"Parabola, ellipse, hyperbola, and circle are called conic sections, or simply conics, because they can all be obtained as plane sections of a circular cone. They were originally studied from that point of view, and nearly all their elementary properties that are known to-day were proved by the Greek geometers more than two thousand years ago. The conic sections are of especial interest because of the fact that the paths of all the heavenly bodies are curves of this kind. This fact was first established by the great German astronomer and mathematician Johannes Kepler, in 1609. He showed that the planet Mars moves in an ellipse; the other planets, including of course the earth, do the same, while many comets move in parabolas or hyperbolas."