Last updated on April 24, 2014 at 17:35 EDT


The Moon — The Moon is the largest satellite of the Earth, and is occasionally called Luna (Latin for moon) to distinguish it from the general use of the word “moon”.

The Moon is distinguished from the satellites of other planets by its initial capital letter; the other moons are described in the natural satellite article. The words moon and month come from the same Old English root word.

The Moon makes a complete orbit of the celestial sphere about every four weeks. Each hour the moon moves in the sky a distance close to its perceived angular size, or by about 0.5. The Moon always remains within a path, called the Zodiac, which extends about 8 on either side of the ecliptic. The moon crosses the ecliptic about once every 2 weeks.

The color image of the Moon to the right was taken by the Galileo spacecraft at 9:35 a.m. PST December 9, 1990, at a range of about 350,000 miles. The color composite uses monochrome images taken through violet, red, and near-infrared filters.

The concentric, circular Orientale basin, 600 miles across, is near the center; the near side is to the right, the far side to the left. At the upper right is the large, dark Oceanus Procellarum; below it is the smaller Mare Humorum. These, like the small dark Mare Orientale in the center of the basin, formed over 3 billion years ago as basaltic lava flows.

At the lower left, among the southern cratered highlands of the far side, is the South Pole-Aitken basin, similar to Orientale but twice as great in diameter and much older and more degraded by cratering and weathering. The cratered highlands of the near and far sides and the Maria are covered with scattered bright, young ray craters. (click here for full-sized image)

Physical characteristics

Since the Moon’s rotational period is exactly the same as its orbital period, we always see the same face of the Moon pointed towards the Earth. This synchronicity is a result of friction having slowed down the Moon’s rotation in its early history, a process known as tidal locking.

As a result of tidal locking, the Earth’s rotation is also gradually being slowed down by the Moon, and the Moon is slowly receding from the Earth as the Earth’s rotational momentum is transferred to the Moon’s orbital momentum.

The gravitional attraction that the Moon exerts on the Earth is the cause of tides in the sea. Tidal flow is synchronised to the Moon’s orbit around the Earth.

The Earth and Moon orbit about a common center of mass, which lies about 4700 km from the Earth’s center. Since the common center of mass of the Earth-Moon system (the barycenter) is located within Earth, Earth’s motion is more commonly described as a “wobble”. When viewed from Earth’s North pole, the Earth and Moon rotate counter clockwise about their axes, Moon orbits Earth counter-clockwise and Earth orbits the Sun counter-clockwise.

The Moon’s orbital plane about the Earth is inclined by 5 degrees with respect to the Earth’s orbital plane about the Sun (the ecliptic plane). The Moon’s orbital plane along with its spin axis rotates clockwise with a period of 18.6 years, always maintaining the 5 degree inclination.

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.

The inclination of the Moon’s orbit makes it rather unlikely that the Moon formed along with the Earth or was captured later; its origin is the subject of strong scientific debate.

The most accepted theory states that the Moon originated from the collision between the young Earth and an impactor the size of Mars (sometimes called Theia) and was formed from material ejected from Earth as a result of the collision. This is called the Giant Impact theory. New simulations published in August 2001 support this theory. This theory is also corroborated by the fact that the Moon has all the same minerals as the Earth, albeit in different proportions.

The geological epochs of the Moon are defined based on the dating of various significant impact events in the Moon’s history.

Tidal forces deformed the once molten Moon into an ellipsoid, with the major axis pointed towards the Earth.


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, KREEP (K-potassium, Rare Earth Elements, and P-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, 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.

Over time, comets and meteorites continually bombard the Moon. Water-rich meteorites and comets, largely water ice, may leave significant traces of water on the lunar surface. Energy from sunlight splits much of this water into its constituent elements hydrogen and oxygen, both of which usually fly off into space immediately. Some water molecules, however, may have literally hopped along the surface and gotten 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. 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. Such components could make space operations as well as human colonization on the Moon possible.

Compared to that of the Earth, the Moon has a very small 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 the Earth, then the impact theory loses some credibility (although there are alternate explanations for why the lunar core might contain less iron).

Blanketed atop the Moon’s crust is a dusty outer rock layer called regolith. Both the crust and regolith are unevenly distributed over the entire Moon. The crust ranges from 38 miles (60 km) on the near side to 63 miles (100 km) on the far side. The regolith varies from 10 to 16 feet (3 to 5 meters) in the maria to 33 to 66 feet (10 to 20 meters) in the highlands. Scientists think that such asymmetry of the lunar crust most likely accounts for the Moon’s off-set center of mass. Crustal asymmetry may also explain differences in lunar terrain, such as the dominance of smooth rock (maria) on the near side of the Moon.

The Moon has a relatively insignificant and tenuous atmosphere. One source of this atmosphere is outgasing – 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.



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