Orbital period

For celestial objects in general, the orbital period is determined by a 360° revolution of one body around its primary, e.g. Earth around the Sun.

Periods in astronomy are expressed in units of time, usually hours, days, or years.

For a perfect sphere of uniform density, it is possible to rewrite the first equation without measuring the mass as: where: For instance, a small body in circular orbit 10.5 cm above the surface of a sphere of tungsten half a metre in radius would travel at slightly more than 1 mm/s, completing an orbit every hour.

For the case of the Earth orbiting around the Sun, this period is referred to as the sidereal year.

Examples of some of the common orbital ones include the following: Periods can be also defined under different specific astronomical definitions that are mostly caused by the small complex external gravitational influences of other celestial objects.

An example of this related period description is the repeated cycles for celestial bodies as observed from the Earth's surface, the synodic period, applying to the elapsed time where planets return to the same kind of phenomenon or location — for example, when any planet returns between its consecutive observed conjunctions with or oppositions to the Sun.

[citation needed] The following table lists the synodic periods of some planets relative to each the Sun and each other:[original research?