[172] The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by Earth, is described by the saros, which has a period of approximately 18 years. [73] This is thought to be due to a concentration of heat-producing elements under the crust on the near side, seen on geochemical maps obtained by Lunar Prospector's gamma-ray spectrometer, which would have caused the underlying mantle to heat up, partially melt, rise to the surface and erupt. [113][115], The gravitational field of the Moon has been measured through tracking the Doppler shift of radio signals emitted by orbiting spacecraft. The crewed Apollo program was developed in parallel; after a series of uncrewed and crewed tests of the Apollo spacecraft in Earth orbit, and spurred on by a potential Soviet lunar human landing, in 1968 Apollo 8 made the first human mission to lunar orbit.
[63][65] The highest elevations of the surface are located directly to the northeast, and it has been suggested might have been thickened by the oblique formation impact of the South Pole–Aitken basin. The Greek goddess of the wilderness and the hunt, Artemis, equated with the Roman Diana, one of whose symbols was the Moon and who was often regarded as the goddess of the Moon, was also called Cynthia, from her legendary birthplace on Mount Cynthus. [130] These gases either return into the regolith because of the Moon's gravity or are lost to space, either through solar radiation pressure or, if they are ionized, by being swept away by the solar wind's magnetic field. [160][162] That angular momentum, lost from the Earth, is transferred to the Moon in a process (confusingly known as tidal acceleration), which lifts the Moon into a higher orbit and results in its lower orbital speed about the Earth. [161] As a result, the timing of the tides at most points on the Earth is a product of observations that are explained, incidentally, by theory. [145] The full Moon's angular diameter is about 0.52° (on average) in the sky, roughly the same apparent size as the Sun (see § Eclipses). The Moon's apparent size in the sky is almost the same as that of the Sun, since the star is about 400 times the lunar distance and diameter.
[164] [120][121] However, early in its history, 4 billion years ago, its magnetic field strength was likely close to that of Earth today. [146] The Moon is visible for two weeks every 27.3 days at the North and South Poles. [156], There has been historical controversy over whether features on the Moon's surface change over time. The ~30-day month is an approximation of the lunar cycle. [179] The Chinese astronomer Shi Shen (fl. When exposed to solar radiation, water quickly decomposes through a process known as photodissociation and is lost to space. [266][267][268][269] [180](pp415–416), In Aristotle's (384–322 BC) description of the universe, the Moon marked the boundary between the spheres of the mutable elements (earth, water, air and fire), and the imperishable stars of aether, an influential philosophy that would dominate for centuries. Today, many of these claims are thought to be illusory, resulting from observation under different lighting conditions, poor astronomical seeing, or inadequate drawings. The splitting of the moon (Arabic: انشقاق القمر) is a miracle attributed to Muhammad. [168], Eclipses only occur when the Sun, Earth, and Moon are all in a straight line (termed "syzygy"). [207] In 1994, the U.S. sent the joint Defense Department/NASA spacecraft Clementine to lunar orbit. These clouds are generated during large impacts in an ambient magnetic field. 2 [247][248][249], In 2020, U.S. President Donald Trump signed an executive order called "Encouraging International Support for the Recovery and Use of Space Resources". [253] In Greco-Roman mythology, the Sun and the Moon are represented as male and female, respectively (Helios/Sol and Selene/Luna);[252] this is a development unique to the eastern Mediterranean[252] and traces of an earlier male moon god in the Greek tradition are preserved in the figure of Menelaus.[252]. What happened after that is unknown. [72][k] This is due partly to the brightness enhancement of the opposition surge; the Moon at quarter phase is only one-tenth as bright, rather than half as bright, as at full moon. This results in it always keeping nearly the same face turned towards Earth. As a result, the Moon would be suspended in the sky over one meridian, as is already currently the case with Pluto and its moon Charon. Now, with accurate measurements of radiation from the surface, scientists have a better idea of what space travelers will have to prepare for once they arrive. Despite this, it is the brightest object in the sky after the Sun. The contrast between the brighter highlands and the darker maria creates the patterns seen by different cultures as the Man in the Moon, the rabbit and the buffalo, among others. [f][140] The Earth and the Moon's barycentre, their common center of mass, is located 1,700 km (1,100 mi) (about a quarter of Earth's radius) beneath Earth's surface. ", "Recent gravity models as a result of the Lunar Prospector mission", "Magnetometer / Electron Reflectometer Results", "LADEE - Lunar Atmosphere Dust and Environment Explorer", "Chapter 5, Appendix J: Impact Upon Lunar Atmosphere", "Lunar Outgassing, Transient Phenomena and The Return to The Moon, I: Existing Data", "NASA's LADEE Spacecraft Finds Neon in Lunar Atmosphere", "Lopsided Cloud of Dust Discovered Around the Moon", "NASA: The Moon Once Had an Atmosphere That Faded Away", "Temperature on the Moon – Surface Temperature of the Moon – PlanetFacts.org", "Global influences of the 18.61 year nodal cycle and 8.85 year cycle of lunar perigee on high tidal levels", "Is the Moon seen as a crescent (and not a "boat") all over the world?
Tally sticks, notched bones dating as far back as 20–30,000 years ago, are believed by some to mark the phases of the Moon. Scale model of the Earth–Moon system: Sizes and distances are to scale. This makes possible both total (with the Moon appearing larger than the Sun) and annular (with the Moon appearing smaller than the Sun) solar eclipses. [242], Although Luna landers scattered pennants of the Soviet Union on the Moon, and U.S. flags were symbolically planted at their landing sites by the Apollo astronauts, no nation claims ownership of any part of the Moon's surface. Larger planets tend to have more satellites, both large and small, than smaller planets.
[1], The discovery of fault scarp cliffs by the Lunar Reconnaissance Orbiter suggest that the Moon has shrunk within the past billion years, by about 90 metres (300 ft). Beside the traces of human activity on the Moon, there have been some intended permanent installations like the Moon Museum art piece, Apollo 11 goodwill messages, Lunar plaque, Fallen Astronaut memorial and other artifacts. Two NASA GRAIL spacecraft began orbiting the Moon around 1 January 2012,[220] on a mission to learn more about the Moon's internal structure. [271] Even today, people who believe in a lunar effect claim that admissions to psychiatric hospitals, traffic accidents, homicides or suicides increase during a full moon, but dozens of studies invalidate these claims.[271][272][273][274][275]. The Moon's far side has a crust that is 50 km (31 mi) thicker than that of the near side. China intends to following this up with a sample return mission (Chang'e 5) in 2020.[212]. [59] The final liquids to crystallise would have been initially sandwiched between the crust and mantle, with a high abundance of incompatible and heat-producing elements. As a result, the slightly greater attraction that the Moon has for the side of Earth closest to the Moon, as compared to the part of the Earth opposite the Moon, results in tidal forces.
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