- For other moons in the solar system see natural satellite. For other uses see Moon (disambiguation).
The Moon is the only natural satellite of Earth. It has no formal name other than "The Moon" although it is occasionally called Luna (Latin for moon) to distinguish it from the generic "moon". Its symbol is a crescent (Unicode: ☾). Apart from the word lunar, the terms selene/seleno and cynthion (from the Lunar deities Selene and Cynthia) refer also to the Moon (aposelene, selenocentric, pericynthion, etc.).
The average distance from the Moon to the Earth is 384,403 kilometres (238,857 miles). The Moon's diameter is 3,476 kilometres (2,160 miles).
Between 1969 and 1972, the U.S. Apollo program sent twelve men to land on the Moon, the first of which were Neil Armstrong and Buzz Aldrin in Apollo 11. The first men sent to the Moon were Frank Borman, James Lovell and William Anders, in Apollo 8. Before and since that time, the Moon has been the target of numerous landing and orbiting space probes, starting with the Soviet Luna 1 in 1959.
The two sides
The Moon is in a synchronous rotation with Earth, which means that one side of the Moon (the "near side") is permanently turned towards Earth. The other side, the "far side", mostly cannot be seen from Earth, except for small portions near the limb which can be seen occasionally due to libration. Most of the far side was completely unknown until the era of space probes. This synchronous rotation is a result of torque having slowed down the Moon's rotation in its early history, a process known as tidal locking.
The far side is sometimes called the "dark side". In this case "dark" means "unknown and hidden" and not "lacking light"; in fact the far side receives (on average) as much sunlight as the near side, but at opposite times. Spacecraft are cut off from direct radio communication with the Earth when on the far side of the Moon.
One distinguishing feature of the far side is its almost complete lack of maria (singular: mare), which are the dark albedo features.
The Moon makes a complete orbit about once a month. Each hour the Moon moves relative to the stars by an amount roughly equal to its angular diameter, or by about 0.5°. The Moon differs from most satellites of other planets in that its orbit is close to the plane of the ecliptic and not in the Earth's equatorial plane.
Several ways to consider a complete orbit are detailed in the table below, but the two most familiar are: the sidereal month being the time it takes to make a complete orbit with respect to the stars, about 27.3 days; and the synodic month being the time it takes to reach the same phase, about 29.5 days. These differ because in the meantime the Earth and Moon have both orbited some distance around the Sun.
The gravitational attraction that the Moon exerts on Earth is the cause of tides in the sea. Tidal flow is synchronized to the Moon's orbit around Earth. This synchronous rotation is only true on average because the Moon's orbit has definite eccentricity. When the Moon is at its perigee, its rotation is slower than its orbital motion, and this allows us to see up to an extra eight degrees of longitude of its East (right) side. Conversely, when the Moon reaches its apogee, its rotation is faster than its orbital motion and reveals another eight degrees of longitude of its West (left) side. This is called longitudinal libration. The tidal bulges on Earth, caused by the Moon's gravity, lag behind the apparent position of the Moon, due to the impedance of the ocean system - effectively the inertia of the water and the friction as it slides over the ocean bottom and into or out of bays and estuaries. As a result, some of the Earth's rotational momentum is gradually being transferred to the Moon's orbital momentum, resulting in the Moon slowly receding from Earth at the rate of approximately 38 mm per year. At the same time the Earth's rotation is gradually slowing, the Earth's day thus lengthens by about 15 µs every year. Because the lunar orbit is also inclined to the Earth's equator, the Moon seems to oscillate up and down (as a person's head does when indicating "yes") as it moves in celestial latitude (declination). This is called latitudinal libration and reveals the Moon's polar zones over about seven degrees of latitude. Finally, because the Moon is only at about 60 Earth radii distance, an observer at the equator who observes the Moon throughout the night moves by an Earth diameter sideways. This is diurnal libration and reveals about one degree's worth of lunar longitude.
Earth and Moon orbit about their barycentre, or common centre of mass, which lies about 4700 km from Earth's centre (about 3/4 of the way to the surface). Since the barycentre is located below the Earth's surface, Earth's motion is more commonly described as a "wobble". When viewed from Earth's North pole, Earth and Moon rotate counter-clockwise about their axes; the Moon orbits Earth counter-clockwise and Earth orbits the Sun counter-clockwise.
