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 month days. The Moon completes one orbit around Earth every 27.3 days (a sidereal month), but due to Earth's orbital motion around the Sun, the Moon does not yet finish a synodic cycle until it has reached the point in its orbit where the Sun is in the same relative position. This table lists the average lengths of five types of astronomical lunar month. These are not constant, so a first-order (linear) approximation of the secular change is provided: Valid for the epoch J2000.0 (1 January 2000 12:00 TT): "Note:" In this table, time is expressed in Ephemeris Time (more precisely