The far side of the Moon, also called the dark side of the Moon, is the hemisphere of the Moon that faces away from Earth; the opposite hemisphere being the near side. Due to tidal locking, the time it takes for the Moon to orbit the Earth once is equal to the time it takes for the Moon to rotate once, thus, the far side of the Moon never fully comes into view from Earth's surface. The far side has sometimes been called the "dark side of the Moon", where "dark" means "unseen" instead of "unilluminated". There is however a common misconception that the dark side of the Moon is so-called because it never receives light, when each location on the Moon experiences two weeks of sunlight while the opposite location experiences night. The far side is actually more reflective than the near side, as it lacks the large areas of darker maria surface.
About 18% of the far side is occasionally visible from Earth due to oscillation and to libration. The remaining 82% 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 in person when they orbited the Moon in December of 1968. Compared to the near side, the far side's terrain is rugged, with a multitude of impact craters and relatively few flat and dark lunar maria ("seas"), giving it an appearance closer to other barren places in the Solar System such as Mercury and Callisto. It has one of the largest craters in the Solar System, the South Pole–Aitken basin.
All crewed and uncrewed soft landings had taken place on the near side of the Moon, until January 3, 2019, when the Chang'e 4 spacecraft made the first landing on the far side. The Chang'e 6 sample-return mission was launched on May 3, 2024, landed in the Apollo basin in the southern hemisphere of the lunar far side and returned to Earth a month later on June 25 with humanity's first lunar samples retrieved from the far side.
Astronomers have suggested installing a large radio telescope on the far side, where the Moon would shield it from possible radio interference from Earth.
Definition
Tidal forces from Earth have slowed 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 crescent-shaped edges 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").
A common misconception holds that the Moon does not rotate on its axis. If that were so, the whole of the Moon would be visible to Earth over the course of its orbit. Instead, its rotation period matches its orbital period, meaning it turns around once for every orbit it makes: in Earth terms, it could be said that its day and its year have the same length (i.e., ~29.5 earth days).
Despite a common misconception, the phrase "dark side of the Moon" does not refer to "dark" as in the absence of light, but rather "dark" as in unseen: until humans were able to send spacecraft around the Moon, this area had never been seen. In reality, both the near and far sides receive, on average, almost equal amounts of light directly from the Sun. This symmetry is complicated by sunlight reflected from the Earth onto the near side (earthshine), and by lunar eclipses, which occur only when the far side is already dark. Lunar eclipses mean that the side facing Earth receives fractionally less sunlight than the far side when considered over a long period of time.
At night under a "full Earth" the near side of the Moon receives on the order of 10 lux of illumination (about what a city sidewalk under streetlights gets; this is 34 times more light than is received on Earth under a full Moon) whereas the far side of the Moon during the lunar night receives only about 0.001 lux of starlight. 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.
Differences
The two hemispheres of the Moon have dramatically different appearances, with the near side covered in multiple, large maria (Latin for 'seas', since the earliest astronomers incorrectly thought that these plains were seas of lunar water).
The far side has a battered, densely cratered appearance with few maria. Only 1% of the surface of the far side is covered by maria, compared to 31.2% on the near side, making the far side brighter. One commonly accepted explanation for this difference is related to a higher concentration of heat-producing elements on the near-side hemisphere, as has been demonstrated by geochemical maps obtained from the Lunar Prospector gamma-ray spectrometer. While other factors, such as surface elevation and crustal thickness, could also affect where basalts erupt, these do not explain why the far side South Pole–Aitken basin (which contains the lowest elevations of the Moon and possesses a thin crust) was not as volcanically active as Oceanus Procellarum on the near side.
It has also been proposed that the differences between the two hemispheres may have been caused by a collision with a smaller companion moon that also originated from the Theia collision. In this model, the impact led to an accretionary pile rather than a crater, contributing a hemispheric layer of extent and thickness that may be consistent with the dimensions of the far side highlands. The chemical composition of the far side is inconsistent with this model.
The far side has more visible craters. This is thought to be a result of the effects of lunar lava flows, which cover and obscure craters, rather than a shielding effect from the Earth. NASA calculates that the Earth obscures only about 4 square degrees out of 41,000 square degrees of the sky as seen from the Moon. "This makes the Earth negligible as a shield for the Moon [and] it is likely that each side of the Moon has received equal numbers of impacts, but the resurfacing by lava results in fewer craters visible on the near side than the far side, even though both sides have received the same number of impacts."Near-side/far-side impact crater counts by David Morrison and Brad Bailey, NASA. http://lunarscience.nasa.gov/?question=3318. Accessed 9 January 2013.
