We only see one side of the Moon because it is tidally locked, so its rotation matches the time it takes to orbit Earth.
Tidal locking
The Moon rotates once for every orbit around Earth. Both take about 27.3 days, so the same side always faces us.
Why
Earth’s gravity created a tidal bulge on the Moon. Over millions of years, the Moon’s spin slowed until it matched its orbit.
The far side
It receives sunlight as the near side does. It has far fewer dark maria. The first photos came from the Soviet Luna 3 in 1959. China’s Chang’e 4 landed there in 2019, the first soft landing.
Libration
Wobbles let us see about 59 percent of the Moon’s surface over time.
Other moons
Most large moons in the solar system are tidally locked.
Earth
In billions of years, Earth might become locked to the Moon.
Tidal locking
The Moon rotates on its axis in exactly the same time it takes to orbit Earth, about 27.3 days. This is called synchronous rotation, or tidal locking. As a result, the same side, called the near side, always faces Earth. It is a coincidence of timing that is a consequence of gravitational forces, not a coincidence in the sense of chance.
Why
Billions of years ago the Moon rotated faster. Earth’s gravity raised tidal bulges in the Moon’s rocky body. Because the Moon rotated faster than it orbited, friction pulled the bulge slightly ahead of the line to Earth, and Earth’s gravity pulled back on the bulge, acting as a brake. Over about tens of millions of years, the spin slowed until it matched the orbital period, and the bulge pointed at Earth permanently.
The far side
The far side is not “dark.” It gets just as much sunlight as the near side, over a 29.5-day cycle. It was first photographed in 1959 by the Soviet probe Luna 3. Its surface differs from the near side: it has far fewer dark volcanic plains (maria), a thicker crust and more craters. In January 2019, China’s Chang’e 4 became the first spacecraft to land on the far side, in the Von Karman crater. In 2024, Chang’e 6 returned samples from the far side.
Libration
We can see about 59% of the Moon’s surface over time, not just 50%, because of libration, a slight wobble caused by the Moon’s elliptical orbit and the tilt of its axis. The Moon appears to rock up to about 7 degrees in longitude and latitude.
Other moons
Tidal locking is common. Most large moons in the solar system, such as Jupiter’s Galilean moons, Saturn’s Titan and Mars’ Phobos, are tidally locked to their planets. Pluto and its moon Charon are mutually locked, each showing the same face to the other.
Earth
Earth’s rotation is also slowing due to tidal friction with the Moon, by about 1.7 milliseconds per century. The Moon recedes from Earth by about 3.8 centimeters a year, measured by lasers bounced off retroreflectors left by Apollo astronauts. In billions of years, Earth could become tidally locked to the Moon, but the Sun will likely expand first.
Orbital and physical numbers
The Moon takes about 27.3 days to spin once on its axis and the same 27.3 days to orbit Earth, so it keeps the same face turned toward us. This is called tidal locking or synchronous rotation, and it happened because Earth’s gravity created tidal bulges on the Moon that slowed its spin over hundreds of millions of years. The far side is not dark, as it receives as much sunlight as the near side, and has more craters and less of the dark volcanic plains known as maria. Because the Moon’s orbit is elliptical and tilted, we see about 59 percent of its surface over time, a movement called libration. The Moon is 384,400 km away on average and drifts away from Earth at about 3.8 cm per year. China’s Chang’e 4 landed on the far side in January 2019, and Chang’e 6 returned samples from the far side in June 2024. Many moons in the solar system are tidally locked, such as Pluto’s Charon, which is locked both ways.
Based on data from NASA and the China National Space Administration
