Moons

What a moon is

Moons, also called planetary satellites, come in a wide range of shapes, sizes and types; most are solid, and only a few have an atmosphere.^[1] They circle planets and dwarf planets, and some even circle asteroids.^[1]

Most moons of the planets probably formed from discs of gas and dust that swirled around the young planets.^[1] Neptune’s large moon Triton orbits backwards, which suggests it may once have been a separate object that Neptune captured.^[7]

How many moons are there?

Counts keep changing as new moons are found, so each figure here carries its date. As of March 25, 2025, more than 891 moons had been confirmed in the solar system.^[1] That total was 421 officially recognised moons of planets (counting Pluto) plus more than 470 around other dwarf planets, asteroids and trans-Neptunian objects, bodies that orbit beyond Neptune.^[1]

Counts for single planets come from different dates:

• Earth has one moon, and Mars two, Phobos and Deimos.^[2]
• Jupiter had 115 known moons as of April 2026.^{[2, 5]}
• Saturn had 274 confirmed moons as of March 2025, and 293 as of June 2026.^{[6, 2]}
• Uranus had 29 known moons as of July 31, 2026, the date its source page was updated.^[2]
• Neptune has about 16 known moons (the source gives no date).^{[7, 2]}

Earth’s Moon

Earth’s Moon probably formed when a body roughly the size of Mars struck Earth and threw a great deal of material into orbit.^[1] Debris from both bodies gathered into the Moon about 4.5 billion years ago, the age of the oldest lunar rocks collected.^[1]

The Moon lies an average of 384,399 km (238,855 miles) from Earth, enough room for about 30 Earths.^[3] It circles Earth once every 27.322 days in a slightly stretched orbit, yet it takes 29.5 days to go from one new Moon to the next.^[3]

The Moon is tidally locked: it turns once on its axis for each trip around Earth, so we only ever see one side.^[3] Even so, a slight wobble called libration lets us glimpse a little of the far side.^[3]

Phases

The Sun always lights half of the Moon, but how much of that lit half we can see changes as the Moon moves around its orbit.^[3] These changes are the phases.^[3] In order, the eight phases are new Moon, waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, third quarter and waning crescent, and the cycle repeats roughly every 29.5 days.^[3] At new Moon, the lit side faces the Sun and the dark side faces us, so the Moon is invisible.^[3] Sometimes sunlight reflecting off Earth’s surface also falls on the Moon’s face.^[3]

The Moon and the tides

The Moon’s gravity drives Earth’s tides, although high tides don’t line up exactly with where the Moon is.^[4] Its pull is strongest on the side of Earth nearest the Moon and weakest on the far side, and it reaches every bit of water.^{[4, 11]} The result is two bulges of water, one facing the Moon and one on the opposite side.^{[4, 11]}

The two sources explain the far-side bulge in different words. One describes the water being pushed and squeezed around the globe by gravity and other forces until it heaps up on both sides.^[4] NOAA, the US National Oceanic and Atmospheric Administration, explains it by inertia: on the far side the water tends to keep moving in a straight line, away from Earth, more strongly than the Moon’s weaker pull holds it back.^[11]

As Earth turns, coastlines pass through both bulges, so most shores get two high and two low tides a day.^{[4, 12]}

The Sun matters too. NOAA calls it a major influence on the size and position of the bulges, with the Moon the stronger of the two.^[11] The Sun is about 27 million times as massive as the Moon but about 390 times farther away, which gives it a little under half the Moon’s tide-raising force.^[4] Twice a month, at new and full Moon, Sun, Moon and Earth line up and produce extra-high spring tides; around first and last quarter they pull at right angles and produce milder neap tides.^[4] Real tides are also shaped by coastlines, water depth, wind and storms.^{[4, 12]}

Famous moons of other worlds

• Mars: Phobos and Deimos are lumpy and dark.^[1] Phobos is slowly creeping closer to Mars and could hit the planet in 40 or 50 million years.^[1]
• Jupiter: Galileo Galilei first saw its four largest moons, Io, Europa, Ganymede and Callisto, in 1610, and they are called the Galilean satellites.^[5] Io is the most volcanically active body in the solar system, and Ganymede is the largest moon of all, bigger even than the planet Mercury.^[5] Europa may hide a liquid-water ocean under its frozen crust.^[5]
• Saturn: Titan, the second-largest moon, is the only one with a thick atmosphere, and it has seas of ethane and methane on its surface.^[1] Both Titan and Enceladus have oceans under their surfaces, and Enceladus sprays out jets of water.^{[1, 6]} Shepherd moons help keep Saturn’s rings in line.^[1]
• Uranus: Its inner moons seem to be about half water ice and half rock, and Miranda’s chopped-up surface shows scars from large impacts.^[1]
• Neptune: Triton, as big as Pluto, was found in 1846 by William Lassell, just 17 days after the planet itself was discovered.^{[1, 7]}
• Pluto: Its system of moons might have formed in an early collision between Pluto and another body of similar size.^[8] Charon, about half Pluto’s size, is the largest moon compared with the world it orbits, and the same side of it always faces Pluto.^[8]

Planets with no moon

Mercury and Venus have no moons.^{[9, 2]} A simple explanation for Mercury is that it sits so close to the Sun that it could not keep a moon, while why Venus has none is still a mystery.^[2] Venus does have a quasi-satellite named Zoozve, found in 2002.^[10] A quasi-satellite is not a moon: it is an asteroid that orbits the Sun while staying close to a planet.^[10]

Common misconceptions

“The Moon only comes out at night.” It is often visible in daylight.^[3]

“During phases, part of the Moon goes dark.” Half the Moon is always lit; what changes is how much of that half faces us.^[3]

“High tide comes when the Moon is overhead.” High tides do not line up exactly with the Moon’s position.^[4]

What this page does not cover

This page does not describe the Moon’s surface or geology, the Apollo missions in detail, eclipses, whether the Moon is drifting away, tide tables or tidal power, or Titan and Enceladus beyond the basics above.

Going further

See Planets for the worlds these moons circle, Solar System for the bigger picture, Orbits and Gravity for why moons stay in their paths, Tides for the oceans’ side of the story and Space Exploration for the missions that study moons.