The story of Newton’s apple is probably legend, but the conceptual leap is real: understanding that the same force pulling the apple down also keeps the Moon in orbit was one of the boldest unifications in the history of human thought.
Until Newton, the sky and the earth were two separate worlds, with different laws. Aristotle had attributed to the heavenly bodies a “naturally circular motion” and to earthly bodies a “naturally rectilinear motion downward.” Newton sweeps away this divide: there is only one law, that of universal gravitation, and the Moon “falls” toward the Earth exactly like the apple, except that it falls fast enough sideways to always miss it.
This is the image of continuous falling: if we launched a projectile ever faster from a mountaintop, it would land ever further away; beyond a certain speed, its falling trajectory would follow the curvature of the Earth without ever touching it. That is an orbit. The Moon does exactly this: it perpetually falls toward the Earth, but its tangential velocity makes it “miss” at every instant.
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Topics: Gravitazione Concepts: Legge di gravitazione universale · Orbite
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