All planets revolve around stars. In the Solar System, all the planets orbit in sync with the Sun's rotation. The period of one revolution of a planet's orbit is known as its sidereal period or year. A planet's year depends on its distance from its star; the farther a planet is from its star, not only the longer the distance it must travel, but also the slower its speed, as it is less affected by the star's gravity. Because no planet's orbit is perfectly circular, the distance of each varies over the course of its year. Its closest distance to is called its (perihelion in the Solar System), while its farthest distance from the star is called its (aphelion in the Solar System). As a planet approaches perihelion, its speed increases as the pull of its star's gravity strengthens; as it reaches aphelion, its speed decreases.
Each planet's orbit is delineated by a set of elements:
The eccentricity of an orbit describes how elongated a planet's orbit is. Planets with low eccentricities have more circular orbits, while planets with a high eccentricities have more elliptical orbits. The planets in our Solar System have very low eccentricities, and are circular orbits. Comets and Kuiper belt objects (as well as several extrasolar planets) have very high eccentricities, and exceedingly elliptical orbits.
The semi-major axis is the distance from a planet to the half-way point along the longest diameter of its elliptical orbit. This distance is not necessarily the same as its apasteron, as no planet's orbit has its star at its exact centre.
In our Solar System, the inclination of a planet tells how far above or below the plane of Earth's orbit (called the ecliptic) a planet's orbit lies. The eight planets of our Solar System all lie close to the ecliptic; comets and Kuiper belt objects like Pluto are at far more extreme angles to it. The points at which a planet crosses above and below the ecliptic are called its ascending and descending nodes. Other orbital elements used to describe the orientation of a planet's orbit within our Solar System include the argument of periapsis and longitude of the ascending node.
