North pole right ascension
North pole declination
|Surface absorbed dose rate||13.2 μGy/h (during lunar daytime)|
|Surface equivalent dose rate||57.0 μSv/h (during lunar daytime)|
|29.3 to 34.1 arcminutes[d]|
|Composition by volume|
The Moon is
The Moon orbits Earth at an
The most widely accepted
Both the Moon's prominence in Earth's sky and its regular cycle of phases have provided cultural references and influences for human societies throughout history. Such influences can be found in language, calendar systems, art, and mythology. The first artificial object to reach the Moon was the Soviet Union's uncrewed Luna 2 spacecraft in 1959; this was followed by the first successful soft landing by Luna 9 in 1966. The only human lunar missions to date have been those of the United States' Apollo program, which landed twelve men on the surface between 1969 and 1972. These and later uncrewed missions returned lunar rocks that have been used to develop a detailed geological understanding of the Moon's origins, internal structure, and subsequent history. The Moon is the only celestial body visited by humans.
Names and etymology
Occasionally, the name Luna /ˈluːnə/ is used in scientific writing and especially in science fiction to distinguish the Earth's moon from others, while in poetry "Luna" has been used to denote personification of the Moon. Cynthia /ˈsɪnθiə/ is another poetic name, though rare, for the Moon personified as a goddess, while Selene /səˈliːniː/ (literally "Moon") is the Greek goddess of the Moon.
The usual English adjective pertaining to the Moon is "lunar", derived from the Latin word for the Moon, lūna. The adjective selenian /səliːniən/, derived from the Greek word for the Moon, σελήνη selēnē, and used to describe the Moon as a world rather than as an object in the sky, is rare, while its cognate selenic was originally a rare synonym but now nearly always refers to the chemical element selenium. The Greek word for the Moon does however provide us with the prefix seleno-, as in selenography, the study of the physical features of the Moon, as well as the element name selenium.
The Greek goddess of the wilderness and the hunt,
Lunar geologic timescale
The lunar geological periods are named after their characteristic features, from most
The prevailing theory is that the Earth–Moon system formed after a
Giant impacts are thought to have been common in the early Solar System. Computer simulations of giant impacts have produced results that are consistent with the mass of the lunar core and the angular momentum of the Earth–Moon system. These simulations show that most of the Moon derived from the impactor, rather than the proto-Earth. However, more recent simulations suggest a larger fraction of the Moon derived from the proto-Earth. Other bodies of the inner Solar System such as Mars and Vesta have, according to meteorites from them, very different oxygen and tungsten isotopic compositions compared to Earth. However, Earth and the Moon have nearly identical isotopic compositions. The isotopic equalization of the Earth-Moon system might be explained by the post-impact mixing of the vaporized material that formed the two, although this is debated.
The impact would have released enough energy to liquefy both the ejecta and the Earth's crust, forming a magma ocean. The liquefied ejecta could have then re-accreted into the Earth–Moon system. Similarly, the newly formed Moon would have had its own lunar magma ocean; its depth is estimated from about 500 km (300 miles) to 1,737 km (1,079 miles).
While the giant-impact theory explains many lines of evidence, some questions are still unresolved, most of which involve the Moon's composition.
After the Moon's formation it settled into a much closer Earth orbit than it has today. Each body therefore appeared much larger in the sky of the other,
Since cooling and stripped of most of
The Moon is a very slightly
Size and mass
The Moon is by size and mass the fifth largest natural satellite of the Solar System, categorizeable as one of its
The Moon's diameter is about 3,500 km, more than a quarter of Earth's, with the face of the Moon comparable to the width of Australia. The whole surface area of the Moon is about 38 million square kilometers, between the size of the Americas (North and South America) and Africa.
The Moon's mass is 1/81 of Earth's, being the second densest among the planetary moons, and having the second highest surface gravity, after Io, at 0.1654 g and an escape velocity of 2.38 km/s (8600 km/h; 5300 mph).
The Moon is a differentiated body that was initially in hydrostatic equilibrium but has since departed from this condition. It has a geochemically distinct crust, mantle, and core. The Moon has a solid iron-rich inner core with a radius possibly as small as 240 kilometres (150 mi) and a fluid outer core primarily made of liquid iron with a radius of roughly 300 kilometres (190 mi). Around the core is a partially molten boundary layer with a radius of about 500 kilometres (310 mi). This structure is thought to have developed through the fractional crystallization of a global magma ocean shortly after the Moon's formation 4.5 billion years ago.
