We see it almost every night, yet most of us could only name a handful of real facts about the Moon. Here’s what’s actually true, the numbers, the mechanics, and the science behind humanity’s most familiar celestial neighbor.

How old is it, and how did it form?

The leading explanation is the giant-impact hypothesis: roughly 4.5 billion years ago, a Mars-sized body (sometimes called Theia) collided with the young Earth. Debris from that collision, vapor, molten rock, and dust, spun into orbit around Earth and gradually clumped together to form the Moon, likely within tens of millions of years of Earth’s own formation. This explains several oddities about the Moon, including why it’s so poor in iron compared to Earth: Earth’s core holds about 30% of its mass, while the Moon’s core is only around 1.6–1.8%.

The basics

The Moon is 3,474 km across, about 27% of Earth’s diameter, with a mass of roughly 7.34 × 10²² kg, just over 1% of Earth’s. Its surface gravity is 1.62 m/s², about one-sixth of what you feel here. It has no rings, no moons of its own, and essentially no atmosphere, just an extremely thin exosphere of gases like helium, neon, and sodium. With nothing to trap heat, surface temperatures swing wildly: from around 127°C in direct sunlight to -173°C in the dark.

Why we always see the same face

The Moon takes 27.32 days to both rotate once on its axis and complete one orbit around Earth, a state called tidal locking, and the reason we only ever see one side of it from Earth. One nuance worth knowing: that 27.32-day figure is the Moon’s rotation and orbit period relative to the stars. The Moon’s actual light-to-dark cycle (from one sunrise to the next, as an astronaut standing on the surface would experience it) is closer to 29.5 days, because Earth itself is also moving around the Sun during that time. Both numbers are “correct”, they’re just answering slightly different questions.

The push and pull of tides

The Moon’s gravity is the primary driver of Earth’s ocean tides, and it also helps stabilize Earth’s axial tilt, the same tilt responsible for our seasons. Without the Moon’s steadying influence, Earth’s tilt could wobble far more chaotically over long timescales. It’s a quiet job, but an essential one for a stable climate.

And the Moon isn’t standing still, either: it’s drifting away from Earth at about 3.8 cm per year, a measurable effect of the same tidal forces it exerts on us, confirmed by laser-ranging experiments using reflectors the Apollo astronauts left on the surface.

One thing worth clarifying: the “no magnetic field” claim

The Moon doesn’t have a global, Earth-like magnetic field generated by an active internal dynamo, that’s true. But it isn’t magnetically silent: many regions of lunar crust are weakly, unevenly magnetized, a leftover signature of the Moon possibly having had a magnetic field billions of years ago before its core cooled. So “no magnetic field” is a fair simplification, but not the whole story.

A short history of getting there

The Soviet Luna program (1959–1976) delivered the first successful missions to the Moon. NASA’s Apollo 11 in 1969 put the first humans on its surface. Since then, robotic missions have expanded the picture dramatically, from the Lunar Roving missions of the 1970s to China’s ongoing Chang’e program, which has returned samples and deployed rovers on the far side, to NASA’s Artemis program, aiming to return humans to the Moon this decade. Ice deposits detected in permanently shadowed craters near the south pole are now one of the most closely watched targets for future exploration, a potential resource for future missions.

The Moon has spent 4.5 billion years quietly shaping our planet’s climate, tides, and night sky. It’s easy to take for granted precisely because it’s so constant, but very little about it is actually simple once you look closely.

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