Figure 1. The Moon imaged through an 11-inch Schmidt-Cassegrain (30 mm eyepiece) and attached Nikon 4-megapixel camera. Courtesy: Jim Wentworth, "Fire in the Sky Observatory," Brainerd, Minnesota.

The size of the Moon and the British Isles compared

Figure 2. The size of the Moon and the British Isles compared.

Earth and Moon shown in their correct relative sizes

Figure 3. The Earth and the Moon shown in their correct relative sizes. They would need to be about 30 feet apart to show the separation distance to the same scale.


Figure 4. Lunar maria visible from Earth.

supermoon, photographed from Mazatlan, Mexico

Figure 5. A supermoon, photographed on May 5, 2012, from Mazatlan, Mexico.

The Moon is Earth's only natural satellite and the only extraterrestrial body to have been visited by humans – 12 in all aboard six Apollo missions between July 1969 and December 1972. It has also been the target of numerous robotic probes including, most recently, Clementine (1994),Lunar Prospector (1999), SMART-1 (2003), Kaguya (2007), Chang'e 1 (2007), Chandrayaan 1 (2008), Lunar Reconnaissance Orbiter, and Lunar Crater Observation & Sensing Satellite (both 2009).


A total of 382 kilograms of rock samples were returned to Earth by Apollo and the Russian Luna programs; in addition, a number of lunar meteorites have been found. Most of these samples have been dated at between 4.6 and 3 billion years (the one exception is a lunar meteorite dated at 2.8 billion years), and provide information about the early history of the Solar System which is missing on Earth due to a lack of rocks more than about 3.8 billion years old.


The Moon is unusually large relative to the size of its primary (being exceeded by Charon alone in this respect). Its crust averages 68 kilometers thick, is thinner on the near side, and varies from essentially zero thickness under Mare Crisium to a depth of 107 kilometers north of the crater Korolev on the lunar farside. Below the crust is a mantle and probably a small core, some 340 kilometers in radius, containing about 2% of the lunar mass.


Curiously, the Moon's center of mass is offset from its geometric center by about 2 kilometers in the direction toward Earth.


The prefix "seleno-" means to do with the Moon, as in selenocentric (with reference to the center of the Moon) and selenography (the study of the Moon's surface and description of its topography).


How big is the Moon compared to Earth?

The Moon's diameter is 3,474 kilometers and Earth's is 12,800 kilometers, so Earth has 3.68 times the diameter of the Moon (see Figure 2 and 3). The surface area of Earth is about 13 times that of the Moon (which has roughly the surface area of Africa), and about 50 Moons could be fitted inside Earth's volume. The Moon's mass is 81 times less than Earth's.


Origin and early history

the big splat
Theia impact hypothesis


The Moon's origin remains a matter of debate, though until recently the widely held view was that it formed over 4.5 billion years ago from debris resulting from a relatively slow, near head-on collision between Earth and a Mars-sized body, called Theia. In 2012 this theory was challenged as a result of computer simulations which showed that the impact may have an involved a much larger and faster-moving object.[1} The larger impactor would have caught Earth only a glancing blow, cleaving off a portion of our planet, which subsequently gathered back together again, from a disk of hot debris, to form the Moon. In this new scenario, very little of the impactor would have contributed to the substance of the Moon – a situation that ties in more neatly with the observation that certain isotope ratios (notably of oxygen and titanium) in lunar surface material are almost identical to those found in Earth rocks.


The outer layers of the Moon, initially molten and comprising a global "magma ocean", cooled to form the 4.5-billion-year-old rocks now found in the lunar highlands. These ancient igneous rocks known as anorthosites are rich in the silicate mineral plagioclase and impart to the lunar highlands their characteristic light color. A period of intense bombardment followed which caused extensive cratering and fragmentation of the crust. About 4 billion years ago, the Moon suffered a number of major impacts which excavated the basins referred to as maria. Subsequent volcanic activity, between about 4 and 2.5 billion years ago, flooded these basins with molten lava which then cooled and solidified to form dark basalt. Since that time, the Moon has changed little except for the occasional impact of a meteorite or comet.



