27 December

Kepler was a German astronomer who became Tycho Brahe's assistant.

Unlike Brahe, Kepler believed firmly in the Copernican system. Copernicus had correctly placed the Sun at the centre of the Solar System, but had assumed the orbits of the planets to be circles. Kepler provided the final piece of the puzzle: after a long struggle, in which he tried to avoid his eventual conclusion, Kepler was forced to accept that the orbits of the planets were not the circles demanded by Aristotle and assumed by Copernicus, but were instead the 'flattened circles' that we now call 'ellipses'.