Earth's Moon

The Moon is the only natural satellite of Earth.

Luna by the Romans, Selene and Artemis by the Greeks, and many other names in other mythologies.

Moon, of course, has been known since prehistoric times. It is the second brightest object in the sky after the Sun. As the Moon orbits around the Earth once per month, the angle between the Earth, the Moon and the Sun changes; we see this as the cycle of the Moon's phases. The time between successive new moons is 29.5 days (709 hours), slightly different from the Moon's orbital period (measured against the stars) since the Earth moves a significant distance in its orbit around the Sun in that time.

The Moon is in synchronous rotation, meaning that it keeps nearly the same face turned toward Earth at all times (There is a small change however called libration). The side of the Moon that faces Earth is called the near side, and the opposite side is called the far side. The far side is also sometimes called the "dark side", which means "unknown and hidden", and not "lacking light" as might seem to be implied by the name; 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 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 the Earth approximately once every 29.5 days. 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. The tidal flow period, but not the phase, is synchronized to the Moon's orbit around Earth. The tidal bulges on Earth, caused by the Moon's gravity, are carried ahead of the apparent position of the Moon by the Earth's rotation, in part because of the friction of the water 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 mm every year. A more detailed discussion follows in the section titled Earth and Moon.

The Moon is in synchronous rotation, meaning that it keeps the same face turned to the Earth at all times. 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.

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 nodding) 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 barycenter, or common center of mass, which lies about 4700 km from Earth's center (about 3/4 of the way to the surface).

Since the barycenter 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 strange 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 plane of the lunar orbit maintains an inclination of 5.145 396 with respect to the ecliptic (the orbital plane of the Earth around the Sun), 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), mostly because of the gravitational perturbation induced by the Sun.

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.

Roughly once every 18.6 years, the declination of the Moon reaches a maximum, which is called the lunar standstill.

In the representations of the Solar system, it is common to draw the trajectory of the Earth from the point of view of the Sun, and the trajectory of the Moon from the point of view of the Earth, in a way that seems to suggest that the trajectory of the Moon circles around the Earth in such a way that sometimes it goes backwards. In fact, this never occurs. Unlike most other moons in the Solar System, the annual trajectory of the Moon is very similar to the one of the Earth and is always curved in the same way, concave towards the Sun, and nowhere looped or even convex towards the Sun.

Tidal effects

The tides on Earth are mostly generated by the Moon's gravitation, with a less significant contribution by the Sun. These gravitational effects are specifically manifested as tidal forces. The combination of the two is responsible for spring and neap tides.

Two tidal bulges, one in the direction of the Moon, and one in the opposite direction form as a result of the tidal forces. The buildup of these bulges and their movement around the earth causes an energy loss due to friction. The energy loss decreases the rotational energy of the Earth.

Since the Earth spins faster than the Moon moves around it, the tidal bulges are dragged along with the Earth's surface faster than the Moon moves, and move "in front of the Moon" .

Because of this, the Earth's gravitational pull on the Moon has a component in the Moon's "forward" direction with respect to its orbit. This component of the gravitational forces between the two bodies acts like a torque on the Earth's rotation, and transfers angular momentum and rotational energy from the Earth's spin to the Moon's orbital movement.

Because the Moon is accelerated in the forward direction, it moves to a higher orbit. As a result, the distance between the Earth and Moon increases, and the Earth's spin slows down. Measurements reveal that the Moon's distance to the Earth increases by 38 mm per year (lunar laser ranging experiments with laser reflectors are used to determine this). Atomic clocks also show that the Earth's day lengthens by about 15 microseconds (s) every year.

However, the formation of tidal bulges on Earth is irregular and not directly related to the frictional energy loss which accompanies the tides. For example, continents on Earth may cause an increase in frictional energy losses and hamper the buildup of tidal bulges.

The lunar surface is also subjected to tides from earth, and rises and falls by around 10 cm over 27 days. The lunar tides comprise a mobile component, due to the Sun, and a selenographically fixed one, due to Earth (the Moon keeps the same face turned to the Earth, but not to the Sun).

