An apse line, or line of apsides, is an imaginary line defined by an orbit's eccentricity vector. It is strictly defined for elliptic, parabolic, and hyperbolic orbits.
![](http://178.128.105.246/cars-http-upload.wikimedia.org/wikipedia/commons/thumb/9/9e/Apogee_%28PSF%29_mul.svg/langzxx-230px-Apogee_%28PSF%29_mul.svg.png)
For such orbits the apse line is found:[1]
- for elliptical orbits – between the orbit's periapsis and apoapsis (also known as the major axis)
- for parabolic and hyperbolic orbits – between the orbit's periapsis and focus
For circular orbits, the apse line is not defined because the 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
editReferences
edit- ^ "Line of apsides – Astronomy". Encyclopædia Britannica. Retrieved 23 August 2015.