Maria are large dark regions on the surface of the Moon, once thought to be seas, now known to be lava-flooded basins (see Figure 4). The term 'mare soil' refers to the sediment on the surface of the lunar maria; it consists of fragments of basalt rocks, broken mineral grains, and glass produced by impact.


Geological activity on the Moon

However, the Moon is not completely inactive. Seismometers left on the lunar surface by the Apollo astronauts have recorded small seismic events, known as "moonquakes", occurring at depths of several hundred kilometers, which probably result from tidal stresses induced by Earth's gravitational pull. In addition, there have been many reports over the years of unusual color changes and cloud-like obscurations associated with certain craters, including Aristarchus and Alphonsus, and referred to as transient lunar phenomena. Most unusually, Ken Mattingley, the Command Module pilot on Apollo 16, reported seeing flashes of light on the dark side of the Moon on two consecutive orbits of his craft.


Mass concentrations, or mascons, associated with the circular maria such Mare Imbrium, and thought to be caused by layers of denser, basaltic lava, were discovered as a result of their gravitational effect on the orbital motions of Lunar Orbiter probes in the 1960s. Local magnetic areas have also been detected around some craters, although the Moon has no global magnetic field.


mascon under Mare Serenitatis
Topography (top) and surface gravity (bottom) for Mare Serenitatis. The gravity shows a plateau and thus a substantial near-surface contribution to the mascon. Also apparent is the smaller mascon Mare Vaporum in the lower left and center part of the figure. Data from Lunar Prospector.


Air and water

It was natural that philosophers and romantics alike should have long dreamt of traveling to the Moon (see Moon, voyages) and there finding intelligent life (see Moon, life). But the possibility of lunar life (except possibly for certain types of hardy microbes) died with the realization that the Moon has neither an atmosphere nor any liquid water (see Moon, water). Recent observations by Lunar Prospector, however, have confirmed the existence of substantial quantities of ice in deep craters at the lunar poles.


Earth-Moon interactions

The gravitational forces between Earth and the Moon cause some interesting effects, the most obvious of which is the tides. The Moon's gravitational attraction is stronger on the side of Earth nearest to the Moon. Since Earth and its oceans are not perfectly rigid, they are stretched along the line toward the Moon. From our perspective, we see two small bulges, one in the direction of the Moon and one directly opposite. The effect is much stronger in the oceans than in the solid crust so the water bulges are higher. Also, because Earth rotates much faster than the Moon moves in its orbit, the bulges move around Earth about once a day producing two high tides per day.


However, as Earth is not completely fluid, Earth's rotation carries our planet's bulges slightly ahead of the point directly beneath the Moon. This means that the force between Earth and the Moon isn't exactly along the line between their centers; the result is a torque on Earth and an accelerating force of the Moon. This causes a net transfer of rotational energy from Earth to the Moon, slowing down Earth's rotation by about 1.5 milliseconds per century and raising the Moon into a higher orbit by about 3.8 centimeters per year.


The asymmetric nature of this gravitational interaction is also responsible for the fact that the Moon rotates synchronously, i.e. it is locked in phase with its orbit so that the same side is always facing toward us. Just as Earth's rotation is now being slowed by the Moon's influence so in the distant past the Moon's rotation was slowed by the action of Earth, but in that case the effect was much stronger. When the Moon's rotation rate was slowed to match its orbital period (such that the bulge always faced toward the Earth) there was no longer an off-center torque on the Moon and a stable situation was achieved. The same thing has happened to most of the other satellites in the Solar System. Eventually, Earth's rotation will be slowed to match the Moon's period, too, as is the case with Pluto and Charon.


The Moon appears to wobble a bit (due to its slightly non-circular orbit) so that a few degrees of the far side can be seen from time to time, but the majority of the far side was completely unknown until Luna 3 photographed it in 1959.






Earthshine is light reflected from Earth and its atmosphere onto the dark part of the Moon when it is not full. Earthshine explains why the full disk of the Moon remains dimly visible even when only part of it is illuminated by sunlight.

