Orbital period

The orbital period is the time taken for a given object to make one complete orbit about another object.

When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.

There are several kinds of orbital periods for objects:

• The sidereal period is the temporal cycle that it takes an object to make a full orbit, relative to the stars. This is the orbital period in an inertial (non-rotating) frame of reference.
• The synodic period is the temporal interval that it takes for an object to reappear at the same point in relation to two or more other objects.
• The draconitic period, or draconic period, is the time that elapses between two passages of the object through its ascending node, the point of its orbit where it crosses the ecliptic from the southern to the northern hemisphere. This period differs from the sidereal period because both the orbital plane of the object and the plane of the ecliptic precess with respect to the fixed stars, so their intersection, the line of nodes, also precesses with respect to the fixed stars.
• The anomalistic period is the time that elapses between two passages of an object at its periapsis, the point of its closest approach to the attracting body. It differs from the sidereal period because the object's semimajor axis typically advances slowly.

Binary stars

Binary star	Orbital period
AM Canum Venaticorum	17.146 minutes
Beta Lyrae AB	12.9075 days
Alpha Centauri AB	79.91 years
Proxima Centauri - Alpha Centauri AB	500,000 years or more

See also

• Geosynchronous orbit derivation
• Sidereal time
• Sidereal year
• Opposition