Synodic month
What is a synodic month?
A synodic month is one full cycle of Moon phases, lasting about 29.5 days. It is the time from one full moon to the next, or from one new moon to the next.
Try the question
Why is the phase month longer than one orbit?
Stop first at one star-relative orbit, then continue until the Moon catches the Sun’s shifted direction.
Watch for: Back to the same stars is not yet back to the same phase.
Two ways to count a month
Start with a full moon. Wait until the next full moon. You have counted one cycle of phases: about 29.5 days.
Now compare the Moon’s path with the distant stars. One trip takes about 27.3 days. This is called a sidereal month.
Why are the times different?
Earth moves around the Sun while the Moon goes around Earth. After one orbit, the Moon needs a little more time to reach the same angle with the Sun. See how that angle changes the phase.
Get more on this
What about a calendar month?
Calendar months last 28 to 31 days. They do not stay in step with Moon phases. Moonrise also changes from day to day, but the amount varies by date and place. Explore changing moonrise times.
See it live
On the hub: Sun–Earth–Moon simulator · Moon