Ajay Kumar at 05:15, 7 August 2023

2023-08-07T05:15:16Z

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{{Short description|Time for Moon to complete one rotation on its axis}}
{{Also|Synodic day}}
[[File:Lunation animation November 2009.gif|thumb|A lunar day completes]]
A '''lunar day''' is the period of time for [[Earth]]'s [[Moon]] to complete one [[rotation]] on its [[axis of rotation|axis]] with respect to the [[Sun]]. Due to [[tidal locking]], this equals the time that the Moon takes to complete one [[orbit of the Moon|orbit]] around [[Earth]] ([[Earth rise]] to Earth set) plus about 2.2 more Earth days to return to the same Moon [[lunar phase|phase]] (due to the Moon's orbit around the Sun). The lunar day is roughly 29 {{Half}} Earth days, the length of a [[lunar month]], and the time of which includes a full [[day-night cycle]].

==Main definition==
Relative to the [[fixed stars]] on the [[celestial sphere]], the Moon takes 27 Earth [[day]]s, 7 hours, 43 minutes, 12 seconds to complete [[orbital period|one orbit]];<ref name="ebmonth">{{cite web|url=http://www.britannica.com/EBchecked/topic/390962/month#ref225844|title=Month|publisher=[[Encyclopædia Britannica]]|accessdate=15 April 2015}}</ref> however, since the Earth–Moon system advances around the Sun at the same time, the Moon must travel farther to return to the same phase. On average, this [[synodic period]] lasts 29 days, 12 hours, 44 minutes, 3 seconds,<ref name="ebmonth" /> the length of a [[lunar month]] on Earth. The exact length varies over time because the speed of the Earth–Moon system around the Sun varies slightly during a year due to the [[orbital eccentricity|eccentricity]] of its elliptical orbit, variances in [[orbital speed|orbital velocity]], and a number of other periodic and evolving variations about its observed, relative, mean values, which are influenced by the gravitational [[perturbation (astronomy)|perturbations]] of the Sun and other bodies in the [[Solar System]].

As a result, daylight at a given point on the Moon would last approximately two weeks from beginning to end, followed by approximately two weeks of ''lunar night''.

== Alternate usage ==

* The term ''lunar day'' may also refer to the period between [[Moonrise and moonset|moonrises]] or [[culmination|high moon]] in a particular location on Earth. This period is typically about 50 minutes longer than a 24-hour Earth day, as the Moon orbits the Earth in the [[prograde motion|same direction]] as the Earth's axial rotation.<ref>{{cite web |url=https://oceanservice.noaa.gov/education/tutorial_tides/tides05_lunarday.html |title=Frequency of Tides - The Lunar Day |publisher=[[NOAA]] |accessdate=15 May 2019}}</ref>
* The term ''lunar day'' is also used in the context of night and day, i.e., opposite to the lunar night. This is common in discussions of the huge difference in temperatures, such as discussion about [[Lunar rover|lunar rovers]]. For example, "the [[Soviet Union|Soviet Union's]] [[Luna programme|Luna missions]] [...] were designed to survive one lunar day (two Earth weeks)."<ref>{{Cite web | url=http://www.bbc.com/future/story/20151218-how-to-survive-the-freezing-lunar-night | title=This is why lunar colonies will need to live underground}}</ref>, or, [[China|China's]] [[Yutu-2]] rover, which landed in January 2019, was designed to survive lunar nights by shutting down.<ref>{{cite news |last1=Urrutia |first1=Doris Elin |title=Goodnight, Chang'e-4! China's Probe on Moon's Far Side Naps for Lunar Night |url=https://www.space.com/moon-far-side-fifth-night-for-china-chang-e-4.html |accessdate=15 November 2019 |work=space.com |publisher=Future US, Inc.|date=16 May 2019}}</ref>

=== Lunar calendars ===

In some [[lunar calendar]]s, such as the [[Vikram Samvat]], a lunar day, or ''[[tithi]]'', is defined as 1/30 of a [[lunar month]], or the time it takes for the [[longitude|longitudinal]] angle between the Moon and the Sun to increase by 12 degrees. By this definition, lunar days generally vary in duration.

==See also==
*[[Lunisolar calendar]]
*[[Timekeeping on Mars#Sols|Sol]], the name for a Martian solar day

==References==
{{Reflist}}

==External links==
* [http://www.lunarium.co.uk/calendar/universal.jsp Lunar days and other lunar data for many different cities]. Lunarium.co.uk.
* [http://lunarclock.org/what-is-lunar-standard-time.php Lunar Standard Time (LST)] {{Webarchive|url=https://web.archive.org/web/20171114220008/http://lunarclock.org/what-is-lunar-standard-time.php |date=2017-11-14 }} lunarclock.org.

[[Category:Orbit of the Moon|Day]]
[[Category:Units of time|Day, lunar]]

Lunar day - Revision history

Ajay Kumar at 05:15, 7 August 2023