# Orbital period

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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 around the Sun (or other celestial objects):

• The sidereal period is the temporal cycle 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.
• The synodic period is the temporal interval that it takes for an object to reappear at the same point in relation to two other objects (linear nodes), e.g., when the Moon relative to the Sun as observed from Earth returns to the same illumination phase. The synodic period is the time that elapses between two successive conjunctions with the Sun-Earth line in the same linear order. The synodic period differs from the sidereal period due to the Earth's orbiting around the Sun.
• The draconitic period, or draconic 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 node is a coinciding of planes rather than a linear coinciding, and the object's line of nodes typically precesses or recesses slowly in relation to orbital cycle.
• The anomalistic period is the time that elapses between two passages of an object at its periapsis (in the case of the planets in the solar system, called the perihelion), 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.
• Also, the Earth's tropical period (or simply its "year") is the time that elapses between two alignments of its axis of rotation with the Sun, also viewed as two passages of the object at right ascension zero. One Earth year has a slightly shorter interval than the solar orbit (sidereal period) because the inclined axis and equatorial plane slowly precesses (rotates in sidereal terms), realigning before orbit completes with an interval equal to the inverse of the precession cycle (about 25,770 years).

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### Relation between the sidereal and synodic periods

Copernicus devised a mathematical formula to calculate a planet's sidereal period from its synodic period.