In the solar system, the Sun is always at one focus of each planet’s elliptical orbit.
Elliptical orbits and foci
Kepler’s First Law states that each planet moves in an elliptical orbit with the Sun at one of the foci of that ellipse. An ellipse has two internal points called foci, and for planetary orbits the Sun’s center occupies one of these points.
Subtle refinement: center of mass
More precisely, orbital mechanics place the center of mass (barycenter) of the system at one focus of the ellipse. In the solar system this barycenter lies very near the Sun’s center, so it is usually stated that “the Sun is at one focus” of every planet’s orbit.
