Orbit

Scientists assumed that the orbits of planets observable in Kepler's times were very circular. Kepler was the first to propose that the planets' orbits were elliptical, thus improving the accuracy and prediction-value of the heliocentric view of the solar system. Comets, on the other hand, have highly eccentric orbits with only one focal point inside the Sun.

The orbit of the Moon is nearly a perfect circle, but due to its high relative weight the center of its orbit is actually closer to the Earth's surface than it is to the exact center of the Earth.