Editing Apse line

[[File:Apogee (PSF) mul.svg|lang=zxx|thumb|230px|The line of apsides of an ellipse connects point 2 and 3 ([[major axis]]).]]

An '''apse line''', or '''line of apsides''', is an imaginary line defined by an [[orbit]]'s [[eccentricity vector]]. It is strictly defined for [[elliptic orbit|elliptic]], [[parabolic trajectory|parabolic]], and [[hyperbolic trajectory|hyperbolic]] [[orbit]]s.

For such orbits the apse line is found:<ref>{{cite web |url=http://www.britannica.com/science/line-of-apsides |title=Line of apsides – Astronomy |website=[[Encyclopædia Britannica]] |accessdate=23 August 2015}}</ref>
* for elliptical orbits{{spaced ndash}}between the orbit's [[periapsis]] and [[apoapsis]] (also known as the [[major axis]]) 
* for parabolic and hyperbolic orbits{{spaced ndash}}between the orbit's [[periapsis]] and [[Focus (geometry)|focus]]

For [[circular orbit]]s, the apse line is not defined because the [[Orbital eccentricity|eccentricity]] is equal to zero. As it is required as a base for the definition of [[true anomaly]], it is usually arbitrarily assumed (as a line pointing into the direction of the vernal [[equinox]]).

== See also ==
* [[Apsidal precession]]
* [[Apsis]]
* [[Eccentricity (orbit)]]
* [[Orbit]]: [[Circular orbit|circular]], [[elliptic orbit|elliptic]], [[parabolic trajectory|parabolic]] and [[hyperbolic trajectory|hyperbolic]]
* [[True anomaly]]

==References==
{{Reflist}}

{{DEFAULTSORT:Apse Line}}
[[Category:Orbits]]