In 1666, Isaac Newton was sitting in his garden when he observed an apple falling straight down. He wondered why it always falls down and not sideways or up. Then it hit him. Maybe the same invisible force pulling the apple to the ground is also what keeps the moon from flying off into space.
Twenty Years of Work
Newton worked on these ideas for 20 years, publishing Principia Mathematica in 1687. This incredible volume introduced the three laws of motion, the law of universal gravitation, laws of planetary motion, calculus, and fluid dynamics.
The apple story is often dismissed as a myth, but it comes from Newton himself. He told it to multiple people late in his life, including William Stukeley, whose 1752 account describes Newton recounting the moment in his own words. The apple did not hit him on the head, as cartoons like to suggest. What actually happened was more interesting. Newton’s insight was not simply that things fall. Everyone knew that. His leap was connecting the force that pulls an apple to the ground with the force that governs the orbit of the moon, hundreds of thousands of kilometers away. Before Newton, no one had proposed that the same law could explain both. The Principia unified terrestrial and celestial mechanics into a single framework, and it remained the foundation of physics for over two centuries until Einstein’s general relativity extended it further.
Further reading:
- Newton’s Principia Mathematica is available in English translation by I. Bernard Cohen and Anne Whitman (University of California Press)
- Stukeley’s original 1752 manuscript recounting the apple story is held by the Royal Society (Royal Society)
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