For such orbits the apse line is found:
- 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).
- Apsidal precession
- Eccentricity (orbit)
- Orbit: circular, elliptic, parabolic and hyperbolic
- True anomaly