Editing Synodic day

{{Short description|Rotation period of a body relative to the primary object it orbits, e.g. solar day}}
{{hatnote group|
{{Distinguish|Orbital period#Synodic period{{!}}Synodic period}}
{{Redirect|Solar day|the measurement of a solar day|Solar time}}
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A '''synodic day''' (or '''synodic rotation period''' or '''solar day''') is the [[rotation period|period]] for a [[astronomical object|celestial object]] to rotate once in relation to the [[star]] it is [[orbit]]ing, and is the basis of [[solar time]].

The synodic day is distinguished from the [[sidereal time|sidereal day]], which is one complete rotation in relation to distant stars<ref>{{cite book |title=Time in Powers of Ten: Natural Phenomena and Their Timescales |first1=T. Hooft |last1=Gerard |first2=Vandoren |last2=Stefan |date=12 May 2014 |publisher=World Scientific |isbn=9789814494939 |url=https://books.google.com/books?id=hy-7CgAAQBAJ&q=synodic+day+definition&pg=PA24}}</ref> and is the basis of sidereal time.

In the case of a [[Tidal locking|tidally locked]] planet, the same side always faces its parent star, and its synodic day is infinite.  Its sidereal day, however, is equal to its orbital period.

==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>

==The Moon==
{{Main|Lunar day}}

Due to [[tidal locking]] with Earth, the [[Moon]]'s synodic day (the [[lunar day]] or synodic rotation period) is the same as its [[Orbital period#Synodic period|synodic period]] with Earth and the Sun (the period of the [[lunar phases]], the [[Lunar month#Synodic month|synodic lunar month]], which is the month of the [[lunar calendar]]).

==Venus==
{{Further|Venus#Orbit and rotation}}

Due to the slow [[retrograde motion|retrograde]] [[rotational speed]] of [[Venus]], its synodic [[rotation period]] of 117 Earth days is about half the length of its [[sidereal time|sidereal]] rotational period (sidereal day) and even its orbital period.<ref name="Venusday">{{cite web | title=How long is a day on Venus? | website=TE AWAMUTU SPACE CENTRE | url=https://www.spacecentre.nz/resources/faq/solar-system/venus/day.html | access-date=2021-06-03}}</ref>

==Mercury==
{{Further|Mercury (planet)#Spin-orbit resonance}}

Due to [[Mercury (planet)|Mercury]]'s slow rotational speed and fast orbit around the Sun, its synodic rotation period of 176 Earth days is three times longer than its sidereal rotational period (sidereal day) and twice as long as its orbital period.<ref name="ESO">{{cite web | title=ESO | website=ESO | url=https://www.eso.org/public/outreach/eduoff/vt-2004/mt-2003/mt-mercury-rotation.html | access-date=2021-06-03}}</ref>

==See also==
*{{section link|Noon|Solar noon}}
*[[Orbital period]]
*[[Rotation period]]
*[[Sidereal time]]
*[[Solar rotation]]
*[[Solar time]]
*[[Noon#Solar noon|Sun transit time]]
*[[Lunar month|Synodic month]]
*[[Day length fluctuations]]

==References==
{{reflist}}

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{{DEFAULTSORT:Synodic Day}}

[[Category:Units of time]]
[[Category:Astronomy]]