Why Does the Moon Always Show the Same Face?

Imagine walking in a circle around a chair while keeping your face towards it. To do that you have to turn your body slowly as you go, completing one full turn for each lap. The Moon does the same thing. It spins on its axis once per orbit, so the side facing Earth at the start of the month is still facing Earth at the end.

If the Moon did not rotate at all, we would see every side of it over a month, just as someone circling the chair while facing a fixed wall shows the chair every side of their head. The fact that we see one side is the proof that it rotates at precisely the orbital rate. Astronomers call this synchronous rotation, or tidal locking.

Early in its history the Moon almost certainly spun faster. What changed it is the same force that raises tides on Earth. The Earth's gravity pulls harder on the near side of the Moon than on the far side, stretching it slightly into an egg shape with a bulge pointing towards us. When the Moon spun faster than it orbited, its rotation carried that bulge slightly ahead of the Earth-Moon line, and the Earth's pull on the offset bulge acted as a brake, dragging the rotation back a little with every turn.

Over millions of years the braking continued until the bulge stayed exactly on the line to Earth, at which point there was nothing left to pull against. The rotation had been locked to the orbit. Because the Moon is small and close, the process finished long ago. The same thing has happened to nearly every large moon in the solar system: the four big moons of Jupiter, Titan around Saturn, and Charon around Pluto all keep one face to their planet.

The lock is not quite perfect from our point of view, and careful observers can see round the edges. The Moon's orbit is slightly elliptical, so it moves faster when nearer the Earth and slower when farther away, while its rotation stays steady; the two fall out of step by a few degrees and then catch up, letting us peek round the eastern and western edges in turn. Its axis is also tilted a little, so we see slightly over the north pole at one point in the month and the south pole at another.

These nodding motions are called librations, and together they bring about 59 percent of the surface into view at one time or another. The remaining 41 percent was unknown until Luna 3 sent back its blurry photographs in October 1959.

Because we never see it, the far side is often called the dark side, and it is nothing of the kind. The Moon turns relative to the Sun once every 29.5 days, so every part of it has about two weeks of daylight followed by two weeks of night. When we see a new Moon, with the near side in darkness, the far side is in full sunlight. The far side is only dark in the sense of being unseen.

It does look different. The near side has the large dark plains of solidified lava, the maria, that make up the familiar face; the far side has almost none and is covered instead in pale, heavily cratered highlands. The best explanation is that the crust on the far side is thicker, so lava from the interior rarely broke through. Why the crust is thicker is still debated, with the heat of the young Earth warming the near side being one candidate. China's Chang'e 4 became the first spacecraft to land on the far side in 2019.

Tidal braking runs in both directions. The Moon raises tides on Earth, the Earth's faster spin carries the tidal bulge ahead, and the Moon's pull on that bulge slows the Earth down while flinging the Moon into a slightly higher orbit. The effects are measurable: the day lengthens by about 2 milliseconds a century, and laser reflectors left by the Apollo astronauts show the Moon receding at 3.8 centimetres a year. Fossil corals with daily growth lines record that 400 million years ago a year had around 400 days.

Given long enough, the Earth would lock to the Moon as well, keeping one face towards it, with a day and a month both around 47 of our current days. The Sun will swell into a red giant long before that happens. The chain of cause and effect:

• •Earth's gravity stretches the Moon into a slight bulge
• •The bulge lagged behind while the Moon spun faster than it orbited
• •The pull on the lagging bulge braked the spin until it matched the orbit
• •One rotation per orbit means one face towards Earth
• •Librations let us see 59 percent; probes have mapped the rest

Tidal locking does not always end with one rotation per orbit. Mercury, the planet closest to the Sun, spins exactly three times for every two trips around it. Its orbit is quite elongated, and the Sun's tidal grip is strongest at the closest point, so the planet settled into a rhythm in which the same long axis points at the Sun at each closest approach. Astronomers only discovered this in the 1960s using radar, having assumed for decades that Mercury kept one face to the Sun like the Moon does to Earth.

Tidal locking matters well beyond the solar system. Many of the rocky planets found around red dwarfs, small cool stars, orbit so close to them that they are expected to be locked, with one side in permanent daylight and the other in permanent night. Whether such a world could keep a stable climate is genuinely debated. A thin atmosphere might freeze out on the dark side, while a thicker one, or an ocean, could carry heat around and leave a mild band near the boundary between day and night. Because red dwarfs are the commonest stars, the answer affects how many habitable planets there might be.

Tidal locking makes the sky look very strange from the lunar surface. If you stood on the near side, the Earth would hang in almost the same spot all month, never rising or setting, only drifting a little as the librations rock the Moon back and forth. It would go through phases opposite to the Moon's: when we see a thin crescent Moon, an astronaut there would see a nearly full Earth. That bright Earth lights the unlit part of the crescent with a faint grey glow called earthshine, which Leonardo da Vinci correctly explained in his notebooks about five centuries ago.

From the far side the Earth never appears at all, and that has practical consequences. A spacecraft there cannot talk directly to home, which is why China placed a relay satellite called Queqiao beyond the Moon in 2018 before landing Chang'e 4. The same shielding makes the far side attractive to astronomers. The Earth is a loud source of radio noise from broadcasting, radar and its own upper atmosphere, and three and a half thousand kilometres of lunar rock block it out, so the far side is often proposed as the quietest nearby place for a radio telescope listening to the early universe.

Because the far side of the Moon always faces away from Earth, a spacecraft there cannot talk to mission control directly, which is one reason no mission landed there until 2019. That January, China's Chang'e 4 touched down in a large crater in the southern part of the far side and released a small rover called Yutu-2.

To keep in contact, China first launched a relay satellite, Queqiao, which sits in a looping orbit beyond the Moon where it can see both the far side and Earth, passing messages between them. The far side's terrain differs from the near side, with fewer of the dark lava plains and more heavily cratered highlands, so landing there opened up new questions about why the Moon's two faces look so different.

The Moon shows one face because it rotates exactly once per orbit, a rate the Earth's tidal pull forced on it long ago. Both sides get the same sunlight, the far side simply faces away from us, and the same slow braking is lengthening the Earth's day and pushing the Moon gently outward.