Since each lunation is approximately29+1⁄2 days,[1] it is common for the months of a lunar calendar to alternate between 29 and 30 days. Since the period of 12 such lunations, alunar year, is 354 days, 8 hours, 48 minutes, 34 seconds (354.36707 days),[1] lunar calendars are 11 to 12 days shorter than thesolar year. In lunar calendars, which do not make use of lunisolar calendars' intercalation, the lunar months cycle through all the seasons of a solar year over the course of a 33–34 lunar-year cycle (see, e.g.,list of Islamic years).
Scholars have argued that ancient hunters conducted regular astronomical observations of the Moon back in theUpper Palaeolithic.[2] Samuel L. Macey dates the earliest uses of the Moon as a time-measuring device back to 28,000–30,000 years ago.[3]
Lunar and lunisolar calendars differ as to which day is the first day of the month. Some are based on the first sighting of thelunar crescent, such as theHijri calendar observed by most of Islam. Alternatively, in some lunisolar calendars, such as theHebrew calendar andChinese calendar, the first day of a month is the day when an astronomicalnew moon occurs in a particular time zone. In others, such as someHindu calendars, each month begins on the day after the full moon.
The length of each lunar cycle varies slightly from the average value. In addition, observations are subject to uncertainty and weather conditions. Thus, to minimise uncertainty, there have been attempts to create fixed arithmetical rules to determine the start of each calendar month. The best known of these is theTabular Islamic calendar: in brief, it has a 30-year cycle with 11leap years of 355 days and 19 years of 354 days. In the long term, it is accurate to one day in about 2,500 solar years or 2,570 lunar years. It also deviates from observation by up to about one or two days in the short term. The algorithm was introduced by Muslim astronomers in the 8th century to predict the approximate date of the first crescent moon, which is used to determine the first day of each month in theIslamic lunar calendar.[4]
Most calendars referred to as "lunar" calendars arelunisolar calendars. Their months are based on observations of the lunar cycle, with periodicintercalation being used to restore them into general agreement with the solar year. The solar "civic calendar" that was used inancient Egypt showed traces of its origin in the earlier lunar calendar, which continued to be used alongside it for religious and agricultural purposes. Present-day lunisolar calendars include theChinese,Korean,Vietnamese,Hindu,Hebrew andThai calendars.
The most common form of intercalation is to add an additional month every second or third year. Some lunisolar calendars are also calibrated by annual natural events which are affected by lunar cycles as well as the solar cycle. An example of this is the lunisolar calendar of theBanks Islands, which includes three months in which the ediblepalolo worms mass on the beaches. These events occur at the last quarter of the lunar month, as the reproductive cycle of the palolos is synchronized with the moon.[5]
^abP. Kenneth Seidelmann, ed. (1992).Explanatory Supplement to the Astronomical Almanac. p. 577.For convenience, it is common to speak of a lunar year of twelve synodic months, or 354.36707 days. (which gives a mean synodic month as 29.53059 days or 29 days 12 hours 44 minutes and 3 seconds)
