Lunar calendar

A lunar calendar is a calendar that is based on cycles of the lunar phases. This can be contrasted with the Gregorian calendar, which is a solar calendar based on the revolution of the Earth around the sun. Because there are slightly more than twelve lunations (synodic months) in a solar year, the period of 12 lunar months (354.367 days) is sometimes referred to as a lunar year.

A common purely lunar calendar is the Islamic (or Hijri Qamari) calendar. A feature of the Islamic calendar is that a year is always 12 months, so the months are not linked with the seasons and drift each solar year by 11 to 12 days. It comes back to the position it had in relation to the solar year approximately every 33 Islamic years. It is used mainly for religious purposes, but in Saudi Arabia it is the official calendar. Other lunar calendars often include extra months added occasionally to synchronize it with the solar calendar.

The oldest known lunar calendar was found in Scotland at Warren Field and dates back to around 8,000 BC.Alexander Marshack, in a controversial reading, believed that marks on a bone baton (c. 25,000 BCE) represented a lunar calendar. Similarly, Michael Rappenglueck believes that marks on a 17,000-year-old cave painting in Lascaux represent a lunar calendar.

Most calendars referred to as "lunar" calendars are in fact lunisolar calendars. That is, months reflect the lunar cycle, but then intercalary months (e.g. "second Adar" in the Hebrew calendar) are added to bring the calendar year into synchronisation with the solar year. Some examples are the Chinese, Hindu, and Thai calendars. Some other calendar systems used in antiquity were also lunisolar.

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