That point in the orbit of a double star where the smaller star is farthest from its primary.
For an orbit around a star, the farthest point from that star.
means the greatest separation of the principals (q. v.) in a binary. It is a homologue of apogee for an Earth satellite, and aphelion for a planet. The term apocentron is used elsewhere in place of apastron to describe the farthest point on an orbit.