Types of orbital period
"Completion of an orbit" has many definitions. In consequence, many definitions of orbital period are known to astronomers.
- The sidereal period is the time required for a body to make one circuit around its primary with respect to the stars. It is the truest definition of an orbital period. In fact it is what is meant by "orbital period" whenever one uses the latter term without qualification.
- The synodic period is the time required for a body to reach the same position in the sky of earth, in relation to the Sun. Hence for any body except the earth itself, sidereal and synodic years are quite different, for the simple reason that the earth has an orbit of its own. Planets and other bodies beyond the earth have a synodic year longer than the earth year, but as an object's semi-major axis lengthens, its synodic year approaches the earth year.
- The draconitic period is the time between two passages of the object through the ascending node of its orbit (the point at which the satellite passes from the south to the north of its primary's equatorial plane).
- The anomalistic period is the time between two passages through periapsis, the point of the object's closest approach to its primary.
- The tropical period is the time between two passages of the object through a right ascension of zero.