Editing Lunar day

{{Short description|Time for Moon to complete one rotation on its axis}}
{{hatnote group|
{{For|the tidal lunar day of about 24 hours and 50 minutes|Tide}}
{{redirect|Moon day|the day of the week named after the Moon|Monday}}
}}
[[File:Lunation animation November 2009.gif|thumb|A full lunar day observed from the Earth, where orbital [[libration]] causes the apparent wobble.]]
A '''lunar day''' is the time it takes for [[Earth]]'s [[Moon]] to complete on its [[axis of rotation|axis]] one [[synodic rotation]], meaning with respect to the [[Sun]]. Informally, a lunar day and a lunar night is each approximately 14 Earth days. The formal lunar day is therefore the time of a full lunar [[Solar day|day-night cycle]]. Due to [[tidal locking]], this equals the time that the Moon takes to complete one [[Synodic period|synodic]] [[orbit of the Moon|orbit around Earth]], a [[Lunar month#Synodic month|synodic lunar month]], returning to the same [[lunar phase]]. The synodic period is about 29.53 Earth days, which is about 2.2 days longer than its [[sidereal rotation|sidereal period]].

==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&ndash;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&ndash;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 lasts approximately two weeks from beginning to end, followed by approximately two weeks of '''lunar night'''.

==Darkness (lunar night)==
Lunar night is the darkest on the far side and during lunar eclipses on the near side (and darker than a moonless night on Earth). On the near side [[Earthshine]] makes the night about 43 times brighter, and sometimes even 55 times brighter than a night on Earth illuminated by the light of the full moon. No person has been on the Moon during its night and experienced earthshine.<ref name="u988">{{cite web | last=Siegel | first=Ethan | title=Ask Ethan: How Bright Is The Earth As Seen From The Moon? | website=Forbes | date=March 18, 2017 | url=https://www.forbes.com/sites/startswithabang/2017/03/18/ask-ethan-how-bright-is-the-earth-as-seen-from-the-moon/ | access-date=April 29, 2025}}</ref>

== 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'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> while 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]]
*[[Mars sol]], the Martian day
*[[Synodic 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.

{{The Moon}}

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