Lunar statistics
mean distance from center of Earth 384,400 km (238,906 miles)
diameter 3,476 km (2,160 miles)
mass (Earth=1) 0.0122
mean density 3.34 g/cm3
surface gravity (Earth=1) 0.165
escape velocity 2.38 km/s (8,568 km/h, 5,325 mph)
orbital period 27.3 days
orbital eccentricity 0.055
orbital inclination 5.1°
axial period 27.3 days (gravitational lock)
max. surface temperature 117 °C (243 °F)
min. surface temperature -163 °C (-261 °F)
albedo 0.07


Some notable features on the Moon
Feature Description
Aitken Basin An impact basin in the south polar region. With a diameter of about 2,500 kilometers, a maximum depth of over 12 kilometers, and an average depth of about 10 kilometers, it is the biggest, deepest impact basin in the solar system
Apennines A mountain range that rises to 4,572 meters at the southeastern edge of Mare Imbrium. The highest escarpment on the Moon, it is higher above the adjacent flatlands than the Himalayan front is above the plains of India and Nepal. The landing site of Apollo 15 was chosen to allow the astronauts to drive from the Lunar Module to the base of the Apennines during two excursions
Bailly The largest crater on the nearside of the Moon with a diameter of 295 kilometers and maximum depth of 3.96 kilometers. A highly eroded structure, it is named after the French astronomer Jean Bailly
Copernicus A 93-kilometer-wide crater that is one of the most prominent features on the lunar nearside. Made less than 1 billion years ago, and thus one of the Moon’s youngest major markings, it has a system of bright rays seen most clearly at full Moon
Imbrium Basin The largest and youngest of the giant impact basins on the nearside of the Moon. The asteroid collision that formed it, about 3.9 billion years ago, went close to breaking the Moon apart; in the event, it threw ejecta over much of the lunar surface and created deep fissures. Through these cracks, lava poured out, filling much of the basin and leaving the 1,300-kilometer-wide dark feature known as Mare Imbrium
Orientale Basin The youngest and best preserved impact basin on the Moon, visible from Earth only at the extreme western limb as a libration feature. Formed some 3.8 to 3.9 billion years ago, it is shows three concentric rings of mountains. Strong radial lineations made by the scouring flow of ejecta are also evident
Tycho A spectacular, 85-kilometer-wide crater associated with the brightest and most extensive ray system on the Moon. In some cases, the rays extend for over 1,500 kilometers; their prominence suggests that Tycho was formed relatively recently, perhaps within the past 3 billion years


Harvest moon and hunter's moon

A harvest moon is the full moon that appears closest in time to the autumnal equinox, in late September or early October in the Northern Hemisphere. For several nights the full moon rises at about the same time (around sunset), and may be bright enough for harvesting to continue into the night. In the Southern Hemisphere this occurs around the spring equinox.


A hunter's moon is the next full moon after the harvest moon; it usually falls in October. Thought of as providing more light for the night hunter, it rises, as does the harvest moon, around the time of sunset for several successive nights in mid- or high-northern latitudes.


Blue moons and supermoons


Blue moon

A blue moon is the second full moon in a month that has two full moons (see lunar phases), or a blue moon is the third full moon in a season of the year that has four full moons (usually each season has only three full moons.)




A supermoon, also known as a perigee full moon, is the Moon when seen from Earth at the biggest and brightest that it can possibly appear. During a supermoon occurrence, the Moon appears up to 14% bigger and 30% brighter than when it is furthest from Earth (see Figure 5).


Supermoons happen when the Moon is both full (i.e. on the opposite side of the Earth from the Sun) and also at a minimum distance from Earth in its elliptical orbit – when it is just 356,400 kilometers (221,457 miles) away, compared to its mean distance of 384,000 kilometers (238,606 miles). The last supermoon occurred on May 5, 2012, when the accompanying photo was taken.


The term 'supermoon' derives from modern astrology and is not used within the astronomical community, which uses the terms perigee full moon or perigee-syzygy moon.



1. Reufer, A., Meier, M. M. M., Benz, W., and Wieler, R. "A hit-and-run Giant Impact scenario." Icarus, in press. Cite as arXiv:1207.5224v1. (July 2012)