Newton · Principia
Do the apple and the Moon fall by the same law?
That time, that place
When plague closed Cambridge in 1665, Newton went home. In that year and a half, he later said, he seized on calculus, the nature of light, and the clue to gravity all at once — and published none of it. Only twenty years later, after Halley came and pressed him, did he write the Principia.
Why this question
Two bodies of knowledge sat apart.
Kepler had found what path the planets follow and how, but not why. Galileo had found how bodies fall to the ground.
And the very idea of joining them was strange: since Aristotle, heaven and earth had been separate realms under separate laws.
What was found
Newton's answer was that it is one law. The force that drops an apple and the force that holds the Moon are the same force.
The Moon is falling too. But it also moves sideways, and the Earth curves away beneath it as fast as it falls. So it falls forever and never arrives.
Set that force to weaken with the square of distance, and all three of Kepler's laws — found from observation — come out by calculation. Heaven and earth had come under one law.
What he did not explain was what gravity is. How do two separated bodies pull on each other with nothing in between? On that he wrote only that he would frame no hypotheses.
Heaven and earth follow different laws
That all three of Kepler's laws follow by calculation from a single force
The question of what gravity is stayed open until Einstein