<P> About 18% of the far side is occasionally visible from Earth due to libration . The remaining 82% remained unobserved until 1959, when the Soviet Union's Luna 3 space probe photographed it . The Soviet Academy of Sciences published the first atlas of the far side in 1960 . In 1968, the Apollo 8 mission's astronauts were the first humans to view this region directly when they orbited the Moon . To date, no human being has ever stood on the surface of the far side of the Moon . </P> <P> Astronomers have suggested installing a large radio telescope on the far side, where the Moon would shield it from possible radio interference from Earth . </P> <P> Tidal forces from Earth have slowed down the Moon's rotation so that 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 . 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"). </P> <P> The idiomatic 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 . 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 which 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 also refer to the fact that communication with spacecraft can be blocked while on the far side of the Moon, during Apollo space missions for example . </P>

The far side of the moon cannot be seen from earth because
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