Sidereal Period

Definition

The sidereal period is the amount of time it takes for one body to revolve around another, measured with respect to a distant “fixed” star. It is also used to describe rotation or other periodic astronomical phenomena. In all cases this is considered in an inertial (non-rotating) frame of reference.

Theory and Application: Sidereal Versus Synodic Period

For Earth’s Moon, the sidereal period is simply the amount of time that elapses between two consecutive conjunctions between the Moon and an individual “fixed” star (other than the Sun, fixed is in quotes because stars do move, but they are far enough away that the motion is not evident over the period of the phenomenon measured). This star provides a reference mark against which we can conveniently time one revolution of the Moon around Earth. This period is known to be 27.32 days. The synodic period is considerably longer, at 29.53 days, and it is the length of time between two consecutive occurrences of a syzygy or...