  The first segment highlights a further component of the historical context in which the Principia was written and read. Galileo's discovery of the phases of Venus in 1613 had provided decisive evidence against the Ptolemaic  system,[6] but it could not provide grounds favoring the Copernican over the Tychonic system. In the latter, Mercury, Venus, Mars, Jupiter and Saturn circumnavigate the Sun, and the Sun circumnavigates the Earth, with the consequence that these seven bodies are at all times in the same position in relation to one another as they are in the Copernican system. Whether any decisive empirical grounds could be found favoring the Copernican over the Tychonic system became one of the most celebrated issues of the seventeenth century. Kepler, Galileo, and Descartes all published major books in the first half of the century purporting to resolve this  issue,[7] Kepler and Descartes basing their arguments on the physical mechanism each had proposed as governing the orbital motion. Nevertheless, the leading observational astronomer of the second half of the century, G. D. Cassini, was a Tychonist. In the first segment of the “Copernican Scholium” Newton identifies the center of gravity of the planetary system as the appropriate point to which all the motion should be referred — the technical issue behind the issue over the two systems — and then announces that the centripetal forces identified in the text of “De Motu” as governing the orbital motion open the way to establishing a slightly qualified form of the Copernican  system.[8] Newton's discovery of this line of reasoning was surely a major factor urging him on to the Principia.