In lunar calendars, a lunar month is the time between two successive syzygies (new moons or full moons). The precise definition varies, especially for the beginning of the month.
This article deals with the definitions of a 'month' that are mainly of significance in astronomy. For other definitions, including a description of a month in the calendars of different cultures around the world, see: month
Variations
In Shona, Middle-Eastern, and European traditions, the month starts when the young crescent moon becomes first visible at evening after conjunction with the Sun one or two days before that evening (e.g., in the Islamic calendar). In ancient Egypt the lunar month began on the day when the waning moon could no longer be seen just before sunrise. Others use calculation, of varying degrees of sophistication, e.g., the Hebrew calendar or the ecclesiastical lunar calendar. Yet others run from full moon to full moon. Calendars count integer days, so months may be 29 or 30 days in length, in some regular or irregular sequence. In India the month from conjunction to conjunction is divided into thirty parts known as tithis. A tithi is between 19 to 26 hours long. The date is named after the tithi ruling at sunrise. When the tithi is shorter than the day, the tithi may jump. This case is called xaya or loap. Conversely a tithi may 'stall' as well, that is - the same tithi would be associated with two consecutive days, a case which is known as vriddhi.
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.[1] 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.[2]
Terminology
There are several types of lunar month. The term lunar month usually refers to the synodic month because it is the cycle of the visible phases of the Moon.
Most of the following types of lunar month, except the distinction between the sidereal and tropical months, were first recognized in Babylonian lunar astronomy.
Sidereal month
The period of the Moon's orbit as defined with respect to the celestial sphere of apparently fixed stars (nowadays the International Celestial Reference Frame (ICRF)) is known as a sidereal month because it is the time it takes the Moon to return to a similar position among the stars (Latin: sidera): 27.321661 days (27 d 7 h 43 min 11.6 s).[3] This type of month has been observed among cultures in the Middle East, India, and China in the following way: they divided the sky into 27 or 28 lunar mansions, one for each day of the month, identified by the prominent star(s) in them.