line of apsides

in astronomy, the line segment that joins the apsides, that is, the two points of the elliptic orbit of a celestial body: the point closest to and the point most distant from the central body. These points lie at the ends of the major axis of an ellipse and thus the axis is the line of apsides. The line of apsides for the orbits of the planets of the solar system is bounded by the perihelion and aphelion, for the orbits of the moon and artificial earth satellites by the perigee and apogee, and for the orbits of binary stars by the periastron and apastron.