Although it is the second most brightest object in the sky, all of the light is reflected from the Sun. Some tiny portion of the Moons, light could be called Earthshine, as noticed during a crescent phase when the ‘dark’ part of the Moon is slightly illuminated. It is light from the Sun reflected by the Earth to the Moon, which, in turn, reflects it back to Earth. The Earth’s single satellite is mythologically associated with Luna (or Diana), the Roman goddess of the hunt, who was also their goddess of the Moon. The sixth largest moon in the Solar System, the Moon is closer in size to its mother planet than any other except Pluto’s moon Charon. For this reason the Earth and the Moon (like Pluto and Charon as well) are occasionally described as bring a double planet. While the larger planets have on the order of a thousand times the mass of their moons, the Earth has 81 times the mass of the Moon and four times the diameter.
The Moon is the only other object in the Solar System to have been visited by human beings from the planet Earth. In 1968 the United States began the Apollo project, a series of space flights during which the Moon became the first body in the Solar System beyond Earth to be explored firsthand by human beings. The Moon was surveyed by human beings from Lunar orbit for the first time by means of two circumlunar manned flights in December 1968 and May 1969, which began the operational phase of the Apollo program. In July 1969 the Apollo II spacecraft became the first vehicle to land human beings on the Moon. The initial landing was followed by six others between November 1969 and December 1972. (A seventh mission was aborted because of hardware failure in April 1970.)
During the Apollo program, 12 American astronauts conducted detailed surveys on the Lunar surface and seismic studies of the Lunar interior. The Apollo program completed detailed mapping of the Moon and provided a wealth of information about its composition and its geologic history.
As perceived from Earth, the Moon appears to go through a series of phases depending upon its reflection of light from the Sun. These phases, which constitute the Lunar ‘day go through a complete cycle every 29 days, 12 hours and 44 minutes. The cycle is also known as the synodic, or Lunar month, as seen on Earth.
When the Moon is fully illuminated it is said to be ‘full’ As the visible face of the Moon rotates away from the Sun it is said to be ‘waning.’ When exactly half the face of the Moon is illuminated, it is called a ‘quarter Moon’ As it becomes less visible it is said to become a ‘crescent Moon.’ and when it becomes dark and the cycle is resumed, the Moon is said to be a ‘new Moon.’ From ‘new,’ the Moon waxes through the crescent phase to the quarter phase, and once again to full. The Sun always illuminates one-half the Moon. Depending upon the relative angle between the Earth and Moon. we see portions of the sunlit side. At lull Moon’ we see the entire sunlit side, and at ‘new Moon none of the the sunlit side is facing the Earth.
The Moon’s period of rotation is 27 days, seven hours and 43 minutes?nearly the same as the period of its revolution around the Earth, so the same side always faces the Earth. Because of the Moon’s slight wobbling, we are able to see slightly more than half of its surface from the Earth. The Moon’s mysterious far side had been a mystery to mankind for centuries, and it was not until the Soviet Luna 3 spacecraft returned photographic images of the ‘dark side of the Moon’ in October 1959 that actual detailed information of the Moon’s ‘other half was revealed to mankind. Though the 41 percent of the Lunar surface that is never visible from the Earth is frequently.’ referred to as the ‘dark side.’ it actually receives as much light from the Sun as the near side.
The Moon’s surface is characterized by rugged mountain ranges and by thousands of meteorite impact craters. In this sense it is very much like the planet Mercury. Unlike Mercury, however, the Moon has large open areas that are called seas (or in Latin, maria) because to the eye they appear darker than the surrounding terrain, and were once thought by Galileo to resemble seas. Almost entirely concentrated on the side facing the Earth, the maria cover 15 percent of the Lunar surface, and were probably once ‘seas of molten rock that flowed out of the Moon’s interior. The gravitational effect of the Earth probably has a great deal to do with the fact that such features are concentrated on the Earthward side.
Unlike the Earth, the Moon has neither magnetic poles nor a significant magnetic field. although rocks in the Lunar crust are weakly magnetized. Probably due to its low mass and density, the Moon never developed an atmosphere, although trace amounts of hydrogen and helium, as well as hints of argon and neon were detected as escaping from the Lunar surface in 1972 by the crew of the American Apollo 17 spacecraft.
The Apollo program studies revealed that the Lunar interior was quite active, with moonquakes being more common on the Moon than earthquakes are on the Earth, although they have not been recorded in excess of two on the Richter scale.
The Moon formed about the same time as the Earth-4.6 billion years ago?and is composed of the same basic materials, hut their early relationship is unclear. One line of thought theorizes that the Moon was formed out of the Earth, either in a single piece that broke loose (perhaps from the Pacific Basin) or in the form of debris that was knocked loose in a collision with an asteroid, and which eventually congealed into the Lunar mass. Another theory holds that the Moon was a separate planet ‘captured’ by the Earth’s gravitational field. A third notion has it that the Earth and Moon were formed in the same way and in the same place and time. Because the Earth was 81 times larger. the Moon became enslaved to its gravity.
Once in place, the Moon’s geology evolved much like that of the Earth. Originally molten, the crust gradually eooled, leaving a molten core like that of the Earth. In the meantime, it was being bombarded by debris from the formation of the Solar System. In addition to smaller craters, huge basins were hammered into the surfaces of both bodies. Some of the first basins to be formed in the Moon were Mare Fecunditatius and Mare Tranqu ilium; they probably formed 4.4 billion years ago. The last basins to be formed were the Mare Imbrium and Mare Orientale. Dating from 3.85 billion years ago, Mare Imbrium is the largest of the Lunar seas, and its origin concludes the Pre-Imbrian period of Lunar geologic evolution.
When the Imbrian Period began 3.85 billion years ago, the Lunar surface was probably pocked entirely and uniformly with impact craters. The semicircular mountain ranges found around the periphery of the maria are the only remnants of the enormous impacts that created them. During this period, however, intense interior heating resulted in vast flows of darker basalt from deep within the Moon. Part of the heating came from meteor impacts, and part from radioactive decay. These flows filled the huge basins, and the Lunar seas briefly were seas?of lava!
When the Moon cooled, and the lava flows ended 33 billion years ago, small scale volcanic activity continued for approximately 1.3 billion years through what is called the Ratosthenian Period during this period, interplanetary debris crashed into the Moon, creating newer, smaller impact craters on the Lunar seas themselves. One of the major craters now visible on the surface, Copernicus, was probably formed one billion years ago, marking the climax of the third period of Lunar geology. Since the formation of Copernicus there has been very little geo- logic activity on the Moon. This fourth period, the present Copernican Period, has also been marked by very little in the way of impact crater formation, although the crater Tycho is thought to have been formed as recently as one million years ago. and the formation of the great crater Giordano Bruno is believed to have been witnessed from Earth in 1178 AD. In July 1972,a 2200-pound meteorite was recorded as having struck the Moon. Throughout the billion years of the Copernican Period the Moon’s surface has remained relatively unchanged because there is no air, no wind and no water to cause erosion of the type that has greatly altered the surfaces of such bodies as the Earth and Mars.