Johannes Kepler was a German mathematician, astronomer, and astrologer.

To provide the foundation for his new approach to astronomy, Kepler adopted the simplest geometrical structure consistent with observations.
He felt justified in assuming that a single, closed (repeating) planar curve would represent the path of the planet; this assumption implied a process these days called idealization (neglect of the presence of other bodies in the universe, as we now know). Such a structural simplification allowed Kepler to examine the orbit of each individual planet in isolation, because all mutual interactions between planets had been eliminated. And because it was expressed geometrically, the solution would potentially be exact - the closed orbit of a single planet in a plane round the fixed Sun.