lunar year

A lunar year is a period of 12 lunar months, which are based on the cycles of the Moon's phases. Because there are about 12.37 lunar months in a solar year, a lunar calendar must periodically add days or even an entire month (intercalation) to stay aligned with the seasons.12

The Sumerians were among the first to use a lunar calendar, with each month beginning on the first day the new Moon was visible. Later, the Metonic cycle, developed by Babylonian astronomers and adopted by the Greeks, established a pattern of adding seven intercalary months over 19 years to keep the lunar and solar years synchronized.2

Today, some religious groups still use lunar or lunisolar calendars, such as the Jewish calendar and the Islamic calendar. Lunisolar calendars, like the Jewish one, track both the Moon's phases for months and the Sun's position for the year, ensuring festivals occur in their proper seasons.234