In the classical fixed-Sun approximation for a planet’s elliptical orbit, where is the Sun relative to the ellipse?

Answer & explanation

At one focus, generally off-center.

In the classical fixed-Sun approximation, a planet’s bound orbit is an ellipse with the Sun at one focus. A focus is generally distinct from the ellipse’s center, though the foci coincide with the center in the circular case. In the full two-body description, both bodies move around their shared barycenter, so this classical formulation is an idealization.