  The view is commonplace that what Newton did was to put forward his theory of gravity to explain Kepler's already established “laws” of orbital motion; and the universality of the law of gravity then ended up explaining the deviations from Keplerian motion by attributing them to gravitational interaction of the planets. This is wrong on several counts, the most immediate of which is that Kepler's “laws” were by no means established before the Principia. The rules for calculating orbital motion that Kepler put forward in the first two decades of the seventeenth century had indeed achieved a spectacular gain in accuracy over anything that had come before. Kepler's rules, however, did not yield comparable accuracy for the motion of the Moon, and even in the case of the planets the calculated locations were sometimes off by as much as a fourth of the width of the Moon. More importantly, by 1680 several other approaches to calculating the orbits had been put forward that achieved the same level of not quite adequate accuracy as Kepler's. In particular, Newton was familiar with seven different approaches to calculating planetary orbits, all at roughly the same accuracy. Only two of these, Kepler's and Jeremiah Horrocks's, used Kepler's area rule — planets sweep out equal areas in equal times with respect to the Sun — to locate planets along their trajectories. Ismaël Boulliau and, following him, Thomas Streete (from whose Astronomia Carolina Newton first learned orbital astronomy) replaced the area rule with a geometric construction. Vincent Wing had adopted still another geometric construction in the late 1660s after having earlier used a point of equal angular motion oscillating about the empty focus of the ellipse; and Nicolaus Mercator in 1676 added still a further geometric  construction.[4] Of these six alternative approaches, only Horrocks and, following him, Streete, took Kepler's 3/2 power rule — the periods of the planets vary as the square root of the cube of their mean distances from the Sun — seriously enough to use the periods rather than positional observations to determine their mean  distances.[5]