The Lunar Cycle: How Long Is a Moon Cycle?
One complete lunar cycle — New Moon back to New Moon — takes about 29.5 days. Scrub the animation above to travel through a full cycle and watch the light grow and fade. That period has a name: the synodic month.
Two kinds of month
The Moon actually orbits Earth in 27.3 days — the sidereal month. But because Earth is also moving around the Sun, the Moon needs about 2.2 extra days to return to the same phase. That phase-to-phase period, 29.53 days, is the synodic month and is what we experience as the lunar cycle.
Why calendars and the Moon drift apart
Twelve synodic months total about 354 days — roughly 11 days short of a solar year. That mismatch is why purely lunar calendars shift against the seasons, and why lunisolar calendars add a leap month to catch up.
Moon age within the cycle
Astronomers track where we are in the cycle as the Moon's age in days: 0 at New Moon, about 7.4 at First Quarter, 14.8 at Full Moon, and 22.1 at Last Quarter, completing near 29.5.
Frequently Asked
How long is a lunar cycle?
About 29.5 days — precisely 29.53 days — measured from one New Moon to the next. This is called the synodic month.
Why is the synodic month longer than the Moon's orbit?
The Moon orbits Earth in 27.3 days, but Earth moves around the Sun in that time too, so the Moon needs about 2.2 extra days to line back up into the same phase.
How many moon cycles are in a year?
About 12.4. That is why some years have 13 Full Moons and occasionally two in one month — a Blue Moon.
What is the moon's age?
The number of days since the last New Moon: 0 at New, roughly 14.8 at Full, and about 29.5 when the cycle completes.