month

Unit of time based on the motion of the Moon around the Earth.

The time taken for the Moon to orbit the Earth (from one new or full Moon to the next) is known as a synodic or lunar month and is 29.53 days. The Western calendar is not based on the lunar month, but rather on the calendar month, a human invention devised to fit the calendar year. However, the Islamic calendar is based on the lunar month, and therefore each year a fixed date goes back by about ten days.

The time for the Moon to complete one orbit around the Earth relative to the stars (the sidereal month) is 27.32 days. The solar month equals 30.44 days, and is exactly one-twelfth of the solar or tropical year, the time taken for the Earth to orbit the Sun.