What is a synodic month?

A synodic month refers specifically to the time it takes for the Moon to complete a full cycle of phases, from one new moon to the next new moon. This period is approximately 29.5 days. The reason this duration is slightly longer than the lunar month, which is 27.3 days (the time it takes for the Moon to orbit Earth once in relation to the stars), is due to the movement of both the Earth and the Moon around the Sun. As the Earth travels in its orbit, the Moon must move a bit further to catch up and return to the same phase, resulting in the longer synodic month cycle.

Understanding the distinction between the synodic month and other related concepts, such as the sidereal month (the time for the Moon to orbit Earth relative to distant stars), helps clarify why it is recognized as approximately 29.5 days. This measurement is crucial for various aspects of both astronomy and calendar systems, reflecting how we track time based on lunar phases.