Lunar calendar

File:2025 Lunar Calendar.png

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, and lunisolar calendars, whose lunar months are brought into alignment with the solar year through some process of intercalationTemplate:Sndsuch as by insertion of a leap month. The most widely observed lunar calendar is the Islamic calendar.Template:Efn 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 Template:Frac days,<ref name=ESAA>Template:Cite book (which gives a mean synodic month as 29.53059 days or 29 days 12 hours 44 minutes and 3 seconds)</ref> 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),<ref name=ESAA /> lunar calendars are 11 to 12 days shorter than the solar 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).

History

Scholars have argued that ancient hunters conducted regular astronomical observations of the Moon back in the Upper Palaeolithic.<ref>Template:Cite journal</ref> Samuel L. Macey dates the earliest uses of the Moon as a time-measuring device back to 28,000–30,000 years ago.<ref>Template:Cite book</ref>

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. 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

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.<ref>Template:Cite web</ref>

List of lunar calendars

• Islamic Hijri calendarTemplate:Efn
• Javanese calendarTemplate:Efn

See also

• List of calendars
• Islamic calendar
• Lunisolar calendar
• Lunar phase
• Epact
• Paschal Full Moon