Barry Schwarz wrote:

But we know the orbit is elliptical with the Sun at one focus.

Close enough for the purposes at hand, but it's worth pointing out
to the original poster that, more properly, it's not the Sun which
resides at the focus of the ellipse. At the focus, in fact, is the mass
center of the Earth/Sun system, also called the barycenter. Of course,
Earth contributes such a negligible portion of the total mass that the
barycenter is practically at the Sun's own mass center.

-Mark Martin