Lunar day
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In space exploration, a lunar day is the period of time it takes for the Moon to complete one full rotation on its axis with respect to the Sun. Equivalently, it is the time it takes the Moon to make one complete orbit around the Earth and come back to the same phase. It is marked from a New Moon to the next New Moon.
With respect to the stars, the Moon takes 27 d 7 h 43.2 min to go around in its orbit, but since the Earth-Moon system has advanced around the Sun in the mean time, the Moon must keep going a while longer to get back to the same phase. On average, this synodic period is 29 days, 12 hours, 44 minutes and 3 seconds long. This is an average figure since the speed of the Earth-Moon system around the Sun varies slightly over a year, due to the eccentricity of the orbit; the Moon's own orbit also undergoes a number of periodic variations about its mean value because of the gravitational perturbations of the Sun.
In lunar calendars, 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°. By this definition, lunar days vary in duration (see tithi).
