Full Moons can appear different sizes when rising compared to high in the sky, but they always have the dark patches in the same spots. Telescopes reveal the same craters month after month. It’s not just a full Moon thing. When the Moon is a crescent or half full, you won’t be able to see the parts in darkness, but the craters, seas, and mountains maintain position. Crossing the Earth for a different view won’t help. Things will be upside down in the other hemisphere, but you won’t spot anything new. We never see the far side from Earth precisely because it is always the far side.
The reason for this is not, as some people assume, that the Moon doesn’t rotate. It does, like every other body in the Solar System. However, it does it once every 29 days, the same rate at which it circles the Earth. Consequently, the same side is always turned towards Earth.
The alignment is not perfect. The Moon’s orbit is not perfectly circular, and as a loyal subject of Kepler’s laws, it moves faster when it is closer to the Earth and slower when it is further away. Meanwhile, its rate of rotation is constant. Consequently, at times, parts of the further hemisphere appear at the edge of what we can see on one side, while part of what is normally the near side disappears at the opposite edge. Nevertheless, this only allows us to see a few extra degrees.
Why the Moon always faces us
This situation is called being “tidally locked” or in “synchronous rotation”. It would be an astonishing coincidence if it had happened by chance, but that is not the case. The Earth and Moon affect each other’s rotation – the Moon is the reason our days are getting longer, although it takes millions of years to produce a noticeable difference.
Given the Earth’s much larger mass, it’s not surprising the Earth’s effect on the Moon is much greater. This influence pushes the length of the lunar day to align with the period of its orbit. If the Moon was rotating more quickly than it circled the Earth, it would be slowed down until the periods matched. If it was rotating more slowly, it would be sped up.
The reason this occurs is that the Earth raises tides on the Moon, just as the Moon does on Earth. If the Moon had real oceans, instead of misnamed basalt plains, these tides would be huge, because of the Earth’s larger gravity. However, even in bare rock, the force pulls the Moon out of shape, with a bulge at the point facing the Earth, and another at the exact opposite point.
After formation, the Earth’s gravity pulled more strongly on the Earthward bulge as the Moon turned, and this pull altered the rate at which the Moon spun until it matched the time it takes to complete an orbit. The rate of rotation is slowly changing to match the Moon’s glacially increasing orbit.
Are most moons tidally locked?
It’s part of being a moon to experience forces like this, adjusting the rate of spin to match its orbital period. The closer a moon is to its planet, the more extreme the gradient between the closest and furthest part, and therefore the stronger the pressure for the rotation rate to fall in line. Consequently, the only way a moon could not be tidally locked is either if it is an immense distance from its planet, or if it was only captured recently.
Some of Jupiter and Saturn’s outermost moons are thought to be recently captured asteroids, so these may not be tidally locked – in many cases we haven’t studied them well enough to tell. However, all the moons whose names you are likely to know keep one face towards their planet.
In the case of Pluto and Charon, it goes further than that. The pair are tidally locked to each other, each rotating every 6.4 Earth days, the period of their mutual orbit. The same is thought to be true for Eris and its moon Dysnomia. If the Sun lasted long enough, The Earth would become tidally locked to the Moon too.
The importance of tidal locking for other stars
Tidal locking is not just a thing for moons. We usually can’t observe it to be sure, but the basic physics says it should happen to close-in planets orbiting other stars. Indeed, Mercury was once thought to be tidally locked – some science fiction from the so-called “golden age” imagines bases built in the permanent cool of the dark side.
For stars with similar masses to the Sun, tidal locking is something that only happens to inner planets too hot to support life anyway. However, that’s not necessarily true for cooler red dwarfs. Their habitable zones are much closer to the star – close enough that we expect tidal locking to be the norm for any that are not freezing cold.
So, what does that do for the prospects of life on such a world? It will mean that a relatively small portion of the planet will have temperatures amenable to life. In some cases, this will be the part of the planet that directly faces the star. Everything else will be locked in permanent ice. Would a modest region like that be sufficient to allow life to develop and survive? We don’t know. In other cases, the side in permanent darkness would be frozen, but the area where the star was directly overhead would be too hot for life. Only in a twilight ring would temperatures be tolerable. We don’t know if life would be sustainable there either.
This is important because worlds like this are not curious exceptions. Red dwarfs easily are the most common stars in the galaxy. Almost certainly, the majority of rocky planets that have at least some portion of them at temperatures suited to liquid water fit into one of the categories described above. Maybe the overwhelming majority. If worlds like this cannot support advanced life, it helps explain why we haven’t encountered any.
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