The Moon is Earth's only natural satellite, and by far the closest major celestial body to us — about 384,400 km (238,900 miles) away on average, close enough that light from it reaches Earth in about 1.3 seconds. It's also unusually large relative to its planet: at roughly a quarter of Earth's diameter, it's the largest moon relative to its host planet of any in the solar system except Pluto's Charon.
Where it came from
The leading theory is that the Moon formed about 4.5 billion years ago, shortly after Earth itself, when a Mars-sized object collided with the young Earth. Debris from that collision is thought to have coalesced into the Moon. This is called the giant-impact hypothesis, and it's supported by, among other things, the striking similarity between Moon rocks and Earth's outer layers.
No air, no weather, no erosion
The Moon has essentially no atmosphere, which means no wind, no weather, and no erosion. Footprints left by Apollo astronauts in the 1960s and 70s are still there today, undisturbed, and will likely remain visible for millions of years unless a meteorite strike happens to land nearby. Temperatures swing enormously between lunar day and night — from around 127°C (260°F) in direct sunlight to -173°C (-280°F) in darkness — because there's no atmosphere to trap or spread heat.
Only twelve people have ever walked there
Between 1969 and 1972, NASA's Apollo program landed six crewed missions on the Moon, and twelve astronauts walked on its surface — all of them American, and all of them men. No human has returned to the lunar surface since Apollo 17 in December 1972.
That's changing. NASA's Artemis program successfully flew astronauts around the Moon in April 2026 (Artemis II) — the first crewed lunar flight since Apollo — without landing. A crewed lunar landing, Artemis III, is currently planned for around 2027–2028, which would mark humanity's return to the Moon's surface after more than 50 years.
Why people care
Beyond its role in human exploration, the Moon quietly shapes life on Earth: its gravity drives our ocean tides, and it's thought to help stabilize the tilt of Earth's axis, which keeps our climate relatively steady over long timescales. It's also the only other world human eyes have seen up close in person — everything else we know about other planets and moons has come from robots and telescopes.