Universal gravitation
Newton's law that every mass attracts every other, with a force that grows with their masses and falls with the square of their distance.
Isaac Newton proposed that the force that makes an apple fall is the same one that holds the Moon in orbit, and that it acts between every pair of masses in the universe. His law of universal gravitation says the pull between two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between their centres. Double the distance and the force drops to a quarter.
Gravity between everyday objects is so weak that it is very hard to detect; what you feel is the pull of the whole Earth. To show that ordinary lumps of matter attract each other, you need something big. In 1774 the Astronomer Royal, Nevil Maskelyne, set up on Schiehallion, a conveniently regular mountain in Perthshire, Scotland. Using the stars as a reference, he measured how far the mountain pulled a hanging plumb line sideways on its north and south sides. The combined deflection was just 11.6 arcseconds, about a three-hundredth of a degree.
From the deflection and the mountain's size, Charles Hutton calculated that the average density of the Earth was about 4,500 kilograms per cubic metre, within 20 per cent of the modern 5,515. The Earth's interior had to be much denser than surface rock. To work out the mountain's volume, Hutton also invented contour lines.
A common misconception is that there is no gravity in orbit. Astronauts float because they and their spacecraft are falling around Earth together.
Coming up in future stories
- They Weighed the Earth With a Mountain 1774 · Schiehallion, Scotland