A falling apple is said to have inspired Isaac Newton to connect it with the motion of the Moon. Newton's theory of universal gravitation was published in 1687. His law 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 the distance between two masses, the attraction between them falls to one-fourth of its former value. In 1687 Newton explained that ocean tides come from the gravitational pull of the Sun and Moon on the oceans. Tides come from a force that falls off with the cube of distance. The Sun is far more massive than the Moon but 390 times farther away, so its tide-generating force is about half the Moon's.

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. There was no practical way to separate G from the mass of Earth. As of November 2026, the 2022 CODATA value of the Newtonian constant of gravitation is 6.674 30 x 10^-11 m^3 kg^-1 s^-2, with a standard uncertainty of 0.000 15 x 10^-11. Anything that has mass also has gravity, and objects with more mass have more gravity. You pull on Earth with the same force that Earth pulls on you.

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 on the Moon is weaker than gravity on Earth. Gravity is slightly stronger over places with more mass underground than over places with less.

Gravity is why things fall down when you drop them or throw them. A dropped object falls faster and faster as it falls. Near Earth's surface, a falling object speeds up by about 9.8 meters per second every second. With no air, heavy objects and light objects fall at the same rate. Galileo explored falling bodies by dropping different weights or sliding them down inclined planes. Legend has it that he dropped heavy and light balls from the leaning tower of Pisa. On the Moon, Apollo 15 commander David Scott released a hammer and a feather together. With no air resistance, the feather fell at the same rate as the hammer.

Newton's gravity could not explain one observation. Mercury's elliptical orbit gradually moves around the Sun, which is called precession. Albert Einstein's general relativity explained it. In general relativity, matter tells spacetime how to curve, and curved spacetime tells matter how to move. An orbiting object is always falling toward the planet, but it moves sideways fast enough that it never hits it. In Einstein's view, a falling astronaut is not pulled or pushed by anything. The astronaut moves along a straight line, a geodesic, through curved spacetime. Astronauts on the space station float because they and the station are all falling together around Earth.

Albert Einstein described gravity as a curve in space that wraps around objects such as stars and planets. Gravity also pulls on light, so a massive object can bend light. Einstein argued that the more strongly you feel gravity, the slower time passes for you. A gravitational wave is an invisible ripple in space. Each LIGO observatory has two arms more than 2 miles (4 kilometers) long. Scientists first directly detected gravitational waves in 2015, using LIGO. The first waves came from two black holes that collided 1.3 billion years ago.

Words to know

• constant: fixed number
• precession: orbit slowly turning
• spacetime: space with time
• geodesic: straight line path