Crystallization of this magma ocean would have created a
The Moon is the second-densest satellite in the Solar System, after Io. However, the inner core of the Moon is small, with a radius of about 350 kilometres (220 mi) or less, around 20% of the radius of the Moon. Its composition is not well understood, but is probably metallic iron alloyed with a small amount of sulfur and nickel; analyses of the Moon's time-variable rotation suggest that it is at least partly molten. The pressure at the lunar core is estimated to be 5 GPa (49,000 atm).
On average the Moon's surface gravity is 1.62 m/s2 (0.1654 g; 5.318 ft/s2), about half of the surface gravity of Mars and about a sixth of Earth's.
The Moon's gravitational field is not uniform. The details of the gravitational field have been measured through tracking the Doppler shift of radio signals emitted by orbiting spacecraft. The main lunar gravity features are mascons, large positive gravitational anomalies associated with some of the giant impact basins, partly caused by the dense mare basaltic lava flows that fill those basins. The anomalies greatly influence the orbit of spacecraft about the Moon. There are some puzzles: lava flows by themselves cannot explain all of the gravitational signature, and some mascons exist that are not linked to mare volcanism.
The Moon has an external magnetic field of less than 0.2 nanoteslas, or less than one hundred thousandth that of Earth. The Moon does not currently have a global dipolar magnetic field and only has crustal magnetization likely acquired early in its history when a dynamo was still operating. However, early in its history, 4 billion years ago, its magnetic field strength was likely close to that of Earth today. This early dynamo field apparently expired by about one billion years ago, after the lunar core had completely crystallized. Theoretically, some of the remnant magnetization may originate from transient magnetic fields generated during large impacts through the expansion of plasma clouds. These clouds are generated during large impacts in an ambient magnetic field. This is supported by the location of the largest crustal magnetizations situated near the antipodes of the giant impact basins.
The Moon has an
Studies of Moon magma samples retrieved by the Apollo missions demonstrate that the Moon had once possessed a relatively thick atmosphere for a period of 70 million years between 3 and 4 billion years ago. This atmosphere, sourced from gases ejected from lunar volcanic eruptions, was twice the thickness of that of present-day Mars. The ancient lunar atmosphere was eventually stripped away by solar winds and dissipated into space.
The surface is exposed to drastic temperature differences ranging from 140 °C to −171 °C depending on the solar irradiance.
Because of the lack of atmosphere, temperatures of different areas vary particularly upon whether they are in sunlight or shadow, making topographical details play a decisive role on local surface temperatures.
Parts of many craters, particularly the bottoms of many polar craters,
Blanketed on top of the Moon's crust is a highly comminuted (broken into ever smaller particles) and impact gardened mostly gray surface layer called regolith, formed by impact processes. The finer regolith, the lunar soil of silicon dioxide glass, has a texture resembling snow and a scent resembling spent gunpowder. The regolith of older surfaces is generally thicker than for younger surfaces: it varies in thickness from 10–15 m (33–49 ft) in the highlands and 4–5 m (13–16 ft) in the maria. Beneath the finely comminuted regolith layer is the megaregolith, a layer of highly fractured bedrock many kilometers thick.
These extreme conditions for example are considered to make it unlikely for spacecraft to harbor bacterial spores at the Moon longer than just one lunar orbit.
The discovery of fault scarp cliffs suggest that the Moon has shrunk by about 90 metres (300 ft) within the past billion years. Similar shrinkage features exist on Mercury. Mare Frigoris, a basin near the north pole long assumed to be geologically dead, has cracked and shifted. Since the Moon doesn't have tectonic plates, its tectonic activity is slow and cracks develop as it loses heat.
The main features visible from Earth by the naked eye are dark and relatively featureless lunar plains called
Almost all maria are on the near side of the Moon, and cover 31% of the surface of the near side compared with 2% of the far side. This is likely due to a concentration of heat-producing elements under the crust on the near side, which would have caused the underlying mantle to heat up, partially melt, rise to the surface and erupt. Most of the Moon's mare basalts erupted during the Imbrian period, 3.3–3.7 billion years ago, though some being as young as 1.2 billion years and as old as 4.2 billion years.
In 2006, a study of
The lighter-colored regions of the Moon are called terrae, or more commonly highlands, because they are higher than most maria. They have been radiometrically dated to having formed 4.4 billion years ago, and may represent
The concentration of maria on the near side likely reflects the substantially thicker crust of the highlands of the Far Side, which may have formed in a slow-velocity impact of a second moon of Earth a few tens of millions of years after the Moon's formation. Alternatively, it may be a consequence of asymmetrical tidal heating when the Moon was much closer to the Earth.
