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The Earth's companion in its orbit around the Sun is the Moon. The Moon orbits the Earth at an average distance of about 240,000 miles (384,000 km) over a period of about 29.5 days. Its monthly cycle of phases provided a natural clock for ancient societies; most human calendar systems are built around, or least contain units based on, the orbital period of the Moon. To this date, the Moon is the only object in the solar system other than the Earth that has been visited by humans.

The Earth's companion in its orbit around the Sun is the Moon. The Moon orbits the Earth at an average distance of about 240,000 miles (384,000 km) over a period of about 29.5 days. Its monthly cycle of phases provided a natural clock for ancient societies; most human calendar systems are built around, or least contain units based on, the orbital period of the Moon. To this date, the Moon is the only object in the solar system other than the Earth that has been visited by humans.

The Full Moon
Mythology and Culture

The Moon features prominently in art and literature. The monthly cycle of the Moon has been implicitly linked to women's monthly cycles by many cultures. Many of the best-known mythologies feature female lunar deities, such as the Greek goddess Selene. The moon is also worshipped in witchcraft, both in its modern form and in medieval times. The purported influence of the moon in human affairs remains a feature of astrology.

In Roman mythology, the moon goddess is Luna, also the Latin word for Moon. The English name for Monday derives from an Old English word meaning "Moon's day". This is a translation of the Latin name dies lunae ("day of the Moon"); compare this with Romance language versions, like the French lundi, Spanish lunes, and Italian lunedi.

Also of significance is that many religions and calendar systems are oriented chronologically by the Moon, as opposed to the sun. Prominent examples include the Hebrew, Islamic, and Chinese calendars. The Moon has religious significance during many Hindu festivals; in Hinduism the word Chandra means Moon.

Phases and Libration

We always see the same side of the Moon because its rotational period is the same as its orbital period. Thus, the Moon always keeps the same face pointed toward the Earth. The Moon does not keep the same face pointed toward the Sun, however. As the Moon orbits the Earth, different parts of the Moon's surface are illuminated; this is what produces the monthly cycle of phases that we see. There is no permanent "dark side" or "day side" of the moon, but there is a permanent "near side" and "far side".

Actually, the Moon does not quite keep the exact same part of its surface pointed toward the Earth. The Moon's orbit is not perfectly circular, nor is it perfectly aligned with the plane of the Earth's orbit. Because of these facts, the Moon's surface appears to "wobble", as seen from Earth, over the course of its monthly cycle. It moves slightly, up and down and from one side to the other, so that we actually see almost 60% of the surface of the Moon from Earth. This "wobble" is called libration.

The Moon - Third Quarter. (Jim Misti)
Lunar Eclipses

By coincidence, the Moon appears to be about the same angular size as the Sun. Periodically the Sun, the Earth, and the Moon are all in a straight line. This results in a solar eclipse (when the Moon's shadow passes across the Earth), or in a lunar eclipse (when the Earth's shadow passes across the Moon). Because the Moon's orbital plane is slightly inclined to the Earth's, eclipses do not happen every month; usually the Moon passes just "above" or "below" the Sun.

Lunar eclipses are more common than solar eclipses because the Earth's shadow is larger than the Moon's shadow. Seen from a given point on the Earth's surface, solar eclipses last only a few minutes, but lunar eclipses may take several hours.

Lunar eclipses occur whenever the Moon passes into the shadow of the Earth. The shadow of the Earth can be divided into two parts, the umbra and the penumbra. In the umbra, the Sun is totally blocked by the Earth. In the penumbra, the Sun is only partially blocked. Therefore, if the Moon is completely inside the umbra, there will be total lunar eclipse. If only part of the Moon passes inside the umbra, there will only be a partial lunar eclipse. If the Moon only passes through the penumbra, there is a penumbral lunar eclipse. If you were on the Moon at that time, you would see a partial solar eclipse.



A lunar eclipse.
For a lunar eclipse to occur, the Moon must be "behind" the Earth as seen from the Sun. If so, then why we do not have a lunar eclipse every month? The reason is that the plane of the Moon's orbit does not coincide with the plane of the Earth's orbit. So, during most full moons, the Moon is either south or north of the orbital plane of the Earth. For the same reason, we do not see a solar eclipse during every new moon.

