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
Year calculated for 2000, and differences in year lengths, compared to 2000, are given for past and future years. In the tables a day is 86,400 SI seconds long. An average Gregorian year is 365.2425 days (52.1775 weeks, hours, minutes or seconds). For this calendar, a common year is 365 days ( hours, minutes or seconds), and a leap year is 366 days ( hours, minutes or seconds). The 400-year cycle of the Gregorian calendar has days and hence exactly weeks. The Great Year, or equinoctial cycle, corresponds to a complete revolution of the equinoxes around the ecliptic. Its length is