The Sleepwalkers: A History of Man's Changing Vision of the Universe

by Arthur Koestler

In order to carry out this operation, he must first of all know at what rate the earth’s gravity diminished with distance. The apple fell from the tree at a known acceleration of approximately ten yards added speed per second; but what would be the acceleration of the distant moon towards the earth? In other words, he had to discover the Law of Gravity – that the force diminishes with the square of distance. In the second place, he had to know the exact value of the moon’s distance. Thirdly, he had to decide whether it was legitimate to treat two huge globes like the earth and the moon in an abstract manner, as if their whole mass were concentrated in a single central point. Lastly, to reduce the mathematical difficulties, the moon’s orbit had to be treated as if it were a circle instead of an ellipse. As a result of all these difficulties, Newton’s first calculations only agreed ‘pretty nearly’ with the facts; and that was not good enough. For nearly twenty years he dropped the whole issue.