# Orbital Period

by on October 26, 2010

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The orbits of planets in the Gliese 581 system are compared to those of our own solar system. The Gliese 581 star has about 30 percent the mass of our Sun, and the outermost planet is closer to its star than the Earth is to the Sun. The 4th planet, G, is a planet that could sustain life. Credit: Zina Deretsky, National Science Foundation

The orbital period is the amount of time that an object takes to make one trip around another object. In most instances it refers to the sidereal period of an abject as it relates to a star or stars. There are several different orbital periods and each is explained in this article.

The sidereal period is the temporal cycle that it takes an object to make one full orbit, relative to the stars ad is considered an object’s orbital period.

The synodic period is the amount of time that it takes an object to reappear at the same point, usually in relation to two other objects. A good example is the Moon and Sun being observed from Earth. The synodic period for the Sun/Moon/Earth relationship is the time that elapses between two conjunctions in a row where the Sun-Earth line is in the same linear order.

The draconitic period is the time that elapses between two passages of the object at its ascending node(the point of an orbit where an object 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.

The anomalistic period is the time that elapses between two passages of an object at its periapsis or perihelion(the point of its closest approach to the attracting body, the Sun in the case of the planets in our Solar System).

Many publications refer to different orbital periods. It is important for an amateur astronomer to become familiar with each type to be able to aptly distinguish each within a conversation or written piece.

We have written many articles about orbital period for Universe Today. Here’s an article about the orbit of comets, and here’s an article about planetary orbits.

If you’d like more info on the orbit of planets, check out NASA’s Solar System exploration page, and here’s a link to NASA’s Solar System Simulator.

We’ve also recorded a series of episodes of Astronomy Cast about every planet in the Solar System. Start here, Episode 49: Mercury.

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