Lunar calendar

A lunar calendar is a special way to keep track of time that uses the Moon’s phases. Instead of counting days based on how long it takes the Earth to go around the Sun, like we do with our regular calendar, a lunar calendar follows the Moon. Each month in a lunar calendar matches the time it takes the Moon to go through its different shapes, called phases.

Because the Moon’s cycle is about 29 and a half days, lunar calendar months switch between 29 and 30 days. However, twelve of these Moon-based months only add up to about 354 days, which is about 11 or 12 days shorter than a full year based on the Sun. This means that over time, the months of a lunar calendar will move through all the seasons.

The most well-known lunar calendar is the Islamic calendar. Different lunar calendars may start their months at different points in the Moon’s cycle, such as the new Moon, the full Moon, or when we first see a thin slice of the Moon, called a crescent. The idea of a lunar new year marks the beginning of this special calendar and is also used in many cultures around the world.

History

Some scholars believe that ancient hunters watched the Moon and made notes about it during the Upper Palaeolithic time. One person named Samuel L. Macey thinks people began using the Moon to measure time as early as 28,000 to 30,000 years ago.

Start of the lunar month

Lunar and lunisolar calendars have different ways to decide the first day of the month. For example, the Hijri calendar, used by most of Islam, starts with the first sighting of the lunar crescent. In other calendars, like the Hebrew calendar and Chinese calendar, the month begins on the day of the astronomical new moon in a specific area. Some Hindu calendars start each month on the day after the full moon.

Length of the lunar month

The time it takes for the Moon to go through its phases changes a little bit each month. Also, seeing the Moon depends on the weather. In Sunni Islam, a new month starts when people see the first thin part of the Moon, called Hilal.

To make things easier, some people made special math rules to guess when each month will start. One well-known way is the Tabular Islamic calendar. This calendar uses a pattern that repeats every 30 years. In some years, the month is a little longer, and in others, it is a little shorter. This helps guess when the first part of the Moon will appear, which decides the first day of each month in the Islamic lunar calendar.

List of lunar calendars

Some cultures use special calendars based on the Moon's phases. Two examples are the Islamic Hijri calendar and the Javanese calendar. These calendars track the Moon instead of the Sun to decide when months begin.

Lunisolar calendars

Main article: Lunisolar calendar

Many calendars called "lunar" are actually lunisolar calendars. Their months follow the Moon's cycles, but they add extra months sometimes to match the solar year. Ancient Egypt used a solar calendar that still kept some lunar traditions for religious and farming reasons. Today, several cultures use lunisolar calendars, including Chinese, Korean, Vietnamese, Hindu, Hebrew, and Thai calendars.

One common way to keep these calendars in sync is by adding an extra month every few years. Some lunisolar calendars also match natural events that change with both the Moon and the Sun. For example, the calendar of the Banks Islands includes months when special palolo worms come to the beaches, which happens at a certain point in the lunar month.