 Orbital Period

The orbital period is the time it takes a planet to make one full orbit around the Sun.There are several kinds of orbital periods for objects around the Sun:

• The sidereal period is the time that it takes the object to make one full orbit around the Sun, relative to the stars. This is considered to be an object's true orbital period.

• The synodic period is the time that it takes for the object to reappear at the same spot in the sky, relative to the Sun, as observed from Earth. This is the time that elapses between two successive conjunctions with the Sun and is the object's Earth-apparent orbital period. The synodic period differs from the sidereal period since Earth itself revolves around the Sun.

• The draconitic period is the time that elapses between two passages of the object at its ascending node, the point of its orbit where it crosses the ecliptic from the southern to the northern hemisphere. It differs from the sidereal period because the object's line of nodes typically precesses or recesses slowly.

• The anomalistic period is the time that elapses between two passages of the object at its perihelion, the point of its closest approach to the Sun.

• The tropical period, is the time that elapses between two passages of the object at right ascension zero. It is slightly shorter than the sidereal period because the vernal point precesses.

The time taken for a body to go once around a closed orbit. The orbital period of a planet is its "year."

For a body in an elliptical orbit of semimajor axis a about a much more massive body of radius R the orbital period P is given by

P = 2pa(3/2)/SQRT(gR²).

This is the mathematical formulation of the third of Kepler's laws of planetary motion.