The vertical motion of the Earth-induced component comes entirely from the Moon's orbital eccentricity; if the Moon's orbit were perfectly circular, there would be solar tides only. The magnitude of the Moon's tides corresponds to a Love number of 0.0266, and supports the idea of a partially melted zone around its core. Moonquake waves lose energy below 1000 km depth, and this may also show that the deep material is at least partially melted.

The Earth's Love number is 0.3, corresponding to a movement of 0.5 metres per day; for Venus the Love number is also 0.3.

Double-planet hypothesis

Several characteristics of the Earth-Moon system distinguish it from the satellite systems of most other planets in the Solar System, including the unusually large relative size of the Moon, its great orbital distance from Earth, and the fact that the Moon's path around the Sun is always concave to the Sun, like that of the Earth (but unlike that of most other satellites in the Solar System). As a result, some observers hold that the Earth-Moon system is a double planet rather than a planet with a satellite. For more information on these alternative views, see the double planet article.

Origin and History

The inclination of the Moon's orbit makes it implausible that the Moon formed along with the Earth or was captured later; its origin is the subject of some scientific debate.

Early speculation proposed that the Moon broke off from the Earth's crust due to centrifugal force, leaving an ocean basin (presumed to be the Pacific) behind as a scar.

This concept requires too great an initial spin of the Earth and the presumption of a Pacific origin is not compatible with the geological standard model, the theory of plate tectonics. Others speculated the Moon formed elsewhere and was captured into its orbit.

Two of the other theories include the coformation or condensation theory and the impact theory, which speculates that the Moon formed from the debris that resulted from a collision between the early Earth and a planetesimal.

The coformation or condensation hypothesis posits that the Earth and the Moon formed together at about the same time from the primordial accretion disk, the Moon forming from material surrounding the coalescing proto-Earth, similar to the way the planets formed around the Sun. Some suggest that this hypothesis fails to adequately explain the depletion of iron in the Moon.

Recently, the giant impact hypothesis has been considered a more viable scientific hypothesis for the moon's origin than the coformation or condensation hypothesis. The Giant Impact hypothesis holds that the Moon formed from the ejecta resulting from a collision between a very early, semi-molten Earth and a planet-like object the size of Mars, which has been referred to as Theia or Orpheus.

The material ejected from this impact would have gathered in orbit around Earth and formed the Moon. This hypothesis is bolstered by two main observations: First, the composition of the Moon resembles that of Earth's crust, whereas it has relatively few heavy elements that would have been present if it formed by itself out of the same material from which Earth formed. Second, through radiometric dating, it has been determined that the Moon's crust formed between 20 and 30 million years after that of Earth, despite its smallness and associated larger loss of internal heat.

The geological epochs of the Moon are defined based on the dating of various significant impact events in the Moon's history. Analysis of craters and Moon rocks show that there was a late heavy bombardment by asteroids around the period 4.0 to 3.8 billion years ago.

Tidal forces deformed the once molten Moon into an ellipsoid, with the major axis pointed towards Earth.In 2005, a team of scientists from Germany, the United Kingdom, and Switzerland measured the Moon's age at 4.527 0.010 billion years, which would imply that it was formed only 3050 million years after the origin of the solar system.

Physical Characteristics


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, called KREEP (potassium, rare earth elements, and 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, aluminium and hydrogen, as determined by spectroscopy.

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 upper parts of the Earth other than a depletion of volatile elements and of iron.


When observed with earth based telescopes, the moon can be seen to have some 30,000 craters having a diameter of at least 1 kilometer, but close up observation from lunar orbit reveals a multitude of ever smaller craters. Most are hundreds of millions or billions of years old; the lack of atmosphere, weather and recent geological processes ensures that most of them remain permanently preserved.

In the lunar terrae, it is indeed impossible to add a crater of any size without obliterating another; this is termed saturation.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.

Blanketed atop the Moon's crust is a dusty outer rock layer called regolith, the result of rocks shattered by billions of years of impacts. Both the crust and regolith are unevenly distributed over the entire Moon. The crust ranges from 60 km (38 mi) on the near side to 100 km (63 mi) on the far side. The regolith varies from 3 to 5 m (10 to 16 ft) in the maria to 10 to 20 m (33 to 66 ft) 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 continuously 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 km.

