Lunar day
In
space exploration, a
lunar day is the period of time it takes for the
Moon to complete one full rotation on its axis. Equivalently, it is the time it takes the Moon to make one complete
orbit around the
Earth, due to
tidal locking. It is marked from a
New Moon to the next New Moon.
By Earth's time standards, a lunar day is officially 29 days, 12 hours, 44 minutes and 3 seconds long.
In lunar calendars, a
lunar day or
tithi is defined as one 30th of a
lunar month , or the time it takes for the
longitudinal angle between the
moon and the
sun to increase by 12
°. By this definition, lunar days vary in duration (see
tithi.)