Have you ever noticed that the face of the full Moon appears the same every time you look at it?
Although the Moon is spherical like the Earth, we only see one side of it from down here. For most of human history, no one had ever seen the so-called “dark side of the Moon,” the hemisphere of the Moon that always faces away from Earth.
Of course, this hemisphere is more accurately named “the far side of the Moon,” as it’s not actually all that dark. Another meaning for the word “dark” is “unknown,” though, so in that sense, the far side was dark to humankind for most of our history. The far side gets just as much sunlight as the side that faces us.
Characteristics of the Far Side of the Moon
The far side of the Moon is very different in character from the side we’re used to looking at. The near side of the Moon is relatively smooth and about a third of it is covered with large maria—dark, basaltic plains created by volcanic eruptions. These maria, Latin for seas, got their name because early astronomers once thought the dark patches on the Moon were bodies of water.
The far side, in contrast, is far rougher and craggier. Its surface is densely pocked with impact craters, and it features few maria compared to its other half. Only about 1% of the far side is covered in the dark formations. The craters on the far side are also quite large. One, the South Pole–Aitken basin, is actually one of the largest known craters in our Solar System.
Why Do We Only See One Side?
But why is there a far side at all? If the Moon rotates on its axis, like the Earth, shouldn’t we see all of it at one point or another?
Due to a phenomenon called tidal locking (also known as gravitational locking, captured rotation, or synchronous rotation), the Moon takes just as long to rotate on its own axis as it does to revolve around the Earth. This causes one hemisphere constantly to face the in toward the Earth.
Tidal locking occurs because the Moon and the Earth are so close together, and their gravitational pull on one another is so strong. We already know the effect the Moon can have on the Earth, influencing our tides, but the Earth’s effect on the Moon is even more dramatic, due to its greater mass.
Due to libration—the seeming oscillation of the Moon as it travels nearer and farther from the Earth in its uneven, elliptical orbit— we can actually see a small portion of the far side of the Moon on occasion. In all, it’s possible to see as much as 59% of the Moon’s surface from the Earth, though not all at once.
The other 41% of the Moon was a complete mystery to us until 1959 when the Soviet Union’s Luna 3 space probe took the first photographs of the far side. The Russian Academy of Sciences published these photos in 1960. The photos revealed only a third of the far side, or about a sixth of the Moon’s total surface. Later probes took more extensive pictures.
The Apollo 8 Crew Was The First To View It
The first humans to view this region with their own eyes were the crew of Apollo 8, which orbited the Moon in 1968. That’s the closest human beings have ever gotten. To this day, no manned lunar mission has landed on the far side of the Moon.*
And for those who can’t get the song out of their head while reading, enjoy The Dark Side of the Moon by Pink Floyd.