· · · ·


Apoapse, Apocentre

A general term used to describe a point in the orbit of a body around a centre of gravity (usually another body, designated the primary). Specifically, the apoapsis is the point in an orbit where a body reaches its greatest possible distance in its orbit. The word combines the Greek prefix apo-, 'away', and the word apsis, 'orbit' (with the plural being apsides). The terms apoapse or apocentre are sometimes used, each the same meaning. The opposite of the apoapsis (that is, the closest point of orbital approach) is known as the periapsis.

The degree of orbital eccentricity, or the elongation of the orbital ellipse, will dictate how much variation there is between the apoapsis and periapsis. For a body with a near circular orbit, there will be little difference between the two. For example, Earth's orbit around the Sun has a very low eccentricity (about 0.02) and so there is very little variation between its apopapsis (varying around 152,100,000 km) and its periapsis (varying around 147,100,000 km). For bodies with more eccentric orbits, such as comets or certain dwarf planets, a very much greater difference can be seen. For example, the dwarf planet Sedna follows a highly eccentric orbit around the Sun that can carry it as far as 936 AU away from its parent star (that is, about 140,000,000,000 km or roughly 1% of a light year).

Because both Earth and Sedna both orbit the Sun, a specialised term is generally used in place of apoapsis. Instead, in specific reference to orbits around the Sun, the term aphelion is usually preferred (the meaning is the same, except that -helion specifcally refers to the Sun). In a similar way, orbits around the Earth have an apogee at their most distant point. There are several other variant terms, such as apastron for an orbit around any star, or apogalacticon for an orbit through or around a galaxy.


Related Entries