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. 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. 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.

Earth's gravity is not the same everywhere, and it can be measured; 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. 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. A 1.0-kilogram mass weighs 9.8 newtons on Earth and only about 1.6 newtons on the Moon.

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. In Einstein's view, a falling astronaut is not pulled or pushed by anything but 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. Einstein also 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. In 2010 NIST physicist David Wineland and colleagues ran two optical clocks about a foot apart in height, and the clocks ticked at slightly different rates because of the slight difference in gravity.

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. 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, which also magnifies. 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.

Gravity can reveal what no one can see. 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 black hole is so dense that gravity just beneath its surface, the event horizon, is strong enough that nothing, not even light, can escape. 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. As of November 2026, scientists do not yet know what dark matter is.

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. LIGO scientists estimate the first merging black holes were about 29 and 36 times the mass of the Sun, 1.3 billion years away. Orbiting pairs of black holes or neutron stars radiate gravitational waves that carry away orbital energy, so over millennia the objects move closer together. LIGO catches only the last few orbits of a pair: by the time its frequency is detectable, the objects are just seconds away from colliding. The Livingston detector recorded the first event 7 milliseconds before the Hanford detector, which showed that the source was in the Southern Hemisphere. By the time the first waves reached Earth, the space-time wobbling they made was 10,000 times smaller than the nucleus of an atom.

Words to know

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