Perihelion and aphelion

The perihelion and aphelion are the nearest and farthest points respectively (apsides) of a body's direct orbit around the Sun.

The perihelion (n/) of any orbit of a celestial body about the Sun is the point where the body comes closest to the Sun. It is the opposite of aphelion (n/), which is the point in the orbit where the celestial body is farthest from the Sun.[1]

Etymology

The words perihelion and aphelion were coined by Johannes Kepler[2] to describe the orbital motion of the planets. The words are formed from the prefixes peri- (Greek: περί, near) and apo- (Greek: ἀπό, away from) affixed to the Greek word for the sun, ἥλιος.[3]

Perihelion and aphelion are sometimes incorrectly used for the orbits of objects about bodies other than the Sun. The correct terms are:

  • For orbits around the Earth: perigee and apogee.
  • For orbits around a star: periastron and apastron.
  • For orbits around any center of mass: periapsis (or pericenter) and apoapsis (or apocenter).