{{cite book | author1 = J. M. A. Danby | author2 = Jean Meeus | date = 1997 | title = Mathematical Astronomy Morsels | publisher = Willmann-Bell | pages = | isbn = 978-0-943396-51-4 | url = https://books.google.com/books?id=rXbvAAAAMAAJ}}</ref> The mean length, however, is 24 hours (with [[Day length fluctuations|fluctuations]] on the order of [[millisecond]]s), and is the basis of [[solar time]]. The difference between the ''mean'' and ''apparent'' solar time is the [[equation of time]], which can also be seen in Earth's [[analemma]]. Because of the variation in the length of the synodic day, the days with the longest and shortest period of daylight do not coincide with the [[solstice]]s near the equator.

As viewed from Earth during the year, the Sun appears to slowly drift along an imaginary path [[coplanarity|coplanar]] with [[Earth's orbit]], known as the [[ecliptic]], on a [[celestial sphere|spherical background]] of seemingly [[fixed stars]].<ref>{{Cite web |url=http://www.nasa.gov/multimedia/imagegallery/image_feature_635.html |title=The Plane of the Ecliptic |date=July 12, 2016 |website=NASA}}</ref> Each synodic day, this gradual motion is a little less than 1° eastward (360° per 365.25&nbsp;days), in a manner known as [[retrograde and prograde motion|prograde motion]].

Certain [[spacecraft]] orbits, [[Sun-synchronous orbit]]s, have [[orbital period]]s that are a fraction of a synodic day. Combined with a [[nodal precession]], this allows them to always pass over a location on Earth's surface at the same [[Solar time#Mean solar time|mean solar time]].<ref>{{Cite web |url=http://nptel.ac.in/courses/105108077/module2/lecture6.pdf |title=SATELLITES AND ORBITS|date=|website=|access-date=}}</ref>