In physics, an orbit is the gravitationally curved trajectory of an object.[1] Orbits are observed by the trajectory of a planet around a star or a natural satellite around a planet. Normally, orbits of this nature are repeating trajectories. To a close approximation, planets and satellites follow elliptic orbits, with the central mass being orbited at a focal point of the ellipse,[2] as described by Kepler's laws of planetary motion.


1024px-Apogee (PSF)

The apsides indicate the nearest and farthest points of an orbiting body around its host. (1) farthest (3) focus (2) nearest apocenter primary pericenter aphelion Sun perihelion apastron star periastron apogee Earth perigee

According to Kepler's first law of planetary motion, all planets, comets, and asteroids in the Solar System have approximately elliptical orbits around the Sun.[3] It is only approximate because of perturbations due to the gravity of other bodies. The Orbital eccentricity measures the flatness (departure from a perfect circle) of the orbit. Since every ellipse has two focus points, an orbiting body has a closest and a farthest point from its parent object, that is known as an apsis.

The apsis of the Sun are called perihelion and aphelion. The perihelion of any orbit of a celestial body orbiting the Sun is the point where the body comes nearest to the Sun. It is the opposite of aphelion, which is the point in the orbit where the celestial body is farthest from the Sun.[4]

  • The apsis of orbits around the Earth are called: perigee and apogee.
  • The apsis of orbits around the Sun are called: perihelion and aphelion.
  • The apsis of orbits around a star are called: periastron and apastron.
  • The apsis of orbits around any center of mass is called: periapsis (or pericenter) and apoapsis (or apocenter).


  1. orbit (astronomy) – Britannica Online Encyclopedia
  2. The Space Place :: What's a Barycenter
  3. "Introductory Astronomy: Ellipses". Washington State University. 
  4. Walker, Peter, ed (1988) (in English) (Hardback). Cambridge Dictionary of Science and Technology. Cambridge, England: Cambridge University Press. pp. 44, 660. ISBN 0-521-39441-4.