  All these approaches followed Kepler in using an ellipse to represent the trajectory. (The primary historical reason for this was Kepler's success in predicting the 1631 transit of Mercury across the Sun.) This, however, does not mean that the ellipse was established as anything more than a mathematically tractable close approximation to the true orbit. In fact, the planetary orbits known then are not all that elliptical. The minor axis of Mercury is only 2 percent shorter than the major axis, the minor axis of Mars, only 0.4 percent shorter, and in all other cases the difference between an ellipse and an eccentric circle was beyond detection. Newton had real grounds for claiming in a letter to Halley in June 1686 a “right” to the ellipse, remarking that “Kepler knew the Orb to be not circular but oval, and guest it to be Elliptical” [C, II, 436]. Entirely independently, the most judicious reader of the first edition of the Principia, Christiaan Huygens, wrote the following summary of the Principia‘s achievement in his notebook upon reading the complimentary copy Newton had sent him: