A periodic comet is a comet which orbits around the Sun. A short-period comet will complete its orbit in less than 200 years. A long-period comet will take more than 200 years to orbit. In most cases the comets are named after the person who discovered them. The Shoemaker-Levy comets were named after Eugene Shoemaker, Carolyn Shoemaker and David Levy. A few comets are named after the person who calculated the orbits, such as 2P/Encke and 27P/Crommelin. It is very difficult to work out the orbit of a long-period comet because the orbit changes. The gravity of planets, or the release of gas or material from the comet itself, changes the orbit. Short-period comets can be predicted very accurately.