Depending on what you measure from, a month is 27.32 days, 29.53 days, 27.55 days, 27.21 days or 27.32 days again — and all five are in use.

The sidereal month returns the Moon to the same star; the synodic month returns it to the same phase and is longer because the Earth has moved on round the Sun. The anomalistic month returns it to perigee, the draconic month to the same node, and the tropical month to the same point in the seasons. Eclipses, supermoons and calendars each depend on a different one of them.