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.

## ApsisEdit

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).

## ReferencesEdit

- ↑ orbit (astronomy) – Britannica Online Encyclopedia
- ↑ The Space Place :: What's a Barycenter
- ↑ "Introductory Astronomy: Ellipses". Washington State University. http://astro.wsu.edu/worthey/astro/html/lec-ellipse.html.
- ↑ 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.