Figure 1-22The Sidereal and Synodic Months The sidereal month is the time it takes the Moon to complete one revolution with respect to the background stars, about 27.3 days. However, because Earth is constantly moving in its orbit about the Sun, the Moon must travel through more than 360° to get from one new Moon to the next. The synodic month is the time between consecutive new Moons or consecutive full Moons, about 29½ days.