Sidereal Versus Synodic Month

The Moon revolves around the Earth, undergoing a series of phases of illumination, from no illumination, to partial illumination, to full illumination, to partial illumination, to no illumination.The time need to this repeating series of illuminations (a lunation) is the traditional month, also known as a synodic month.A synodic month, measured from say New Moon to the following New Moon, is approximately 29.5 days.The actual time needed for the Moon to revolve around the Earth, as measured with reference to the fixed stars, is less: only about 27.3 days. This time period is called a sidereal month (sidereal means "of the stars").This animation shows clearly why there is a two days difference between the synodic and sidereal months. The Moon must revolve more than one revolution around the Earth to get lined up with the Sun again (New Moon to New Moon), because the Earth has advanced significantly in its orbit over a sidereal month's time.