Why do the planets orbit the sun in ellipses rather than circles, as had long been thought?

Question

Why do the planets orbit the sun in ellipses rather than circles, as had long been thought?

Answer 1

A circular orbit is a special case of an elliptical one (well I guess all orbits are conic sections when the central force is an inverse square type so the elliptical orbit is the most general bound orbit.)  It takes a special arrangement of conditions to create and maintain a circular orbit.  All the other influences are acting to pull it away from truly circular, even if only by a small amount.