What is defined as a sidereal month?

A sidereal month refers to the time it takes the Moon to complete one full orbit of 360 degrees around Earth relative to the fixed stars. This period is about 27.3 days. While the Moon’s phases, such as the time between two new moons, have their own durations, they are not what defines the sidereal month.

Understanding this concept is crucial as it distinguishes between the Moon's orbital motion and other lunar-related phenomena like eclipses or the lunar phases, which are based on the relative positions of the Sun, Earth, and Moon. By focusing solely on the Moon's orbit, the definition points to the Moon's position relative to distant stars after completing its circuit around Earth.