In astrodynamics apse line (or line of apsides) is a common name for an imaginary line defined by orbit's eccentricity vector. It is strictly defined for elliptic, parabolic and hyperbolic orbits. For those orbits it can be imagined as a line:

• between orbit's periapsis and apoapsis _ for circular orbits
• between orbit's periapsis and focus - for parabolic and hyperbolic orbits

Note: For circular orbits apse line is not defined as eccentricity is equal zero. As it is required as a base for definition of true anomaly it is usually arbitrarily assumed (usually as a line pointing into direction of vernal equinox)

See also

• Apsis
• Orbit: circular, elliptic, parabolic and hyperbolic
• Eccentricity (orbit) and eccentricity vector
• True anomaly