Earth's orbit

Whether or not the path of a planet around the sun is elliptical (apart from some slight deviations due to other planets) depends on its total energy and the angular momentum which were fixed in the early stage of formation of the solar system. It can be derived using the Newton's universal gravitation law. However, it was originally mentioned by Kepler who deduced it from the huge amount of data on planetary motion gathered mostly by Tycho Brahe (around 20 years of sky-watching) and partly by himself. This fact is usually stated as the Kepler's first law of planetary motion.