A lunar day is the period of time for
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surf ...
's
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width ...
to complete one
rotation on its
axis with respect to the
Sun. Due to
tidal locking, it is the time the Moon takes to complete one
orbit
In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such a ...
around
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surf ...
(
Earth rise to Earth set) plus about 2.2 more Earth days to return to the same Moon
phase (due to the Moon's orbit around the Sun). The lunar day is roughly 29 Earth days, the length of a
lunar month, and the time of which includes a full
day-night cycle
A diurnal cycle (or diel cycle) is any pattern that recurs every 24 hours as a result of one full rotation of the planet Earth around its axis. Earth's rotation causes surface temperature fluctuations throughout the day and night, as well as we ...
.
Main definition
Relative to the
fixed stars on the
celestial sphere, the Moon takes 27 Earth
day
A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours, 1440 minutes, or 86,400 seconds. In everyday life, the word "day" often refers to a solar day, which is the length between two so ...
s, 7 hours, 43 minutes, 12 seconds to complete
one orbit;
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,
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
eccentricity of its elliptical orbit, variances in
orbital velocity, and a number of other periodic and evolving variations about its observed, relative, mean values, which are influenced by the gravitational
perturbations
Perturbation or perturb may refer to:
* Perturbation theory, mathematical methods that give approximate solutions to problems that cannot be solved exactly
* Perturbation (geology), changes in the nature of alluvial deposits over time
* Perturbatio ...
of the Sun and other bodies in the
Solar System
The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
.
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
moonrises or
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
same direction as the Earth's axial rotation.
* 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 rovers. For example, "the
Soviet Union's Luna missions ..were designed to survive one lunar day (two Earth weeks).", or,
China's Yutu-2 rover, which landed in January 2019, was designed to survive lunar nights by shutting down.
Lunar calendars
In some
lunar calendar
A lunar calendar is a calendar based on the monthly cycles of the Moon's phases ( synodic months, lunations), in contrast to solar calendars, whose annual cycles are based only directly on the solar year. The most commonly used calendar, t ...
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
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
A lunisolar calendar is a calendar in many cultures, combining lunar calendars and solar calendars. The date of Lunisolar calendars therefore indicates both the Moon phase and the time of the solar year, that is the position of the Sun in the ...
*
Sol
Sol or SOL may refer to:
Astronomy
* The Sun
Currency
* SOL Project, a currency project in France
* French sol, or sou
* Argentine sol
* Bolivian sol, the currency of Bolivia from 1827 to 1864
* Peruvian sol, introduced in 1991
* Peruvian sol ( ...
, the name for a Martian solar day
References
{{Reflist
External links
Lunar days and other lunar data for many different cities Lunarium.co.uk.
Lunar Standard Time (LST)lunarclock.org.
Day
A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours, 1440 minutes, or 86,400 seconds. In everyday life, the word "day" often refers to a solar day, which is the length between two so ...
Day, lunar