A moon is an object that orbits a planet or something else that is not a star. Besides planets, moons can circle dwarf planets, large asteroids, and other bodies. Objects that orbit other objects are also called satellites, so moons are sometimes called natural satellites. People have launched many artificial satellites into orbit around Earth, but these are not considered moons.

The planet or body that a moon orbits is called its primary. Just as gravity holds the planets in our solar system in orbit around the sun, gravity also keeps moons in orbit around their primaries.

Many moons formed at the same time as their primary. Gravity pulled bits of dust and gas together into larger and larger clumps of material. Eventually, the smaller clump of material (moon) began orbiting the larger clump (primary).

Some moons formed in other ways. Earth's moon may have formed when an object the size of Mars crashed into the planet. The collision sprayed a huge amount of material into orbit around Earth. This material slowly accumulated into one large body, our moon. Other moons in our solar system were once asteroids, chunks of rock that are too small to be planets. These asteroids came too close to their primary and were pulled into orbit by the force of gravity.

Most moons are made of rock, but many also contain a large amount of ice, gas, and other chemicals. Europa, a large moon orbiting Jupiter, has an icy surface that may cover a liquid oceanof water.

Some moons have volcanic or geologic activity. For example, scientists have observed volcanic plumes rising 300 kilometers (190 miles) from the surface of Io, another one of Jupiters moons. Other moons, including Earths moon, show little or no signs of geologic activity, though they may have been more active in the past.

As of 2010, astronomers had discovered 166 moons circling planets in our solar system. Ninety-nine of these have been discovered since 2000. Jupiter has the most known moons, with 63. Saturn has 60 named moons, Uranus has 27, and Neptune has 13. Mars has just 2, and Earth has only 1. Venus and Mercury have no moons.

Another six moons in our solar system circle dwarf planets. Dwarf planets are planetlike objects that do not fit the full definition of a planet. Pluto is the most famous dwarf planet. Pluto has three moons. Many other moons in our solar system orbit smaller bodies. Because moons are relatively small, none have yet been discovered outside the solar system, but there are likely trillions of moons throughout the universe.

moon
A full moon happens about once a month.

Man in the Moon
The surface of Earth's moon is pockmarked with millions of craters left when asteroids and other space rocks crashed into its surface over millions of years. Sometimes, from Earth, the pattern of craters looks like a face peering down.

Supermoon
The largest moon in the solar system is Ganymede, which orbits Jupiter. Its diameter, or maximum distance across, is 5,262 kilometers (3,270 miles), larger than the planet Mercury. In 1610, Italian astronomer Galileo Galilei discovered Ganymede and three other planet-size moons circling Jupiter. They were the first moons discovered orbiting a planet other than Earth.

accumulate
Verb

to gather or collect.

artificial satellite
Noun

object launched into orbit.

asteroid
Noun

irregularly shaped planetary body, ranging from 6 meters (20 feet) to 933 kilometers (580 miles) in diameter, orbiting the sun between Mars and Jupiter.

collision
Noun

crash.

Noun

bowl-shaped depression formed by a volcanic eruption or impact of a meteorite.

diameter
Noun

width of a circle.

dust
Noun

microscopic particles of rocks or minerals drifting in space. Also called cosmic dust or space dust.

dwarf planet
Noun

celestial body that is nearly spherical but does not meet other definitions for a planet.

Europa
Noun

moon of Jupiter.

Galileo Galilei
Noun

(1564-1642) Italian scientist best known for his work in astronomy.

Ganymede
Noun

moon of Jupiter, largest in the solar system.

gas
Noun

state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion.

geologic
Adjective

having to do with the physical formations of the Earth.

gravity
Noun

physical force by which objects attract, or pull toward, each other.

liquid
Noun

state of matter with no fixed shape and molecules that remain loosely bound with each other.

Mars
Noun

fourth planet from the sun, between Earth and Jupiter.

Mercury
Noun

smallest planet in the solar system, and closest to the sun.

Moon
Noun

Earth's only natural satellite.

Noun

natural satellite of a planet.

natural satellite
Noun

moon, or a celestial body that orbits another celestial body (its primary).

Neptune
Noun

eighth planet from the sun in our solar system.

Verb

to move in a circular pattern around a more massive object.

peer
Verb

to glance or gaze.

Noun

large, spherical celestial body that regularly rotates around a star.

Pluto
Noun

dwarf planet in our solar system.

pockmarked
Adjective

scarred with many small indentations.

primary
Adjective

first or most important.

rock
Noun

natural substance composed of solid mineral matter.

satellite
Noun

object that orbits around something else. Satellites can be natural, like moons, or made by people.

Saturn
Noun

sixth planet from the sun.

solar system
Noun

the sun and the planets, asteroids, comets, and other bodies that orbit around it.

star
Noun

large ball of gas and plasma that radiates energy through nuclear fusion, such as the sun.

trillion
Noun

1,000,000,000,000.

universe
Noun

all known matter, energy, and space.

Uranus
Noun

large, gaseous planet in the solar system, seventh from the sun.

Venus
Noun

planet in the solar system, second from the sun.

volcanic
Adjective

having to do with volcanoes.

volcanic plateau
Noun

flat, elevated landform created by layers of lava from volcanic eruptions.