Geometrical Approach to Kepler’s Laws of Planetary Motion

The elementary pen-and-string method to draw ellipsis has been devised to examine planetary orbits on the basis of the Kepler’s laws. Besides qualitative features of the orbits, quantitative dependence of the orbital shape on the quantities appearing in the Kepler’s laws can also be analyzed with simple geometrical procedures. The method thus provides a relevant intermediate step to students prior to the study of the rigorous theory of central force problems.