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Lunar calendar

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"Lunar date" redirects here. For uses not on Earth, see timekeeping on the Moon.
Lunar calendar year 2025

A lunar calendar is a calendar based on the monthly cycles of the Moon's phases (synodic months, lunations), in contrast to solar calendars, whose annual cycles are based on the solar year. The most widely observed purely lunar calendar is the Islamic calendar.[a] A purely lunar calendar is distinguished from a lunisolar calendar, whose lunar months are brought into alignment with the solar year through some process of intercalation – such as by insertion of a leap month. The details of when months begin vary from calendar to calendar, with some using new, full, or crescent moons and others employing detailed calculations.

Since each lunation is approximately 29+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, a lunar year, is 354 days, 8 hours, 48 minutes, 34 seconds (354.36707 days),[1] purely lunar calendars are 11 to 12 days shorter than the solar year. In purely lunar calendars, which do not make use of 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).

History

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Scholars have argued that ancient hunters conducted regular astronomical observations of the Moon back in the Upper 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]

Start of the lunar month

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Lunar and lunisolar calendars differ as to which day is the first day of the month. Some are based on the first sighting of the lunar crescent, such as the Hijri calendar observed by most of Islam. Alternatively, in some lunisolar calendars, such as the Hebrew calendar and Chinese calendar, the first day of a month is the day when an astronomical new moon occurs in a particular time zone. In others, such as some Hindu calendars, each month begins on the day after the full moon.

Length of the lunar month

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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 the Tabular Islamic calendar: in brief, it has a 30-year cycle with 11 leap 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 the Islamic lunar calendar.[4]

List of lunar calendars

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See also

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Notes

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  1. ^ Iran operates Solar Hijri calendar, which is purely solar.
  2. ^ Calendar used by the large majority of Muslims outside Iran
  3. ^ After 1633 CE reform

References

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  1. ^ a b P. 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)
  2. ^ Gurshtein, Alex (2005-01-01). "Did the Pre-Indo-Europeans Influence the Formation of the Western Zodiac?". Journal of Indo-European Studies. 33: 106.
  3. ^ Macey, Samuel L. (1994). Encyclopedia of Time. Taylor & Francis. p. 75. ISBN 9780815306153.
  4. ^ van Gent, R. H. (October 2023). "The Arithmetical or Tabular Islamic Calendar". Mathematical Institute, Utrecht University. Retrieved 4 January 2024.
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