What is the form of a planetary orbit? The astronomers from Ptolemy to Copernicus had a clear (but wrong) answer to this question: A planet moves on a circle or at least on an orbit which can be explained by superposition of circular motions. It was Johannes Kepler who finished with this wrong idea in 1609. After he had analysed the large and precise observational data of Tycho Brahe, he found that the planets orbit on ellipses. The points of an ellipse are characterized by the property that the sum of their distances to the so-called foci is constant. Kepler's first law of undisturbed planetary motion: The orbit of each planet is an ellipse and the Sun is at one focus.
 The following HTML5 app illustrates this law.