Newer research suggests that heat from Earth at the time when the Moon was formed is the reason the near side has fewer impact craters. The lunar crust consists primarily of plagioclases formed when aluminium and calcium condensed and combined with silicates in the mantle. The cooler far side experienced condensation of these elements sooner and so formed a thicker crust; meteoroid impacts on the near side would sometimes penetrate the thinner crust here and release basaltic lava that created the maria, but would rarely do so on the far side.
The far side exhibits more extreme variations in terrain elevation than the near side. The Moon's highest and lowest points, along with its tallest mountains measured from base to peak, are all located on the far side.The Moon's Highs and Lows
Exploration Early exploration
Until the late 1950s, little was known about the far side of the Moon. Librations periodically allowed limited glimpses of features near the lunar limb on the far side, but only up to 59% of the total surface of the Moon. These features were seen from a low angle, hindering useful observation (it proved difficult to distinguish a crater from a mountain range). The remaining 82% of the surface on the far side remained unknown, and its properties were subject to much speculation.
An example of a far side feature that can be seen through libration is the Mare Orientale, which is a prominent impact basin spanning almost , yet this was not even named as a feature until 1906, by Julius Franz in Der Mond. The true nature of the basin was discovered in the 1960s when rectified images were projected onto a globe. The basin was photographed in fine detail by Lunar Orbiter 4 in 1967. Before space exploration began, astronomers expected that the far side would be similar to the side visible to Earth.
On 7 October 1959, the Soviet probe Luna 3 took the first photographs of the lunar far side, eighteen of them resolvable, covering one-third of the surface invisible from the Earth.Луна (спутник Земли), Great Soviet Encyclopedia The images were analysed, and the first atlas of the far side of the Moon was published by the USSR Academy of Sciences on 6 November 1960.АТЛАС ОБРАТНОЙ СТОРОНЫ ЛУНЫ, Ч. 1, Moscow: USSR Academy of Sciences, 1960 It included a catalog of 500 distinguished features of the landscape. Chronology, 1804–1980, to the 150th anniversary of GAISh – Moscow State University observatory. MSU In 1961, the first globe (1: scale) Moon maps and globes, created with the participation of Lunar and Planetary Research Department of SAI. SAI containing lunar features invisible from the Earth was released in the USSR, based on images from Luna 3.
On 20 July 1965, another Soviet probe, Zond 3, transmitted 25 pictures of very good quality of the lunar far side, with much better resolution than those from Luna 3. In particular, they revealed chains of craters, hundreds of kilometers in length, but, unexpectedly, no mare plains like those visible from Earth with the naked eye. In 1967, the second part of the Atlas of the Far Side of the Moon was published in Moscow,Atlas Obratnoy Storony Luny, p.2, Moscow: Nauka, 1967 based on data from Zond 3, with the catalog now including 4,000 newly discovered features of the lunar far side landscape. In the same year, the first Complete Map of the Moon (1: scale) and updated complete globe (1: scale), featuring 95 percent of the lunar surface, were released in the Soviet Union. Moon Maps. MSU
As many prominent landscape features of the far side were discovered by Soviet space probes, Soviet scientists selected names for them. This caused some controversy, though the Soviet Academy of Sciences selected many non-Soviet names, including Jules Verne, Marie Curie and Thomas Edison. The International Astronomical Union later accepted many of the names.
Further survey missions
On 26 April 1962, NASA's Ranger 4 space probe became the first spacecraft to impact the far side of the Moon, although it failed to return any scientific data before impact.
The first truly comprehensive and detailed mapping survey of the far side was undertaken by the American uncrewed Lunar Orbiter program launched by NASA from 1966 to 1967. Most of the coverage of the far side was provided by the final probe in the series, Lunar Orbiter 5.
The far side was first seen directly by human eyes during the Apollo 8 mission in December, 1968. Astronaut William Anders described the view:
It has been seen directly by all 24 men who flew on Apollo 8 and Apollo 10 through Apollo 17 and the three men and one woman on Artemis II, and photographed by multiple lunar probes. Spacecraft passing behind the Moon were out of direct radio communication with the Earth, and had to wait until the orbit allowed transmission. During the Apollo missions, the main engine of the Service Module was fired when the vessel was behind the Moon, producing some tense moments in Mission Control before the craft reappeared.