Some water molecules, however, may have literally hopped along the surface and become 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[8] 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 vapor. Lunar Prospector results, however, indicate the presence of hydrogen in the permanently shadowed regions, which could be in the form of water ice.

Magnetic field

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.


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.

Occultation of stars

The Moon is continuously blocking our view of the sky directly behind it. When a bright star or planet passes behind the Moon it is occulted or hidden from view. A solar eclipse is an occultation of the Sun. Because the Moon is close to Earth, occultations of stars are not visible everywhere. Because of the moving nodes of the lunar orbit, each year different stars are occulted.

Observation of the Moon

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. When the Moon is in a quarter phase, its brightness is not one half of a full Moon. It is only about 1/10 of that, because the amount of solar radiation reflected towards the Earth is highly reduced by the shadows projected by the higher parts of the Moon over the lower ones.

The Moon appears larger when close to the horizon. This is a purely psychological effect. The angular diameter of the Moon from Earth is about one half of one degree, and is actually about 1.5% smaller when the Moon is near the horizon than when it is high in the sky (because it is further away by up to 1 Earth radius).

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.

From 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 is highest in the sky when it is full, and the full moon is highest in winter. The orientation of the Moon's crescent side also depends on the latitude of the observing site. Close to the equator an observer can see a boat Moon.

We can use the Moon to visualize Earth's trajectory: When the Moon is its third quarter, it is moving in its orbit in front of the Earth. As the distance from the Earth to the moon is about 384 404 km and the Earth's orbital speed is about 107 000 km/h, the Moon is at a point where the Earth will be about 3 and a half hours later. And when the Moon is in its first quarter, it is "where we were" about 3 and a half hours ago.

Like the Sun, the Moon can also give rise to an optical effect known as a halo. For more information on how the Moon appears in Earth's sky - Lunar phase.

Observation of the Moon

The first leap in Lunar observation was caused by the invention of the telescope. Especially Galileo Galilei made good use of this new instrument and observed mountains and craters on the Moon's surface.

The Cold War-inspired space race between the Soviet Union and the United States of America led to an acceleration. What was the next big step depends on the political viewpoint: In the US (and the West in general) the landing of the first humans on the moon in 1969 is seen as a culmination, indeed of the space race in general.

The first man to walk on the lunar surface was Neil Armstrong, commander of the American mission Apollo 11, first setting foot on the moon at 02:56 UTC on July 21, 1969. 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.

On the other hand, many scientifically important steps, such as the first photographs of the until then unseen far side of the moon in 1959, were first achieved by the Soviet Union. Moon samples have been brought back to Earth by three Luna missions (Luna 16, 20, and 24) and the Apollo missions 11 through 17 (excepting Apollo 13, which aborted its planned lunar landing).

From the mid-1960's to the mid-1970's there were 65 moon landings (with 10 in 1971 alone), but after Luna 24 in 1976 they stopped. The Soviet Union started focusing on Venus and space stations and the US on Mars and beyond.

In 1990 Japan visited the moon with the Hiten spacecraft, becoming the third country to orbit the Moon. The spacecraft released the Hagormo probe into lunar orbit, but the transmitter failed rendering the mission scientifically useless.

On January 14, 2004, US President George W. Bush called for a plan to return manned missions to the Moon by 2020. The European Space Agency has 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.

The People's Republic of 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.

Japan has two planned lunar missions, LUNAR-A and Selene, and a manned lunar base is planned by the Japanese Space Agency (JAXA). India is to launch an unmanned mission Chandrayaan-1 in 2007.

Scientific Understanding

A 5,000 year old rock carving at Knowth, Ireland may represent the Moon, which would be the earliest depiction discovered.

In many prehistoric and ancient cultures, the Moon was thought to be a deity or other supernatural phenomenon. Among the first 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. His atheistic view of the heavens was one cause for his imprisonment and eventual exile.

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.

Additional Information

The Moon in Mythology

Moon in Art and Literature The Moon as a Muse

The Moon in Astrology

New Moon