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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 that make it
possible to give a plausible reconstruction how he made his
discoveries. 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 afterwards Newton accomplished this formulation giving a
geometrical expression for the passage of time, thus laying the
foundation for the Principia. On Dec 10, 1684, four months after a
visit of Edmond Halley, Newton sent the first manuscript for his
book to the London Royal Society, "designedly abstruse to be
understood only by able Mathematicians". I will show, however, that
with simply a pencil and a ruler, and without Calculus, good
approximations of orbits for central forces can be obtained
graphically, which also clarifies the content of the Principia.
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