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 phase eastern horizon at one moment sees it on the western horizon about 12 hours later. This adds an oscillation to the apparent progression of the lunar phases. They appear to occur more slowly when the Moon is high in the sky than when it is below the horizon. The Moon appears to move jerkily, and the phases do the same. The amplitude of this oscillation is never more than about four hours, which is a small fraction of a month. It does not have any obvious effect on the appearance of the Moon. However, it does affect accurate calculations of