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 date is named after the "tithi" ruling at sunrise. As the "tithi" is shorter than the day the date sometimes jumps. In common law, a "lunar month" traditionally meant exactly 28 days or four weeks, thus a contract for 12 months ran for exactly 48 weeks. In the United Kingdom, the lunar month was formally replaced by the calendar month for deeds and other written contracts by the Law of Property Act 1925 and for all other legal purposes by the Interpretation Act 1978. There are several types of lunar month. The term "lunar month" usually refers to the synodic