The Earth's Moon

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Our nearest neighbor in space, the Moon is the only natural satellite of the Earth. Apart from the Sun, it is the brightest object in the sky. It is 384,400 kilometers from Earth and has a diameter of 3,476 kilometers. Great plains stretch over the moon's surface, dotted with huge mountains and scarred by numerous craters.

The Moon, of course, has been known since prehistoric times. The moon was called Luna by the Romans, Selene and Artemis by the Greeks, and many other names in other mythologies.

As the Moon orbits around the Earth once per month, the angle between the Earth, the Moon and the Sun changes and 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 (as measured against the stars) since the Earth moves a significant distance in its orbit around the Sun in that time.

Topography

The study of lunar topography began with the invention of the telescope. Galileo studied the moon through a telescope in 1609. Later astronomers made maps of its physical features, discovering mountains and plains, some large craters, and long valleys. Some of them thought the plains on the moon were covered with water and called them 'maria' (Latin for seas). This name has persisted, though it's now known that there's no water on the moon's surface.

Maria and Mountains

The dark surface regions (mare regions), located mainly on the side observable from Earth, represent basaltic (volcanic) flooding of basins created by major asteroidal impacts. Apollo and Luna sample isotopic dating places mare basalts in the range of 3-4 thousand million years, in contrast to 4.2-4.5 thousand million for highland samples.

The brighter surface regions (highlands) represent the original lunar crustal material shaped by saturation bombardment of meteoritic material. The brighter, densely cratered, mountainous highland regions (sometimes called terrae), are found mainly on the southern part of the Moon's near side, and over the entire far side.

Craters

Much of the Moon's surface is covered with craters. These are the result of impacts by meteors. The largest are about 200 km in diameter, the smallest are only about a metre across. Most of these craters were formed between 3000 and 4000 million years ago. Many of the craters were made by meteoric impacts. Some of the craters are large enough to encircle several cities as big as New York, with mountains higher than the Rockies rising up from their floors.

When a meteor strikes Earth, it makes a crater, just as it does on the Moon. But on Earth, wind, rain, and the motions of the crust erase or fill in these craters. Since the Moon has no atmosphere, there is no rain or wind to erode the craters. And because the Moon's interior is no longer hot and active like Earth's, there are no active volcanoes on the Moon. So craters formed by meteorites on the Moon last a long time.

Physical Properties

The average distance of the moon from the earth is about 239,000 miles (384,000 km). The moon's diameter is about 2,160 mi (3,476 km) or about one-fourth that of earth. The moon's volume is about one-fiftieth that of the earth. The mass of the earth is 81 times greater than the mass of the moon. Thus the average density of the moon is only three-fifths, and the pull of gravity at the lunar surface only one-sixth that of the earth.

The Moon has no atmosphere. Any early atmosphere that the Moon might have had, has escaped from the Moon's feeble gravitational pull. This is only one sixth that at the surface of the Earth. Because of the lack of any atmosphere, the temperature of the Moon's surface varies between -180 C and 110 C. The Moon offers little protection from the solar wind, cosmic rays or micrometeorites and so it is not surprising that there is no form of life on the Moon.

Tides and Gravity

The pull of the moon's gravity causes the tides. The Moon's gravitational attraction is stronger on the side of the Earth nearest to the Moon and weaker on the opposite side. This causes two small bulges in the sea water, one in the direction of the Moon and one directly opposite. Because the Earth rotates much faster than the Moon moves in its orbit, the bulges move around the Earth about once a day giving two high tides per day.

But, the Earth is not completely fluid either. The Earth's rotation carries the Earth's bulges slightly ahead of the point directly beneath the Moon. This means that the force between the Earth and the Moon is not exactly along the line between their centers, producing a torque on the Earth and an accelerating force on the Moon.

This causes a net transfer of rotational energy from the Earth to the Moon, slowing down the Earth's rotation by about 1.5 milliseconds/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 is locked in phase with its orbit so that the same side is always facing toward the Earth. Just as the 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 the 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 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, the Earth's rotation will be slowed to match the Moon's period, too, as is the case with Pluto and Charon.

Actually, the Moon appears to wobble a bit (due to its slightly noncircular 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 the Soviet spacecraft Luna 3 photographed it in 1959. There is no "dark side" of the Moon; all parts of the Moon get sunlight half the time (except for a few deep craters near the poles).

