Mass is a quantity of matter, set by the numbers of atoms and molecules a thing contains. It does not change from place to place. Weight is different. Earth's gravity pulls on the mass in your body, and that gives you weight. 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. On a planet with less mass than Earth, you would weigh less.

Isaac Newton's theory of universal gravitation was published in 1687. Newton's law says the force of attraction between any two massive bodies is proportional to their masses. It is also inversely proportional to the square of the distance between their centers. In plain numbers, if you double the distance between two masses, the attraction between them falls to one-fourth of its former value. If you double a mass, you double the attraction. Anything that has mass also has gravity, and things with more mass have more gravity. You pull on Earth with the same force that Earth pulls on you.

Does a heavy thing fall faster than a light one? 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. A dropped thing falls faster and faster as it falls. Near Earth's surface, a falling thing speeds up by about 9.8 meters per second every second.

Gravity does exist in space. It keeps spacecraft in orbit around Earth and Earth in orbit around the Sun. A thing in motion stays in motion unless something pushes or pulls on it. An orbiting thing is always falling toward the planet, but it moves sideways fast enough that it never hits it. Astronauts on the space station float because they and the station are all falling together around Earth. Gravity draws things toward the center of a planet or other body. Planets travel around the Sun on paths shaped like ovals, called ellipses. Gravity keeps the Moon moving around Earth.

The Moon's gravity pulls on the sea and helps make ocean tides. In 1687 Newton explained that ocean tides come from the gravitational pull of the Sun and Moon on the oceans. Gravity gets weaker with distance. 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.

Gravity just beneath a black hole's 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. One type of black hole is born when massive stars run out of fuel and explode. Our Milky Way galaxy has a supermassive black hole at its center. As of November 2026, it is about 4 million times the Sun's mass.

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 thing can bend light. Massive things bend light from more distant things, an effect called gravitational lensing. Einstein argued that the more strongly you feel gravity, the slower time passes for you. A gravitational wave is an invisible ripple in space. 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.

Words to know

• mass: amount of matter
• newtons: units of force
• tides: sea rising, falling
• horizon: black hole edge