We only see one side of the Moon because it's tidally locked to Earth. It spins on its axis exactly once for every trip it makes around us, about every 27.3 days, so the same hemisphere always faces our way. Earth's gravity slowed the young Moon's spin until the two periods matched, a fate shared by many moons in the solar system. The side we never see from Earth, the far side, isn't dark, though: it gets just as much sunlight as the near side, only at different times.
One spin per orbit: what tidal locking means
It's tempting to think a Moon that always shows the same face isn't spinning at all. It is, just slowly. Try this with a chair in the middle of a room. Walk a full circle around it while keeping your face toward it the whole time. By the time you're back where you started, you've also turned all the way around once, facing every wall in the room in turn. That's the Moon: one rotation on its axis for each orbit, perfectly matched.
NASA calls this synchronous rotation. The Moon takes about 27.3 days to orbit Earth relative to the stars, and it takes the same time to turn once on its axis. The cycle of phases is a little longer, about 29.5 days, because Earth is also moving around the Sun, but the spin and the orbit stay locked together.
How Earth's gravity locked the Moon
The Moon didn't start out this way. Early in its history it spun faster, and the lock was created by tides, though not tides of water.
Earth's gravity pulls harder on the near side of the Moon than on the far side, because gravity weakens with distance. That difference stretches the Moon very slightly along the line toward Earth, raising a pair of bulges in its rock. When the young Moon was spinning faster than it orbited, its rotation kept carrying those bulges out of line with Earth. Earth's gravity kept tugging them back, and that constant drag acted like a brake, flexing the rock and turning spin energy into heat. Over time the Moon's rotation slowed until the bulges stayed pointed at Earth, and from then on there was nothing left to brake.
The same process is still working the other way. The Moon raises tides on Earth, in the oceans and the rock, and their drag is very slowly lengthening Earth's day and pushing the Moon outward. NASA says the Moon is getting about an inch farther from Earth each year, a figure measured by bouncing lasers off reflectors left on the surface by Apollo astronauts. We explain the ocean side of that relationship in how the moon affects tides.
Tidal locking is common. Most of the large moons in the solar system, including Jupiter's four big moons and Saturn's Titan, keep one face toward their planet. Pluto and its moon Charon go one step further: each is locked to the other, so they always show each other the same face.
Libration: the wobble that shows a little more
Even with the lock, we don't see exactly half the Moon. Over time, observers on Earth can see a little more than half, about three-fifths of the surface by NASA's reckoning, thanks to a slow rocking motion called libration. It comes from three small effects:
- An oval orbit. The Moon spins at a steady rate, but it moves faster along its orbit when it's closer to Earth and slower when it's farther away. The spin and the orbit drift slightly out of step, so we see a little around the east and west edges in turn.
- A tilt. The Moon's equator is tilted by a few degrees to the plane of its orbit, so we see slightly over its north pole for part of the month and slightly under its south pole for the rest.
- Our own position. As Earth turns, you view the Moon from a slightly different angle at moonrise than at moonset, which shows a sliver more on each side.
Time-lapse films of the Moon over a month make libration easy to see: the face nods and turns as if the Moon were glancing around.
There is no dark side of the Moon
The phrase "dark side of the Moon," made famous by a 1973 rock album, is a misunderstanding. NASA calls it misleading. The far side gets day and night just like the near side, each lasting about two weeks. In fact, at new moon, when the side facing us is dark and we can't see the Moon at all, the far side is in full sunlight. (The few days when the Moon vanishes from our sky are what many people call the dark moon, explained in dark moon vs new moon.)
"Far side" is the accurate term. The only sense in which it was ever dark was to us: until the space age, nobody on Earth had any way of knowing what it looked like. For how the lit and unlit halves create the phases we see, read the Moon's phases explained.
What the far side looks like
When the first pictures arrived, they were a surprise. The Soviet spacecraft Luna 3 photographed the far side in 1959. In 1968 the crew of Apollo 8 became the first people to see it with their own eyes as they orbited the Moon, and in 2019 China's Chang'e 4 made the first soft landing there.
The far side looks very different from the face we know. The near side is marked by large dark patches, the maria, which are old impact basins flooded with lava billions of years ago. The far side has very few of them. It's mostly bright, rugged highland covered in craters on top of craters. NASA notes that the crust is much thicker there, up to about 37 miles (60 kilometers), which is one reason scientists think lava had a harder time reaching the surface on that side.
The same face for everyone on Earth
One lovely consequence of the lock is that every person who has ever looked up at the Moon has seen the same face. The dark maria form the shapes that cultures have read for thousands of years: a man in the Moon in much of Europe, a rabbit in the Moon across China, Japan and Korea, and in Mesoamerican stories too. People in the Southern Hemisphere see the same features, only turned upside down compared with the view from the north.
That fixed face is why the Moon's markings are so familiar and why a full moon looks like an old friend. For tonight's phase, see our moon phase today page, or follow the cycle in moon phases in order.
Questions readers ask
Does the Moon rotate?
Yes. It turns once on its axis about every 27.3 days, the same time it takes to orbit Earth. Because the two match, the same side always faces us, which can make it look as if it isn't rotating at all.
Will we ever see the far side of the Moon from Earth?
Not from the ground. The lock is stable, so the far side will stay out of view, apart from the small extra slivers that libration shows us. Spacecraft have photographed and mapped it in detail, and you can see those images online.
Is the far side of the Moon always dark?
No. It has day and night like the near side, each about two weeks long. It's fully lit at new moon, when the side facing Earth is in shadow. "Dark side" only ever meant unknown.
Is Earth tidally locked to the Moon?
Not yet. Earth still spins about once a day, much faster than the Moon orbits. The Moon's tides are very slowly slowing Earth's rotation, but the process is so gradual that it would take far longer than the Sun's remaining lifetime to finish.
