Anapse line, orline of apsides, is an imaginary line defined by anorbit'seccentricity vector. It is strictly defined forelliptic,parabolic, andhyperbolicorbits.

For such orbits the apse line is found:^[1]

• for elliptical orbits – between the orbit'speriapsis andapoapsis (also known as themajor axis)
• for parabolic and hyperbolic orbits – between the orbit'speriapsis andfocus

Forcircular orbits, the apse line is not defined because theeccentricity is equal to zero. As it is required as a base for the definition oftrue anomaly, it is usually arbitrarily assumed (as a line pointing into the direction of the vernalequinox).

• Apsidal precession
• Apsis
• Eccentricity (orbit)
• Orbit:circular,elliptic,parabolic andhyperbolic
• True anomaly

• Orbits