The siderial month is the time it takes for the Moon to complete one full orbit of the Earth, measured with respect to the distant stars. The major assumption in determing a sidereal month is that the distant stars are fixed relative to Earth, and for the most part they are stationary. The sidereal month is the Moon's true orbital period and is equal to 27.3 days. That is, it takes the Moon 27.3 days to be in the same position relative to the distant stars.

The synodic month is the time it takes for the Moon to complete one cycle of phases. For example, it is the time it takes the Moon to complete one full cycle of phases. That is, the time between successive new moons. Therefore the synodic month is measured with respect to the Sun and is approximately 29.5 days.

Why is there a difference between the sidereal and synodic periods? Well, the Earth keeps orbiting the Sun while the Moon is going through its phases. Thus, to go from one new moon to the next the Moon must travel more than 360o along its orbit. The synodic month is therefore approximately two days longer than the sidereal month.