In A Nutshell
Most of us are aware that we Earthlings see only one side of the Moon. Until the Soviet’s Luna 3 snapped a handful of pictures of it in 1959, no one had ever laid eyes on half the lunar surface. This is because the Moon rotates at the same speed it orbits the Earth (every 27.3 days) and keeps the same side facing us. But because the Moon orbits the Sun along with the Earth, the far side is not any darker—or lighter—than the side we’re familiar with. However, Luna 3 and subsequent Apollo missions revealed that that the far side does not have the coffee-colored maria or lunar seas that cover much of the near side.
The Whole Bushel
For almost all of human existence, we’ve only seen about two-thirds of the Moon’s 36 million-square-kilometer total surface. This is because the Earth and the Moon are tidal locked, with the gravity of each slowing the rotation of the other.
A trillion years from now, the Earth’s rotation will be the same as a lunar month (which by then will be about 1,000 hours long) and the Earth and Moon will waltz around the Sun constantly showing each other the same face. But Earth’s greater gravitation has already slowed the Moon’s rotation so that it takes the same number of days as it takes to orbit us, revealing to Earthlings only one lunar hemisphere.
Logically that would mean we see 50 percent of the Moon’s total surface, but because of the elliptical nature of the lunar orbit (and the slight difference it has with the solar orbit), we actually see about 59 percent of its surface.
For centuries, humans have called the unseen far side of the Moon the “dark” side, as if the Earth produced its own light and directed it like a flashlight on the near side. Mark Twain once said that “everyone is a moon, and has a dark side which he never shows to anybody.” But, as the old saw declares that the Sun shines even on a dog’s butt, so it also does on the lunar far side.
With the coming of the space age, we learned that the far side is very different than the near side. For one thing, the far side is pockmarked with a lot more impact craters.
The near side is smoother, especially around the dark marias that give the surface a dark patchy look. Many a poet has imagined these patches to be the facial features of the Man on the Moon. These lunar seas are made of black basaltic lava rock and ejecta from ancient volcanic eruptions and cover about 30 percent of the near side, but only 2 percent of the far side.
Since then, scientists have been scratching their heads about the disparity in craters and maria. They initially thought that the Earth and its gravity shielded the lunar near side from asteroids and comets but could not protect the far side. But since the distance between the two celestial bodies is 40 times greater than the size of the Earth, there is plenty of room for space rocks to insert themselves between them. Indeed, the math indicates that the Earth would block only 1 percent of the space rocks that aimed for the near side.
One of the stronger theories as to why there’s a disparity is that the same gravitational forces that locked the Moon into a 27-day rotation has also pulled the near side toward the Earth like a wad of gum pulled through clenched teeth.
The Moon is oblong, almost football-shaped, with the near side’s crust considerably thinner than the far side. And when a space rock did impact the near side’s surface, it cracked it open and allowed lava and ejecta to cover neighboring craters or form them into shallow basins. The thicker far side’s crust bore the impacts without the resulting volcanic eruptions.
In a sense then, the dark patches on the near side are the Moon Man’s bruises and black eyes in his fight with the universe.
Show Me The Proof
Featured image via Wikipedia
Hayden Planetarium: The Tidal Force
Discovery News: Why Doesn’t the Moon Spin?
KIRO TV: For the first time, we see ‘dark’ side of the moon
Medium: The Two Faces of the Moon
Nature: Gravity maps reveal why the dark side of the Moon is covered in craters