The Moon's gravity exerts an attractive force on the Earth's oceans, causing them to rise and fall over the course of a day. The tides are probably the most well-known phenomenon we experience on the Earth that is due to the influence of the Moon. Less well known is the reason we experience two high tides per day. The Moon's gravity exerts a stronger attractive force on the near side of the Earth, less at the center of the Earth, and even less on the far side. Relative to the center of the Earth, the effective forces pull water on the Earth both toward and away from the Moon. They lead to two bulges of water on the Earth's surface - and two high tides per day.

Why there are two tides per day.
Similarly, the Earth's gravity exerts a tidal force on the solid structure of the Moon. This is much stronger that the Moon's tides on the Earth because the Earth is so much more massive than the Moon. This tidal force is what slowed the Moon's rotation over millions of years, causing the Moon to always keep the same face pointed toward the Earth. The equality of the Moon's rotation and revolution periods is not a coincidence.

Because the Earth is rotating, the Earth's tidal bulge does not point exactly toward the Moon; instead, it points about 10° ahead of the Moon's position. The Moon's gravity tugs back on the bulge, which very slowly causes the Earth's rotation rate to slow. Similarly, the gravity of the bulge tugs the Moon forward, which very slowly causes the Moon to accelerate in its orbit. Due to this tidal interaction, both the length of the day and the month are slowly increasing, and the Moon is slowly receding from the Earth.



Tidal interactions slow the Earth's rotation and lengthen the Moon's orbital period.
Hundreds of millions of years from now, the Earth and Moon will both be "tide-locked" to each other: the Earth will rotate at exactly the same rate that the Moon orbits. The Earth will always keep the same face toward the Moon, just as the Moon, today, always keeps the same face toward the Earth. We encounter many other examples of such tidal locking in other pairs of objects in the Solar System.

Surface and Geology

The Moon's surface is airless, lifeless, and completely covered by craters. A small telescope or even binoculars will show hundreds of craters, ridges, mountain peaks on the Moon's surface. Once thought to be volcanic in nature, almost all of the Moon's craters are the result of asteroid and comet impacts millions or billions of years ago. Large, dark, flat, smooth areas known as maria (Latin for seas) are pools of solidified lava, filling the basins caused by particularly large impacts.


Unlike the Earth, the Moon's surface displays almost no geological activity. Without any plate tectonics, volcanism, earthquakes, wind, or liquid water to cause erosion, impact features can last for billions of years. On the Earth, by contrast, such processes erase most craters only a few million years after they formed.

The Moon's low surface gravity - about 1/6th of the Earth's - prevents it from retaining any substantial atmosphere. The lack of an atmosphere produces extreme changes in temperature, from -200 C (-330 F) at night near the poles, to 120 C (240 F) during the day at the equator. All of the moon's liquid water disappeared long ago under such conditions.



The Lunar Alps and crater Plato. (Jim Misti)
The Moon has two faces, one of which remained remained a mystery until 1959, when the Soviet Luna 3 spacecraft first snapped images of it. We discovered that the Moon's far side is very different from its near side. For instance, the widespread plains of volcanic rock called maria that cover much of the near side are all but nonexistent on the far side. In addition, while the surface of the near side is mostly low and flat, the far side is often high and mountainous, elevated 1.2 miles (1.9 km) higher than the near side on average.

Water on the Moon

Nevertheless, it was thought that small reservoirs of water might still exist as ice buried at the bottom of deep craters near the lunar poles, where sunlight never reaches the crater floors. Such places are among the coldest in the Solar System, measured at -396 F (-240 C) by NASA's Lunar Reconnaissance Orbiter.

The speculation that water may accumulate in these craters was confirmed in late 2009 by NASA's LCROSS satellite, which deliberately crashed its rocket booster into the permanently shadowed region of Cabeus crater near the moon's south pole. After the rocket struck, the satellite flew through the debris cloud, measuring an amount of water vapor equal to about 25 gallons in liquid form; it then impacted in the same spot, four minutes later.

Earlier in 2009, India's Chandrayyan-1 mission observed spectroscopic evidence that the Moon's entire surface is covered with trace amounts of water - about 32 ounces for every ton of lunar soil. This confirmed earlier observations by NASA's Deep Impact and Cassini missions. The lunar trace surface water is probably formed by hydrogen ions in the solar wind continuously bombarding silicate minerals (which contain oxygen) on the Moon's surface. The daily formation and evaporation of this trace vapor could provide another water source for the cold traps in the Moon's permanently shadowed polar regions.

