A falling apple is said to have inspired Isaac Newton to connect it with the motion of the Moon. Newton's 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 falls to one-fourth of its former value. 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.

The law has a number inside it: the gravitational constant 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, and 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.

Earth's gravity is not the same everywhere. 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, and GRACE-FO is its successor. GRACE watched changes in sea level and could detect changes in Earth's crust brought on by earthquakes. 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.

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 Probe B was designed to measure two key predictions of general relativity by monitoring the orientations of ultra-sensitive gyroscopes relative to a distant guide star.

Gravity also pulls on light, so a massive object can bend light. In 1911 Einstein challenged astronomers to watch total solar eclipses and compare star positions with and without possible warping by the Sun. In 1917 the Joint Permanent Eclipse Committee chose two stations to watch the 1919 eclipse: Sobral in Brazil and the island of Principe. In 1919 the first measurements made against the Newtonian theory were taken, and they arguably aligned with Einstein's general relativity.

Massive objects bend light from more distant objects, an effect called gravitational lensing. Lensing also magnifies: it lets telescopes see objects that would otherwise be too far away and too faint to be seen. In 2020 ESA/Hubble released an image of GAL-CLUS-022058s that showed the largest ring-shaped lensed image of a galaxy, an Einstein ring, discovered up to that time.

Einstein argued that the more strongly you feel gravity, the slower time passes for you. Atomic clocks on GPS satellites run 38 microseconds per day faster than clocks on Earth. Orbital speed slows them 7 microseconds, and weaker gravity speeds them up 45.

A black hole is so dense that gravity just beneath its surface, the event horizon, is strong enough that nothing, not even light, can escape. Black holes do not suck in other matter. From far enough away, their gravity acts just like that of any other object of the same mass. Our Milky Way galaxy has a supermassive black hole at its center. Astronomers tracked the orbits of several stars near the center of the Milky Way to prove it houses a supermassive black hole, a discovery that won the 2020 Nobel Prize. As of November 2026, it is about 4 million times the Sun's mass.

A gravitational wave is an invisible ripple in space. In 1974 Russell Hulse and Joseph Taylor found a binary pulsar. In 1982 Taylor and Joel Weisberg found its orbit slowly shrinking because of gravitational waves, and Hulse and Taylor won the 1993 Nobel Prize in Physics. Scientists first directly detected gravitational waves in 2015, using LIGO. The first waves it detected came from two black holes that collided 1.3 billion years ago.

In 1933 Fritz Zwicky found that galaxies in the Coma Cluster moved too quickly for the gravity of their observed ordinary matter, and he suggested an invisible form of matter. In the 1970s Vera Rubin studied stars at the outer edges of spiral galaxies, which moved as if held in place by a large amount of unseen matter. The Bullet Cluster, studied in 2006, gave some of the best direct evidence for dark matter. As of November 2026, scientists estimate that ordinary matter is only about 5% of the universe, dark matter about 27%, and the rest is thought to be dark energy. As of November 2026, scientists do not yet know what dark matter is.

Words to know

• precession: orbit slowly turning
• lensing: light bending
• microseconds: millionths of seconds