It may seem curious that the inclination of the lunar orbit and the tilt of the Moon's axis of rotation are listed as varying considerably. One must be reminded here that the orbital inclination is measured with respect to the primary's equatorial plane (in this case the Earth's), and that the axis of rotation's tilt is measured with respect to the normal to the satellite's orbital plane (the Moon's). For most planetary satellites, but not for the Moon, these conventions model physical reality and the values are therefore stable.
The Earth and the Moon form in fact a "binary planet": each one is more closely tied to the Sun than to the other. The plane of the lunar orbit maintains an inclination of 5.145 396° with respect to the ecliptic (the orbital plane of the Earth), and the lunar axis of rotation maintains an inclination of 1.5424° with respect to the normal to that same plane. The lunar orbital plane precesses quickly (i.e. its intersection with the ecliptic rotates clockwise), in 6793.5 days (18.5996 years), because of the gravitational influence of the Earth's equatorial bulge. During that period, the lunar orbital plane thus sees its inclination with respect to the Earth's equator (itself inclined 23.45° to the ecliptic) vary between 23.45° + 5.15° = 28.60° and 23.45° - 5.15° = 18.30°. Simultaneously, the axis of lunar rotation sees its tilt with respect to the Moon's orbital plane vary between 5.15° + 1.54° = 6.69° and 5.15° - 1.54° = 3.60°. Note that the Earth's tilt reacts to this process and itself varies by 0.002 56° on either side of its mean value; this is called nutation.
The points where the Moon's orbit crosses the ecliptic are called the "lunar nodes": the North (or ascending) node is where the Moon crosses to the North of the ecliptic; the South (or descending) node where it crosses to the South. Solar eclipses occur when a node coincides with the new Moon; lunar eclipses when a node coincides with the full Moon.
The Moon's periods
With respect to the distant stars (13.368 passes per tropical year)
||With respect to the Sun (phases of the Moon, 12.368 cycles per tropical year)
With respect to the vernal point (precesses in ~26,000 a)
With respect to the perigee (precesses in 3232.6 d = 8.8504 a)
||With respect to the ascending node (precesses in 6793.5 d = 18.5996 a)
Other properties of the Moon's orbit
|Metonic cycle (repeat phase/day)
|Mean distance from Earth
||~384 403 km
|Distance at perigee
||~364 397 km
|Distance at apogee
||~406 731 km
||0.0549003 = 3° 8' 44"
|Period of regression of nodes
|Period of rotation of line of apsides
|Saros cycle (repeat eclipses)
||18 years 10/11 days
|Mean inclination of orbit to ecliptic
|Mean inclination of lunar equator to ecliptic
The inclination of the Moon's orbit makes it rather unlikely that the Moon formed along with Earth or was captured later; its origin is the subject of strong scientific debate.
Early speculation proposed that the Moon broke off from the Earth's crust due to centrifugal force, leaving an ocean basin behind as a scar. This concept requires too great an initial spin of the Earth. Others speculated the Moon formed elsewhere and was captured into its orbit.
Some propose Coformation or Condensation theory, the concept that the Earth and the Moon formed at about the same time from the accretion disk. This theory fails to explain the depletion of iron in the Moon. Yet different groups propose that the Moon formed from a debris field around Earth resulting from an asteroid or planetesimal collision.
The currently accepted theory is the Giant Impact theory, in which the Moon originated from the ejecta from the collision between a semi-molten Earth and something the size of Mars (speculatively called Theia).
The geological epochs of the Moon are defined based on the dating of various significant impact events in the Moon's history.
Tidal forces deformed the once molten Moon into an ellipsoid, with the major axis pointed towards Earth.
More than 4.5 billion years ago, the surface of the Moon was a liquid magma ocean. Scientists think that one component of lunar rocks, KREEP (K-potassium, Rare Earth Elements, and P-phosphorus), represents the last chemical remnant of that magma ocean. KREEP is actually a composite of what scientists term "incompatible elements": those which cannot fit into a crystal structure and thus were left behind, floating to the surface of the magma. For researchers, KREEP is a convenient tracer, useful for reporting the story of the volcanic history of the lunar crust and chronicling the frequency of impacts by comets and other celestial bodies.