A major geologic process that has affected the Moon's surface is
High-resolution images from the Lunar Reconnaissance Orbiter in the 2010s show a contemporary crater-production rate significantly higher than was previously estimated. A secondary cratering process caused by distal ejecta is thought to churn the top two centimeters of regolith on a timescale of 81,000 years. This rate is 100 times faster than the rate computed from models based solely on direct micrometeorite impacts.
Lunar swirls are enigmatic features found across the Moon's surface. They are characterized by a high albedo, appear optically immature (i.e. the optical characteristics of a relatively young regolith), and often have a sinuous shape. Their shape is often accentuated by low albedo regions that wind between the bright swirls. They are located in places with enhanced surface magnetic fields and many are located at the antipodal point of major impacts. Well known swirls include the Reiner Gamma feature and Mare Ingenii. They are hypothesized to be areas that have been partially shielded from the solar wind, resulting in slower space weathering.
Presence of water
Liquid water cannot persist on the lunar surface. When exposed to solar radiation, water quickly decomposes through a process known as
In years since, signatures of water have been found to exist on the lunar surface.
In May 2011, 615–1410 ppm water in
Analysis of the findings of the Moon Mineralogy Mapper (M3) revealed in August 2018 for the first time "definitive evidence" for water-ice on the lunar surface. The data revealed the distinct reflective signatures of water-ice, as opposed to dust and other reflective substances. The ice deposits were found on the North and South poles, although it is more abundant in the South, where water is trapped in permanently shadowed craters and crevices, allowing it to persist as ice on the surface since they are shielded from the sun.
In October 2020, astronomers reported detecting molecular water on the sunlit surface of the Moon by several independent spacecraft, including the Stratospheric Observatory for Infrared Astronomy (SOFIA).
The Earth and the Moon form the Earth-Moon satellite system with a shared center of mass, or barycenter. This barycenter stays located at all times 1,700 km (1,100 mi) (about a quarter of Earth's radius) beneath the Earth's surface, making the Moon seemingly orbit the Earth.
The orbital eccentricity is 0.055, indicating a slightly elliptical orbit. The
The Moon makes a complete orbit around Earth with respect to the fixed stars, its
Unlike most satellites of other planets, the Moon's orbital plane is closer to the
The gravitational attraction that Earth and the Moon (as well as the Sun) exert on each other manifests in a slightly greater attraction on the sides of closest to each other, resulting in
The lunar solid crust experiences tides of around 10 cm (4 in) amplitude over 27 days, with three components: a fixed one due to Earth, because they are in
The cumulative effects of stress built up by these tidal forces produces
The most commonly known effect of tidal forces are elevated sea levels called ocean tides.
The tides are two bulges in the Earth's oceans, one on the side facing the Moon and the other on the side opposite. As the Earth rotates on its axis, one of the ocean bulges (high tide) is held in place "under" the Moon, while another such tide is opposite. As a result, there are two high tides, and two low tides in about 24 hours. Since the Moon is orbiting the Earth in the same direction of the Earth's rotation, the high tides occur about every 12 hours and 25 minutes; the 25 minutes is due to the Moon's time to orbit the Earth.
If the Earth were a water world (one with no continents) it would produce a tide of only one meter, and that tide would be very predictable, but the ocean tides are greatly modified by other effects:
- the frictional coupling of water to Earth's rotation through the ocean floors
- the inertia of water's movement
- ocean basins that grow shallower near land
- the sloshing of water between different ocean basins
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.
Delays in the tidal peaks of both ocean and solid-body tides cause torque in opposition to the Earth's rotation. This "drains" angular momentum and rotational kinetic energy from Earth's rotation, slowing the Earth's rotation. That angular momentum, lost from the Earth, is transferred to the Moon in a process known as tidal acceleration, which lifts the Moon into a higher orbit while lowering orbital speed around the Earth.
Thus the distance between Earth and Moon is increasing, and the Earth's rotation is slowing in reaction.
This tidal drag makes the rotation of the Earth and the orbital period of the Moon very slowly match. This matching first results in tidally locking the lighter body of the orbital system, as is already the case with the Moon. Theoretically, in 50 billion years, the Earth's rotation will have slowed to the point of matching the Moon's orbital period, causing the Earth to always present the same side to the Moon. However, the Sun will become a red giant, engulfing the Earth-Moon system, long before then.
