Lunar month circular, the speed of Earth's progression around the Sun varies during the year. Thus, the angular rate is faster nearer periapsis and slower near apoapsis. The same is so for the Moon's orbit around the Earth. Because of these variations in angular rate, the actual time between lunations may vary from about 29.18 to about 29.93 days. The long-term average duration is days (29 d 12 h 44 min 2.8016 s). The synodic month is used to calculate eclipse cycles. It is customary to specify positions of celestial bodies with respect to the vernal equinox. Because of Earth's precession of
Lunar day daylight at a given point on the Moon would last approximately two weeks from beginning to end, followed by approximately two weeks of night. In some lunar calendars, such as the Hindu calendar, a lunar day, or "tithi", is defined as 1/30th 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. Lunar day A lunar day is the period of time for Earth's Moon to complete one rotation on its axis with respect to the Sun.