The lunar crust is composed of a variety of primary elements, including uranium, thorium, potassium, oxygen, silicon, magnesium, iron, titanium, calcium, aluminum and hydrogen. When bombarded by cosmic rays, each element bounces back into space its own radiation, in the form of gamma rays. Some elements, such as uranium, thorium and potassium, are radioactive and emit gamma rays on their own. However, regardless of what causes them, gamma rays for each element are all different from one another — each produces a unique spectral "signature", detectable by a spectrometer.
A complete global mapping of the Moon for the abundance of these elements has never been performed. However, some spacecraft have done so for portions of the Moon; Galileo did so when it flew by the Moon in 1992.  The overall composition of the Moon is believed to be similar to that of the Earth other than a depletion of volatile elements and of iron.
The Moon is covered with tens of thousands of craters having a diameter of at least 1 kilometre. Most are hundreds of millions or billions of years old; the lack of atmosphere or weather or recent geological processes ensures that most of them remain permanently preserved.
The largest crater on the Moon, and indeed the largest known crater within the solar system, forms the South Pole-Aitken basin. This crater is located on the far side, near the south pole, and is some 2,240 km in diameter, and 13 km in depth.
The dark and relatively featureless lunar plains are called maria, Latin for seas, since they were believed by ancient astronomers to be water-filled seas. They are actually vast ancient basaltic lava flows that filled the basins of large impact craters. The lighter-colored highlands are called terrae. Maria are found almost exclusively on the Lunar nearside, with the Lunar farside having only a few scattered patches. Scientists think that such asymmetry of the lunar crust was caused by the synchronization between the Moon's rotation and orbit about the Earth. This synchroniation exposes the far side of the Moon to more asteroid and meteor impacts than the near, thereby allowing the maria to remain uncratered for many hundreds of millenia.
Blanketed atop the Moon's crust is a dusty outer rock layer called regolith. Both the crust and regolith are unevenly distributed over the entire Moon. The crust ranges from 60 km (38 miles) on the near side to 100 km (63 miles) on the far side. The regolith varies from 3 to 5 metres (10 to 16 feet) in the maria to 10 to 20 metres (33 to 66 feet) in the highlands.
In 2004, a team led by Dr. Ben Bussey of Johns Hopkins University using images taken by the Clementine mission determined that four mountainous regions on the rim of the 73 km wide Peary crater at the Moon's north pole appeared to remain illuminated for the entire Lunar day. These unnamed "mountains of eternal light" are possible due to the Moon's extremely small axial tilt, which also gives rise to permanent shadow at the bottoms of many polar craters. No similar regions of eternal light exist at the less-mountainous south pole, although the rim of Shackleton crater is illuminated for 80% of the lunar day. Clementine's images were taken during the northern Lunar hemisphere's summer season, and it remains unknown whether these four mountains are shaded at any point during their local winter season.
Presence of water
Over time, comets and meteorites continually bombard the Moon. Many of these objects are water-rich. Energy from sunlight splits much of this water into its constituent elements hydrogen and oxygen, both of which usually fly off into space immediately. However, it has been hypothesized that significant traces of water remain on the Moon, either on the surface, or embedded within the crust. The results of the Clementine mission suggested that small, frozen pockets of water ice (remnants of water-rich comet impacts) may be embedded unmelted in the permanently shadowed regions of the lunar crust. Although the pockets are thought to be small, the overall amount of water was suggested to be quite significant — 1 km³.
Some water molecules, however, may have literally hopped along the surface and gotten trapped inside craters at the lunar poles. Due to the very slight "tilt" of the Moon's axis, only 1.5°, some of these deep craters never receive any light from the Sun — they are permanently shadowed. Clementine has mapped () craters at the lunar south pole () which are shadowed in this way. It is in such craters that scientists expect to find frozen water if it is there at all. If found, water ice could be mined and then split into hydrogen and oxygen by solar panel-equipped electric power stations or a nuclear generator. The presence of usable quantities of water on the Moon would be an important factor in rendering lunar habitation cost-effective, since transporting water (or hydrogen and oxygen) from Earth would be prohibitively expensive.
