Orbit
An orbit is an object's rotation around another. Orbits are caused by the gravity of a large mass attracting a smaller mass. The smaller mass is prevented from falling into the large mass by a large velocity perpendicular to the object it is orbiting around. The concept of orbiting in the solar system has changed drastically over time. Scientists assumed that the orbits of planets observable in Kepler's times were 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 mass the focus of its orbit (its barycenter) is actually closer to the Earth's surface than it is to the exact center of the Earth.