Composition

The moon has no free water and essentially no atmosphere, and no weather exists to change its surface; yet it is not totally inert. Evidence from Clementine suggested that there may be water ice in some deep craters near the Moon's south pole which are permanently shaded. This has now been confirmed by Lunar Prospector. There is apparently ice at the north pole as well.

The Moon's crust averages 68 km thick and varies from essentially 0 km under Mare Crisium to 107 km north of the crater Korolev on the lunar far side. Below the crust is a mantle and probably a small core (roughly 340 km radius and 2% of the Moon's mass). Unlike the Earth's mantle, however, the Moon's mantle is only partially molten. Curiously, the Moon's center of mass is offset from its geometric center by about 2 km in the direction toward the Earth. Also, the crust is thinner on the lunar near side.

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.

Origin

The Moon's origin is uncertain. There were three main theories:

• that the Moon and the Earth formed at the same time from the Solar Nebula;
• that the Moon split off of the Earth;
• that the Moon formed elsewhere and was subsequently captured by the Earth.

New and detailed information from the Moon rocks led to the theory that a Mars-sized object collided with the Earth soon after it was formed, a geyser of molten material spewed into space, and entered orbit around Earth. Some of this material fell back to Earth, but much of it coalesced to form the Moon.

Lunar evolution models based on Lunar Orbiter mapping of the Moon and on Apollo and Luna sample analyses suggest five principal episodes: accretion and large-scale melting; crustal separation and concurrent massive meteoritic bombardment; partial melting at depth diminished bombardment with further melting at depth and emplacement of mare basalts and cessation of volcanism and gradual internal cooling.

The popular current theory for the creation of the Moon involves the impact of a Mars-sized object with the Earth, occasioning a catastrophic disruption of the Earth, and the accretion of the Moon in Earth orbit from debris torn from the Earth's mantle.

Orbit

As the Moon orbits the Earth it goes through a sequence of phases as the proportion of the visible illuminated hemisphere changes. The Moon shines by reflecting the light from the Sun and shows the characteristic phases during each orbit of the Earth. Near New Moon, when the sunlit portion of the Moon is small, the phenomenon of `the old Moon in the young Moon's arms' is often seen. This is caused by sunlight being reflected towards the Moon by the Earth and being reflected back again to the Earth. We are seeing Earthshine, the equivalent of moonlight on the Earth.

The lunar orbit is elliptical. The moon and the Earth orbit around their common center of gravity. The moon's orbital period around the earth, and also its rotation period, is 27.322 days. Since the period of the orbit is the same as the Moon's rotational period (or lunar day), we always see the same side of the Moon facing us. The equality of rotational and orbital rates is due to tidal despinning of the Moon into a stable synchronous period, so that the same hemisphere of the Moon always faces the Earth.

This is not a coincidence but something that happened naturally in the evolution of the earth/moon relationship. It is a characteristic shared by the moons of the other planets; for example Titan rotates on its axis once every 15 days 23 hours, the same time that it takes to complete one orbit around Saturn.

Eclipses

The true angular size of the moon's diameter is about 1/2°, which also happens to be the sun's apparent diameter. This coincidence makes possible total solar eclipses. A solar eclipse is when the sun is hidden by the moon; a lunar eclipse is when the moon is in Earth's shadow.

The orbital plane of the Moon is inclined to that of the Earth about the Sun and so eclipses are only seen when New Moon or Full Moon occur when the Moon is near to the crossing points of these planes.

NASA - Astronaut,Rover, Flag On Moon - ©Spaceshots Buy this Poster at AllPosters.com

Moon Landing

Three astronauts travelled to the moon in the Apollo 11 spacecraft, launched from Earth by a giant Saturn rocket. The main capsule orbited the moon, while astronauts Neil Armstrong and Edwin Aldrin flew down in a small landing craft. The first man on the moon was Neil Armstrong, on July 20th 1969.

From 1969 to 1972, six Apollo missions sent 12 astronauts to the Moon's surface. They gathered lunar rocks and soil and brought them back to Earth. They also left behind four special mirrors aimed at Earth. Astronomers beamed pulses of laser light to the Moon and measured the time it takes for the light to reflect back to Earth. This technique has established the exact distance from Earth to the Moon to within a fraction of an inch.

Cold-hearted orb that rules the night
Removes the colours from our sight,
Red is grey and yellow white
But we decide which is right
And which is an illusion.

- Moody Blues lyrics

The Moon is the largest 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." The words moon and month come from the same Old English root word.