The equatorial Moon rock collected by Apollo astronauts contained no traces of water. Neither the Lunar Prospector nor more recent surveys, such as those of the Smithsonian Institution, have found direct evidence of lunar water, ice, or water vapour. Lunar Prospector results, however, indicate the presence of hydrogen in the permanently shadowed regions, which could be in the form of water ice.
Compared to that of Earth, the Moon has a very weak magnetic field. While some of the Moon's magnetism is thought to be intrinsic (such as a strip of the lunar crust called the Rima Sirsalis), collision with other celestial bodies might have imparted some of the Moon's magnetic properties. Indeed, a long-standing question in planetary science is whether an airless solar system body, such as the Moon, can obtain magnetism from impact processes such as comets and asteroids. Magnetic measurements can also supply information about the size and electrical conductivity of the lunar core — evidence that will help scientists better understand the Moon's origins. For instance, if the core contains more magnetic elements (such as iron) than Earth, then the impact theory loses some credibility (although there are alternate explanations for why the lunar core might contain less iron).
The Moon has a relatively insignificant and tenuous atmosphere. One source of this atmosphere is outgassing — the release of gases, for instance radon, which originate deep within the Moon's interior. Another important source of gases is the solar wind, which is briefly captured by the Moon's gravity.
By what can only be a truly extraordinary coincidence, the angular diameters of the Moon and the Sun as seen from Earth overlap in their variation, so that both total and annular solar eclipses are possible. In a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye.
Since the distance between the Moon and the Earth is very slightly increasing over time, the angular diameter of the Moon is decreasing. This means that several million years ago the Moon always completely covered the Sun on solar eclipses so that no annular eclipses occurred. Likewise, in several million years the Moon will no longer cover the Sun completely and no total eclipses will occur.
Eclipses happen only if Sun, Earth and Moon are lined up. Solar eclipses can only occur at new moon; lunar eclipses can only occur at full moon.
See also Solar eclipse and Lunar Eclipse.
Observation of the Moon
The Moon (and also the Sun) appear larger when close to the horizon. This is a purely psychological effect (see Moon illusion). The angular diameter of the Moon from Earth is about one half of one degree.
Various lighter and darker colored areas (primarily maria) create the patterns seen by different cultures as the Man in the Moon, the rabbit and the buffalo, amongst others. Craters and mountain chains are also prominent lunar features.
During the brightest full moons, the Moon can have an apparent magnitude of about −12.6. For comparison, the Sun has an apparent magnitude of −26.8.
The Moon is most clear at night, but can sometimes be seen during the day.
For any location on Earth, the highest altitude of the Moon on a day varies between the same limits as the Sun, and depends on season and lunar phase. For example, in winter the Moon comes highest when it is full, and the full moon comes highest in winter.
Like the Sun, the Moon can also give rise to an optical effect known as a halo.
See also: Lunar phase.
Exploration of the Moon
The first man-made object to reach the Moon was the unmanned Soviet probe Luna 2, which crashed into it on September 14, 1959, at 21:02:24 Z. The far side of the Moon was first photographed on October 7, 1959 by the Soviet probe Luna 3. Luna 9 was the first probe to soft land on the Moon and transmit pictures from the Lunar surface on February 3, 1966. The first artificial satellite of the Moon was the Soviet probe Luna 10 (launched March 31, 1966).
On December 24, 1968 the crew of Apollo 8, Frank Borman, James Lovell, and William Anders became the first human beings to see the far side of the Moon.
Humans first landed on the Moon on July 20, 1969 as the culmination of a Cold War-inspired space race between the Soviet Union and the United States of America. The first man to walk on the lunar surface was Neil Armstrong, commander of the American mission Apollo 11. The last man to stand on the Moon was Eugene Cernan, who as part of the mission Apollo 17 walked on the Moon in December 1972. See also: A full list of lunar astronauts.
The Apollo 11 crew left a 9 by 7 inch (23 by 18 cm) stainless steel plaque on the Moon, to commemorate the landing and provide basic information of the visit to any other beings who may eventually see it. The plaque reads:
- Here men from the Planet Earth first set foot upon the Moon, July 1969, A.D.
