Universal gravitation
The force by which every object with mass attracts every other. Its strength is proportional to the product of the two masses and inversely proportional to the square of the distance.
In the Principia of 1687, Isaac Newton showed that if the force that makes things fall to the ground is the same force that keeps the Moon circling the Earth, all three of Kepler's laws of planetary motion follow[1]. The physics of the Earth and the physics of the heavens were joined under one law.
The same book contains a thought experiment: a cannonball fired horizontally from a mountaintop fast enough would never finish falling and would circle the Earth for ever. That is the principle of the artificial satellite. Spacecraft trajectories are still calculated almost entirely with this law. Finer matters, such as a small drift in Mercury's orbit or the clock corrections GPS needs, require Einstein's general relativity.
Related terms
- Kepler's lawsThree laws of planetary motion: planets move on ellipses with the Sun at one focus, sweep faster when closer to it, and have orbital periods tied to orbit size.
- Three-body problemThe problem of finding how three bodies move under their mutual gravity. Two can be solved with a formula; three in general cannot, and tiny differences can change the future completely.
- RelativityEinstein's theory that measurements of time and space depend on the observer's motion and on gravity — special relativity (1905) and general relativity (1915–16).
On the timeline
Sources
- I. Newton, “Philosophiæ Naturalis Principia Mathematica,” London (book) (1687).