Newton's *Principia* ignited the Scientific Revolution, but the work-sheets and sketches showing how he composed his masterpiece have been lost. Fortunately, he left behind enough clues to make it possible to give a plausible reconstruction of how he did it. Surprisingly, such a reconstruction has not been attempted before. In the winter of 1679, Robert Hooke initiated a correspondence with Newton outlining the physics of planetary motion. But Hooke was unable to formulate his concepts in mathematical form, and afterward, Newton accomplished this formulation, which allowed him to give a geometrical expression for the passage of time, thus laying the foundations for the *Principia*. On Dec.10, 1684, four months after a visit of Edmond Halley, Newton sent the first manuscripts for the *Principia* to the London Royal Society, which he had made "designedly abstruse to be understood only by able Mathematicians". This lack of clarity remains up to the present time. In his talk, I will show, however, that with simply a pencil and a ruler, and without any calculus, good approximations of orbits for central forces can be calculated graphically that also clarify the content of the *Principia*.

Aut. Qtr. Colloq. committee: R. Blandford (Chair), A. Kapitulnik, R. Laughlin, L. Senatore

Location: Hewlett Teaching Center, Rm. 200