- We came in peace for all mankind
The plaque depicts two hemispheres of planet Earth, and is signed by the three astronauts, as well as US President Richard Nixon.
Moon samples have been brought back to Earth from these six manned missions as well as from three Luna missions (nrs. 16, 20, and 24).
In February 2004, US President George W. Bush called for a plan to return manned missions to the Moon by 2020. The European Space Agency and People's Republic of China both have plans to launch probes to explore the Moon in the near future, too. European spacecraft Smart 1 was launched September 27, 2003 and entered lunar orbit on November 15, 2004 . It will survey the lunar environment and create an X-ray map of the Moon.   China has expressed ambitious plans for exploring the Moon and is investigating the prospect of lunar mining, specifically looking for the isotope Helium-3 for use as an energy source on Earth.  For more information about China's first Moon mission, see Chang'e program. Japan and India are on the waiting list for the Moon, too. Japan already outlined its upcoming missions to our neighbour Lunar-A  and Selene . Even a manned lunar base is planned by the Japanese Space Agency (JAXA). India will first try an unmanned orbiting satellite, called Chandrayan.
For escaping at the surface of the Moon from the Moon and the Earth, the escape velocity is the square root of the sum of the squares of the separate escape velocities of 2.4 and 1.5 km/s, or 2.8 km/s. Thus, using the orbital speed of 1.1 km/s, a delta-v of 2.4 km/s, just enough for escaping the Moon, is more than enough to escape Earth as well.
Human understanding of the Moon
Myth and folk culture
See Moon (mythology).
See Moon (astrology)
A 5,000 year old rock carving at Knowth, Ireland may represent the Moon, in which case it is the earliest depiction yet discovered.
In many prehistoric and ancient cultures, the Moon was thought to be a deity or other supernatural phenomenon. One of the first persons in the Western world to offer a scientific explanation for the Moon was the Greek philosopher Anaxagoras, who reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former. For teaching this heresy Anaxagoras was imprisoned by the authorities and sentenced to death.
By the Middle Ages, before the invention of the telescope, more and more people began to recognize the Moon as a sphere, though they believed that it was "perfectly smooth".
In 1609, Galileo Galilei drew one of the first telescopic drawings of the Moon in his book Sidereus Nuncius and noted that it was not smooth but had craters. Later in the 17th century, Giovanni Battista Riccioli and Francesco Maria Grimaldi drew a map of the Moon and gave many craters the names they still have today.
On maps, the dark parts of the Moon's surface were called maria (singular mare) or "seas", and the light parts were called terrae or continents. The possibility that the Moon could contain vegetation and be inhabited by "selenites" was seriously considered by some major astronomers even into the first decades of the 19th century.
In 1835, the Great Moon Hoax fooled some people into thinking that there were exotic animals living on the Moon. Almost at the same time however (during 1834–1836), Wilhelm Beer and Johann Heinrich Mädler were publishing their four-volume Mappa Selenographica and the book Der Mond in 1837, which firmly established the conclusion that the Moon has no bodies of water nor any appreciable atmosphere.
There remained some controversy over whether features on the Moon could undergo changes. Some observers claimed that some small craters had appeared or disappeared, but in the 20th century it was determined that these claims were illusory, due to observing under different lighting conditions or due to the inadequacy of earlier drawings. It is however known that the phenomenon of outgassing occasionally occurs.
During the Nazi era in Germany, the Welteislehre theory, which claimed the Moon was made of solid ice, was promoted by Nazi leaders.
The far side of the Moon remained completely unknown until the Luna 3 probe in 1959, and was extensively mapped by the Lunar Orbiter program in the 1960s.
Lunar location listings
Ben Bussey and Paul Spudis, The Clementine Atlas of the Moon, Cambridge University Press, 2004, ISBN 0521815282.
Patrick Moore, On the Moon, Sterling Publishing Co., 2001 edition, ISBN 0304354694.
- Paul D. Spudis, The Once and Future Moon, Smithsonian Institution Press, 1996, ISBN 1-56098-634-4.
Myth and folklore