Time for Moon to complete one rotation on its axis
For the tidal lunar day of about 24 hours and 50 minutes, seeTide."Moon day" redirects here. For the day of the week named after the Moon, seeMonday.
A full lunar day observed from the Earth, where orbitallibration causes the apparent wobble.
Alunar day is the time it takes forEarth'sMoon to complete on itsaxis onesynodic rotation, meaning with respect to theSun. The synodic period is about 29.53 Earth days, which is about 2.2 days longer than itssidereal period.
Informally, a lunar day and a lunar night are each approximately 14 Earth days. The formal lunar day is therefore the time of a full lunarday-night cycle.
Relative to thefixed stars on thecelestial sphere, the Moon takes 27 Earthdays, 7 hours, 43 minutes, 12 seconds to completeone orbit;[1] 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, thissynodic period lasts 29 days, 12 hours, 44 minutes, 3 seconds,[1] the length of alunar 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 theeccentricity of its elliptical orbit, variances inorbital velocity, and a number of other periodic and evolving variations about its observed, relative, mean values, which are influenced by the gravitationalperturbations of the Sun and other bodies in theSolar 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 oflunar night.
The night side of the Moon, illuminated byearthshine, becomes visible next to the narrow crescent (11 percent, age of the Moon = 3.3 days) with ash-greymoonlight.
Lunar night is the darkest on thefar side (darker than a moonless night on Earth, due to no atmospheric effects likeairglow).
On thenear sideEarthshine[2] 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. Only during lunar eclipses the night is on the near side darker than on Earth.
No person has been on the Moon during its night and experienced earthshine.[3]
The termlunar day may also refer to the period betweenmoonrises orhigh 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 thesame direction as the Earth's axial rotation.[4]
The termlunar 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 aboutlunar rovers. For example, "the Soviet Union'sLuna missions [...] were designed to survive one lunar day (two Earth weeks)",[5] while China'sYutu-2 rover, which landed in January 2019, was designed to survive lunar nights by shutting down.[6]
In somelunar calendars, such as theVikram Samvat, a lunar day, ortithi, is defined as 1/30 of alunar month, or the time it takes for thelongitudinal angle between the Moon and the Sun to increase by 12 degrees. By this definition, lunar days generally vary in duration.