Origin of the Moon

The Moon's orbital plane is closer to the Ecliptic (the Earth's orbital plane) than to the Earth's equator. This is unusual - most other large moons orbit their parent planet close to that planet's equatorial plane. The Moon's density is about half that of the Earth, similar to the rocks in the Earth's mantle.

The Moon is also larger, compared to the Earth, than most moons of the other planets. At 2160 miles (3474 km) in diameter, the Moon is more than 1/4 the size of the Earth, and contains almost 1/80th the mass of the Earth. Earth's Moon is the fifth largest in the solar system, after Jupiter's Ganymede, Callisto, Io, and Saturn's Titan - and all of those moons contain less than 1/10,000th the mass of their parent planet. Only Pluto and Charon come closer to being an equal pair than the Earth and Moon.
Today, the prevailing theory of the Moon's origin is that the Moon formed from the giant impact of a Mars-sized object with the Earth near the end of the time of the Earth's formation, 4.5 billion years ago. This impact blasted much of the Earth's surface material into orbit around the proto-Earth, which later coalesced to form the Moon through a process of accretion. Computer models show that this process would result in an object consistent with the Moon's current composition and unusual orbital properties.

Computer modeling in 2011 indicated that the Earth may once have had two Moons, both formed from the same giant impact. Rubble might have also coalesced into a smaller companion moon at Trojan point, 60 degrees from the larger Moon in its orbit. At such a point, the gravitation of the Earth and Moon essentially balance out, letting objects remain in relatively stability.

Eventually, however, the Trojan moon would have destabilized, and careened into the larger Moon at relatively low speed, splattering itself as a thick extra layer of solid crust instead of forming a crater. The remains of this Trojan moon would make up the highlands now seen on the far side. At the same time, the impact would have pushed an underground ocean of magma toward the near side - explaining why phosphorus, rare-earth metals, and radioactive potassium, uranium and thorium are concentrated in the crust there.

Lunar Exploration

Exploration of the Moon by spacecraft was inspired by the Cold War between the US and USSR in the 1950s and 1960s. The first man-made object to impact the lunar surface (Luna 2), and to photograph the Moon's far side (Luna 3), were launched by the Soviet Union in 1959. The USSR also launched the first unmanned vehicle to perform a successful soft lunar landing (Luna 9), and to orbit the Moon (Luna 10), in 1966.

But the first manned landing on the Moon - seen by many as the culmination of the "space race" - belongs to the United States. Neil Armstrong, commander of the Apollo 11 mission, became the first human being to walk on the Moon on July 20th, 1969.

Five additional manned Apollo missions (12, 14, 15, 16, and 17) landed on the Moon during the years 1969 - 1972; Apollo 13 was forced to abort its mission due to an explosion on the spacecraft; all three astronauts aboard survived. The last person to walk on the Moon was Gene Cernan, commander of Apollo 17, on December 14th, 1972. No one has set foot on it since. In the years after Apollo, the US and USSR both turned their attention to unmanned exploration of the planets; no manned missions since the 1970s have travelled beyond low Earth orbit.

Apollo 17 astronaut Jack Schmitt inspecting a huge boulder that has rolled down an adjacent hillside. (NASA)
In 1994, the U.S. returned to the Moon (in the robotic sense) with the Clementine spacecraft; this obtained the first global, multi-spectral topographic map of the Moon. NASA's Lunar Prospector mission followed in 1998, and indicated the presence of water ice on the floors of permanently shadowed craters near the lunar poles. In 2004, the U.S. announced plans for a manned return to the Moon by 2020, including a permanent outpost at one of the lunar poles. China has also expressed ambitious plans for lunar exploration, successfully launching its first astronaut in 2003, and its first unmanned lunar orbiter in 2007.


Also in 2007, Japan launched a lunar orbiter, Kaguya, which photographed the moon in high resolution. Kaguya's imagery was soon followed by NASA's Lunar Reconnaissance Orbiter in 2009. The LRO cameras were able to image the Moon's entire surface in such high detail that the Apollo lunar module descent stages, and even lunar rover tracks, were visible.

The Google Lunar X Prize, announced by the internet search giant in 2007, hopes to encourage privately-funded lunar exploration by offering 20 million US dollars to anyone who can land a robotic rover on the Moon. As of 2012, the prize remains unclaimed.