Apse line

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.

For such orbits the apse line is found:

For circular orbits apse line is not defined, because 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).