anonymously.) His idea was that the sun was stationary at the ‘ener and thatthe earth ad the planets moved in citelar orbits round the sun, Neal a century pased before his ie was taken seriously. Then two astronomers—the German, Johannes Kepler, and the Kalin, Galileo Galiei—started publicly to support the ican theory, despite the fact thatthe orbits i predicted dd not gute match the ones observed. The death blow to the Aristotelan/olemaic theory came in 1609, In that yar, Galileo started observing the night sky with a telescope, which had just tren invented. When he looked athe planet Jpite, Galileo found tha it was accompanied by several smal satelites or moons that cotbited around it. This implied that everything didnot have to ‘orbit directly around the ert, as Aristotle and Prokemy had thought. (de-was, of course, sill possible to belive that the earth was stationary at the center of the universe and that the moons of Jupiter moved on extremely complicated paths around the earth, saving the appearance that they orbied Jupiter. However, Cperni- 's theory was much simple.) At the same time, Johannes Kepler had modified Copernicus theory, suggesting that the planes moved ‘not in circles but inelipss (an elise isan elongated cicle). The predictions now finally matched the observations. ‘As far as Kepler was concerned elipical orbits were merely hoe hyporhes ther repugnant one at that, because ellipses were clearly less perfec than cites. Having. discovered tmx by accident that eipca obi ithe observations well, he ould not reconcile them with his idea that the planets were to orbit the sun by magnetic forces. An explanation wa ‘only much later, in 1687, when Sir Isaac Neweon published his Philosophiae Naturals Principia Mathematica, probably the most important single work ever published in the physical ene, In it [Newton not only pt forward a theory of how bois move in space tnd ime, but he aso develope the complicated mathematics needed to analyze those motions In ation, Newton postulated a law of universal gravitation according to which each Body i the universe acted toward every other body by a force that was stronger the more massive the bodies andthe closer they were to each other. this same force that caused objet to fall othe ground. (The