Far side of the Moon

Far side of the Moon, photographed by Apollo 16

The far side of the Moon is the hemisphere of the Moon that always faces away from Earth. The far side's terrain is rugged with a multitude of impact craters and relatively few flat lunar maria. It has one of the largest craters in the Solar System, the South Pole–Aitken basin. Both sides of the Moon experience two weeks of sunlight followed by two weeks of night; the far side is sometimes called the "dark side of the Moon", meaning unseen rather than lacking light.[1][2][3][4]

About 18 percent of the far side is occasionally visible from Earth due to libration. The remaining 82 percent remained unobserved until 1959, when it was photographed by the Soviet Luna 3 space probe. The Soviet Academy of Sciences published the first atlas of the far side in 1960. The Apollo 8 astronauts were the first humans to see the far side with the naked eye when they orbited the Moon in 1968. All manned and unmanned soft landings had taken place on the near side of the Moon, until 3 January 2019 when the Chang'e 4 spacecraft made the first landing on the far side.[5]

Astronomers have suggested installing a large radio telescope on the far side, where the Moon would shield it from possible radio interference from Earth.[6]

Definition

Due to tidal locking, the inhabitants of the central body (Earth) will never be able to see the satellite's (Moon) green area

Tidal forces from Earth have slowed down the Moon's rotation to the point where the same side is always facing the Earth—a phenomenon called tidal locking. The other face, most of which is never visible from the Earth, is therefore called the "far side of the Moon". Over time, some parts of the far side can be seen due to libration.[7] In total, 59 percent of the Moon's surface is visible from Earth at one time or another. Useful observation of the parts of the far side of the Moon occasionally visible from Earth is difficult because of the low viewing angle from Earth (they cannot be observed "full on").

The phrase "dark side of the Moon" does not refer to "dark" as in the absence of light, but rather "dark" as in unknown: until humans were able to send spacecraft around the Moon, this area had never been seen.[1][2][3][4] While many misconstrue this to think that the "dark side" receives little to no sunlight, in reality, both the near and far sides receive (on average) almost equal amounts of light directly from the Sun. However, the near side also receives sunlight reflected from the Earth, known as earthshine. Earthshine does not reach the area of the far side that cannot be seen from Earth. Only during a full Moon (as viewed from Earth) is the whole far side of the Moon dark. The word "dark" has expanded to refer also to the fact that communication with spacecraft can be blocked while the spacecraft is on the far side of the Moon, during Apollo space missions for example.[8]

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