The Moon has fascinated mankind throughout the ages. By simply viewing with the naked eye, one can discern two major types of terrain: relatively bright highlands and darker plains. By the middle of the 17th century, Galileo and other early astronomers made telescopic observations, noting an almost endless overlapping of craters. It has also been known for more thana century that the Moon is less dense than the Earth. Although a certain amount of information was ascertained about the Moon before the space age, this new era has revealed many secrets barely imaginable before that time. Current knowledge of the Moon is greater than for any other solar system object except Earth. This lends to a greater understanding of geologic processes and furtherappreciation of the complexity of terrestrial planets.
The Moon is the closest astronomical object to the Earth. With the Earth it forms what is almost a double planet as no planet has a satellite which is as large in comparison to the size of the planet. The Moon has a diameter of 3476 km and orbits the Earth at a mean distance of 384000 km. It orbits the Earth in 27.322 days and always keeps the sameface pointed towards the Earth.
The Moon shines by reflecting the light from the Sun and shows the characteristic phases during each orbit of the Earth. (See Phases of the Moon.) 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 beingreflected back again to the Earth. We are seeing Earthshine, the equivalent of moonlight on the Earth.
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 (See Eclipses).
The gravitational pull of the Moon and the Sun are responsible for the Tides. The Moonhas 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 notsurprising that there is no form of life on the Moon.
It is convenient to give first a general idea of the motion of the moon as seen by an observer on the earth. The sidereal month is 27-32 days and in this interval the moon makes' on the average, a complete revolution relative to the stars. In the meantime, the earth is moving in its orbit round the sun and it is not till after the lapse of anadditional 2-2i days that the moon is in the same position relative to the sun's direction; the synodic month being 29-53 days. The moon being an opaque body, shining by reflected light, only that portion of the hemisphere, which is illuminated and turned towards the earth, can be seen. At new moon the hemisphere turned towards the earth is wholly illuminated by sunlight, while at full moon the fullyilluminate hemisphere is turned towards the earth. At intermediate phases the hemisphere turned towards the earth is only partially illuminated. Near new moon the moon appears as a narrow crescent with the horns turned away from the sun; near full moon the defective edge is that farthest from the sun's direction.
Green Cheese or Granite - Moon Geology
The Moon's surface is characterized bylight mountainous regions interspersed with dark maria (see Fig. 1). The `Man in the Moon' is formed from patches of these two types of terrain. The maria are vast impact basins which have been filled with basaltic rocks some 3000 million years ago. 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...