‘story that Newton was inspired by an apple hitting his head is almost certainly apocryphal, All Newton himself ever sa the idea of gravity came to him as he sat “in ‘mood and “was occasioned by the fall of an apple") Newton went on to show that, according. ro his law, gravity causes the ‘moon to move in an eliptical orbit around the earth and causes the emplative earth and the planets o follow elliptical paths around the sun, ‘The Copernican model got rd of Ptolemy's celestial spheres, tnd with them, che ide thatthe universe had a natural boundary. ‘axis became natural t0 suppose that the fixed stars were objects like our sun but very much father away. Newton realized that, according to his theory of gravity, the stars should attract each othe, soit seemed they could not remain essentially motionless. Would they not all fall together at some point? In a leter in 1691 to Richard Bentley, another leading thinker of his day, Newton argued that this would indeed happen it there were only finite numberof stars distributed over a finite ‘region of space, But he reasoned that if, on the other hand, there ere an infinite numberof stars, distributed more o less uniformly ‘over infinite space, this would not happen, because there would ‘ot be any central point for them t0 fll to. ‘This argument is an instance of the pitfalls that you can encounter in talking about infinity. In an infinite universe, every point can be regarded as the center, because every point has an Infinite number of stats on each side of it. The correct approach, it was realied only much later, is to consider ee finite situation, in which the stars al fallin on eachother, and then to ask how things change if one adds more stars roughly uniformly distributed out tide this region. According to Newton's law, the extea stars would make no difference at all to the original ones on average, s0 the ‘stars would fall in just as fast. We can add as many stars as we like, but chey wil sil always collapse in on themselves. We now know it is imposible to have an infinite static model of the ‘universe in which gravity is always atractive. Iti an interesting reflection on the gener imate of thought