  These were remarkable steps forward at the time, but they and the questions behind them form only an initial part of the context in which Newton went on to write the Principia. Shortly after the “De Motu” tract went off to London, Newton revised the tract and added two further passages. The question precipitating this revision appears to have been about the effect the inverse-square centripetal forces directed toward Jupiter, as implied by its satellites, have on the Sun. Newton first added two principles that he first called “hypotheses” and then changed to “laws”: