Lunar day
A lunar day is the time it takes for
sidereal period
.
Main definition
Relative to the
synodic period lasts 29 days, 12 hours, 44 minutes, 3 seconds,[1] 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 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.
Alternate usage
- The term lunar day may also refer to the period between same direction as the Earth's axial rotation.[2]
- 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)",[3] while China's Yutu-2 rover, which landed in January 2019, was designed to survive lunar nights by shutting down.[4]
Lunar calendars
In some lunar calendars, 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
- Mars sol, the Martian day
References
- ^ a b "Month". Encyclopædia Britannica. Retrieved 15 April 2015.
- NOAA. Retrieved 15 May 2019.
- ^ "This is why lunar colonies will need to live underground".
- ^ Urrutia, Doris Elin (16 May 2019). "Goodnight, Chang'e-4! China's Probe on Moon's Far Side Naps for Lunar Night". space.com. Future US, Inc. Retrieved 15 November 2019.
External links
- Lunar days and other lunar data for many different cities. Lunarium.co.uk.
- Lunar Standard Time (LST) Archived 2017-11-14 at the Wayback Machine lunarclock.org.