Main Page | See live article | Alphabetical index

Orbital period

The orbital period is the time it takes a planet (or another object) to make one full orbit.

There are two main kinds of orbital periods of objects orbiting the sun:

Relation between sidereal and synodic period

Copernicus devised a mathematical formula to relate a planet's sidereal period with its synodic period.

Using the abbreviations

P = planet's sidereal period
S = planet's synodic period
E = the sidereal period of Earth

During the time S, the Earth moves over an angle of (360°/E)S (assuming a circular orbit) and the planet moves (360°/P)S.

Let us consider the case of an inferior planet, i.e. a planet that will complete one orbit more than Earth before the two return toe the same position relative to the sun.

(360°)/P)S = (360°/E)S + 360°

and using algebra we obtain

1/P = 1/E + 1/S

For a superior planet one derives likewise:

1/P = 1/E - 1/S

Related topics