focus, orbital

Orbital focus.

An orbital focus is one of two points on the major axis of an ellipse whose separation from the other focus determines the shape of an elliptical orbit. At one of the foci is the body being orbited; for example, the Sun lies at one of the foci of Earth's orbit. The other focus is unoccupied.


If the orbit is parabolic or hyperbolic, there is only one focus and it is occupied by the orbited body.