aphelion

Aphelion is the point in the orbit of an astronomical object around the Sun at which the object is farthest from the Sun. For bodies in bounded orbits, such as planets, asteroids, and comets, the orbit is approximately an ellipse with the Sun at one focus; the farthest point along the orbit is called aphelion, the opposite point being perihelion.

In an elliptical orbit, with semi-major axis a and eccentricity e, the distance at aphelion is a(1+e) and at perihelion is a(1−e). If the orbit is circular (e = 0), the aphelion and perihelion distances are the same as the orbital radius. The orbital speed is greatest at perihelion and least at aphelion, in accordance with Kepler’s second law.

Earth provides a notable example. Its orbital semi-major axis is about 1 astronomical unit (AU), and its eccentricity is about 0.0167, yielding a perihelion near 0.983 AU (roughly 147 million kilometers) in early January and an aphelion near 1.017 AU (about 152 million kilometers) in early July. Other bodies with larger eccentricities, such as Pluto, have more distant aphelia and perihelia on the order of tens of AU.

The term aphelion is used for orbits around the Sun; there is no finite aphelion distance for hyperbolic or parabolic trajectories. The word is derived from Greek, meaning “away from the Sun.”