Orbital period : Synodic period

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

When regarding planets, there are two main types of orbital periods.

• The sidereal period is the time that it takes an object to make one full orbit, relative to the stars. This is considered to be an object's true orbital period. (See also: sidereal year)
• The synodic period is the time that it takes for an object to make one full orbit, relative to the sun (as observed from Earth). This is the time which elapses between two successive conjuctions and is the object's apparent orbital period. (See also: synodic month)

History of the Orbital Period

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

P = planet's sidereal period in years

S = planet's synodic period in years

E = the sidereal period of Earth in years

• Earth moves around it's orbit at 360°/E per day.
• A planet moves around it's orbit at 360°/P per day.
  • During a time, S, the Earth moves (360°/E)S and the planet moves (360°/P)S

Let us consider the case of an inferior planet.

• The inferior planet will complete one full orbit before the Earth has completed it's orbit.
  • (360°)/P)S = (360°/E)S + 360°

Using algebra...

1/P = 1/E + 1/S

For a superior planet we would obtain:

1/P = 1/E - 1/S

For example:

Jupiter has a synodic period of 1.092 years but a sidereal period of 11.87 years. Thus, every 1.092 years the Earth orbits 360°+ and "laps" Jupiter.

Related topics

• sidereal time
• synodic month