Lunar calendar

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.

Iranian Islamic calendar dedicated to Qajar ruler Naser al-Din Shah in 1280, Linden Museum, Stuttgart, Germany

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

A lunisolar calendar was found at Warren Field in Scotland and has been dated to c. 8000 BC, during the Mesolithic period.^[2][3] Some scholars argue for lunar calendars still earlier—Rappenglück in the marks on a c. 17,000 year-old cave painting at Lascaux and Marshack in the marks on a c. 27,000 year-old bone baton—but their findings remain controversial.^[4][5] Scholars have argued that ancient hunters conducted regular astronomical observations of the Moon back in the Upper Palaeolithic.^[6] Samuel L. Macey dates the earliest uses of the Moon as a time-measuring device back to 28,000–30,000 years ago.^[7]

Start of the lunar month

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 (and, historically, the Hebrew calendar). In some lunisolar calendars, such as the 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

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 CE to predict the approximate date the first crescent moon, which is used to determine the first day of each month in the Islamic lunar calendar.^[8]

List of lunar calendars

• Islamic Hijri calendar
• Javanese calendar

Lunisolar calendars

Main article: Lunisolar calendar

Most calendars referred to as "lunar" calendars are in fact lunisolar calendars. Their months are based on observations of the lunar cycle, with intercalation being used to bring them into general agreement with the solar year. The solar "civic calendar" that was used in ancient 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 the Chinese, Korean, Vietnamese, Hindu, Hebrew and Thai calendars.

Synodic months are 29 or 30 days in length, making a lunar year of 12 months about 11 to 12 days shorter than a solar year. Some lunar calendars do not use intercalation, for example the lunar Hijri calendar used by most Muslims. For those that do, such as the Hebrew calendar, and Buddhist Calendars in Myanmar, 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 lunar calendar of the Banks Islands, which includes three months in which the edible palolo 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.^[9]

In the East Asian cultural sphere, the lunisolar Chinese calendar strongly influenced the traditional calendars used in neighbouring East Asian countries including notably Vietnam and Korea. It was believed to have been first established by Emperor Huang Ti of China in 2600 BC.^[10]

See also

•  Solar System portal

• List of calendars
• Lunar phase
• Epact
• Paschal Full Moon

Notes

1. Iran operates Solar Hijri calendar, which is purely solar.

References

1. 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. Nancy Owano, Scotland lunar-calendar find sparks Stone Age rethink, Phys.org, 27 July 2013 9 August 2013 at the Wayback Machine
3. Gaffney, V.; et al. (2013). "Time and a Place: A luni-solar 'time-reckoner' from 8th millennium BC Scotland". Internet Archaeology (34). doi:10.11141/ia.34.1. from the original on 18 July 2013. Retrieved 16 July 2013. In doing so the monument anticipates problems associated with simple lunar calendars by providing an annual astronomic correction in order to maintain the link between the passage of time indicated by the Moon, the asynchronous solar year, and the associated seasons.
4. James Elkins, Our beautiful, dry, and distant texts (1998) 63ff.
5. "Oldest lunar calendar identified". BBC News. 2000-10-16. Retrieved 2013-03-14.
6. Gurshtein, Alex (2005-01-01). "Did the Pre-Indo-Europeans Influence the Formation of the Western Zodiac?". Journal of Indo-European Studies. 33: 106.
7. Macey, Samuel L. (1994). Encyclopedia of Time. Taylor & Francis. p. 75. ISBN 9780815306153.
8. van Gent, R. H. (October 2023). "The Arithmetical or Tabular Islamic Calendar". Mathematical Institute, Utrecht University. Retrieved 4 January 2024.
9. R.H.Codrington. The Melanesians: Their anthropology and folklore (1891) Oxford, Clarendon Press
10. "Over the Moon - Chinese/Lunar New Year | Epicurious.com". Epicurious. Retrieved 2023-01-20.

External links