Position and appearance
The Moon's highest
The apparent orientation of the Moon depends on its position in the sky and the hemisphere of the Earth from which it is being viewed. In the
The distance between the Moon and Earth varies from around 356,400 km (221,500 mi) (perigee) to 406,700 km (252,700 mi) (apogee), making the Moon's apparent size fluctuate. On average the Moon's angular diameter is about 0.52°, roughly the same apparent size as the Sun (see § Eclipses). In addition, a purely psychological effect, known as the Moon illusion, makes the Moon appear larger when close to the horizon.
Despite the Moon's tidal locking, the effect of libration makes about 59% of the Moon's surface visible from Earth over the course of one month.
The Moon originally rotated at a faster rate, but early in its history its rotation slowed and became tidally locked in this orientation as a result of frictional effects associated with tidal deformations caused by Earth. With time, the energy of rotation of the Moon on its axis was dissipated as heat, until there was no rotation of the Moon relative to Earth. In 2016, planetary scientists using data collected on the 1998-99 NASA Lunar Prospector mission, found two hydrogen-rich areas (most likely former water ice) on opposite sides of the Moon. It is speculated that these patches were the poles of the Moon billions of years ago before it was tidally locked to Earth.
Illumination and phases
Half of the Moon's surface is always illuminated by the Sun (except during a lunar eclipse). Earth also reflects light onto the Moon, observable at times as Earthlight when it is again reflected back to Earth from areas of the near side of the Moon that are not illuminated by the Sun.
With the different positions of the Moon, different areas of it are illuminated by the Sun. This illumination of different lunar areas, as viewed from Earth, produces the different
On 14 November 2016, the Moon was at full phase closer to Earth than it had been since 1948. It was 14% closer and larger than its farthest position in apogee. This closest point coincided within an hour of a full moon, and it was 30% more luminous than when at its greatest distance because of its increased apparent diameter, which made it a particularly notable example of a "supermoon".
At lower levels, the human perception of reduced brightness as a percentage is provided by the following formula:
When the actual reduction is 1.00 / 1.30, or about 0.770, the perceived reduction is about 0.877, or 1.00 / 1.14. This gives a maximum perceived increase of 14% between apogee and perigee moons of the same phase.
There has been historical controversy over whether observed features on the Moon's surface change over time. Today, many of these claims are thought to be illusory, resulting from observation under different lighting conditions, poor astronomical seeing, or inadequate drawings. However, outgassing does occasionally occur and could be responsible for a minor percentage of the reported lunar transient phenomena. Recently, it has been suggested that a roughly 3 km (1.9 mi) diameter region of the lunar surface was modified by a gas release event about a million years ago.
Albedo and color
The Moon has an exceptionally low albedo, giving it a reflectance that is slightly brighter than that of worn asphalt. Despite this, it is the brightest object in the sky after the Sun.[j] 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. Additionally, color constancy in the visual system recalibrates the relations between the colors of an object and its surroundings, and because the surrounding sky is comparatively dark, the sunlit Moon is perceived as a bright object. The edges of the full moon seem as bright as the center, without limb darkening, because of the reflective properties of lunar soil, which retroreflects light more towards the Sun than in other directions. The Moon's color depends on the light the Moon reflects, which in turn depends on the Moon's surface and its features, having for example large darker regions. In general the lunar surface reflects a brown-tinged gray light.
Viewed from Earth the air filters the reflected light, at times giving it a red color depending on the angle of the Moon in the sky and thickness of the atmosphere, or a blue tinge depending on the particles in the air, as in cases of volcanic particles. The terms blood moon and blue moon do not necessarily refer to circumstances of red or blue moonlight, but are rather particular cultural references such as particular full moons of a year.
Eclipses only occur when the Sun, Earth, and Moon are all in a straight line (termed "
Because the Moon's orbit around Earth is inclined by about 5.145° (5° 9') to the orbit of Earth around the Sun, eclipses do not occur at every full and new moon. For an eclipse to occur, the Moon must be near the intersection of the two orbital planes. 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.
Because the Moon continuously blocks the view of a half-degree-wide circular area of the sky,[l] the related phenomenon of occultation occurs when a bright star or planet passes behind the Moon and is occulted: hidden from view. In this way, a solar eclipse is an occultation of the Sun. Because the Moon is comparatively close to Earth, occultations of individual stars are not visible everywhere on the planet, nor at the same time. Because of the precession of the lunar orbit, each year different stars are occulted.
History of exploration and human presence
Pre-telescopic observation (before 1609)
It is believed by some that 20–30,000 year old
Although the Chinese of the
During the Middle Ages, before the invention of the telescope, the Moon was increasingly recognised as a sphere, though many believed that it was "perfectly smooth".