Newton's theory of universal gravitation was published in 1687. The Royal Society planned to publish Newton's Principia but was short of funds, so Edmond Halley agreed to pay the expenses himself. The law inside the book says the force of attraction between any two massive bodies is proportional to their masses and inversely proportional to the square of the distance between their centers. If you double a mass, you double the attraction. You pull on Earth with the same force that Earth pulls on you. The law has a number inside it: the gravitational constant, called G. It is very small, about 6.7 x 10^-11 in metric units, and it has to be measured by experiment. Before Henry Cavendish, scientists could work out only the product of G and the mass of Earth.
A falling apple is said to have inspired Isaac Newton to connect it with the motion of the Moon. Newton predicted that, if gravity weakens with the square of distance, the Moon falls toward Earth in one second 1/3600th as far as an apple falls in its first second. To stay in orbit the Moon must fall about 1.4 millimeters toward Earth each second. An orbiting object is always falling toward the planet, but it moves sideways fast enough that it never hits it. Near Earth's surface, a falling object speeds up by about 9.8 meters per second every second. Gravity keeps the Moon moving around Earth.
Mass is a quantity of matter, set by the numbers of atoms and molecules an object contains, so it does not change from place to place. Weight is the gravitational force acting on an object, so it varies with gravity. A person at low elevation in New Orleans weighs slightly more than one in Denver with the same mass. A 1.0-kilogram mass weighs 9.8 newtons on Earth and only about 1.6 newtons on the Moon. Gravity is slightly stronger over places with more mass underground than over places with less. From 2002 to 2017 NASA's two GRACE spacecraft measured variations in Earth's gravity. GRACE watched changes in sea level and could detect changes in Earth's crust brought on by earthquakes.
Newton's gravity could not explain one observation. Mercury's elliptical orbit gradually moves around the Sun, which is called precession. General relativity explained it. Albert Einstein described gravity as a curve in space that wraps around objects such as stars and planets. In general relativity, matter tells spacetime how to curve, and curved spacetime tells matter how to move. Gravity also pulls on light, so a massive object can bend light. In 1919 the first measurements made against the Newtonian theory were taken, and they arguably aligned with Einstein's general relativity. Massive objects also bend light from more distant objects, an effect called gravitational lensing, which also magnifies: it lets telescopes see objects that would otherwise be too far away and too faint to be seen.
Words to know
- Principia: Newton's book
- precession: orbit slowly turning
- lensing: light bending
