Question

Why is sun at foci in the solar system?

Answer 1

Actually, both the Sun and the planets move around each other with their center of mass lying at the focus of the elliptical orbits. However, since the Sun contains 99.9% of the mass of the solar system, the center of mass is located almost at the Sun and so it looks like the planets are going to the Sun. So the sun is at foci in the solar system.