Moon

I

Introduction

Moon, the natural satellite of the Earth (the term is also sometimes applied to the satellites of the other planets in the solar system). The diameter of the Moon is about 3,480 km (2,160 mi), or about one quarter that of the Earth, and 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 free water and essentially no atmosphere, so there is no weather to change its surface; yet it is not totally inert.

The Moon moves around the Earth at an average distance of 384,403 km (238,857 mi), and at an average speed of 3,700 km/hr (about 2,300 mph). It completes one revolution in an elliptical orbit about the Earth in 27 days, 7 hr, 43 min, 11.5 sec according to sidereal time (that is, by reference to the stars). For the Moon to go from one phase to the next similar phase, a period known as one lunar month, requires 29 days, 12 hr, 44 min, 2.8 sec. The Moon rotates on its axis once in the same period as its sidereal period of revolution, accounting for the fact that the same face of the Moon is always turned towards the Earth. Although the Moon appears bright to the eye, it reflects into space only 7 per cent of the light that falls on it. This reflectivity, or albedo, of 0.07 is similar to that of coal dust.

II

The Moon Seen From the Earth

At any one time, an observer can see only 50 per cent of the Moon’s entire surface. However, from time to time an additional 9 per cent can be seen around the apparent edge because of the relative motion called libration. This is caused by slight differences in the angle of view from the Earth at different relative positions of the Moon along its inclined elliptical orbit.

The Moon shows progressively changing phases as it moves along its orbit around the Earth. Half the Moon is always in sunlight, just as half the Earth has day while the other half has night. The phases of the Moon depend on how much of the sunlit half can be seen from the Earth at any one time. In the phase called the new moon, the face is completely in shadow. About a week later, the Moon is at first quarter, resembling a luminous half-circle; another week later, at full moon, the Moon shows its fully lit surface; a week afterwards, at last quarter, the Moon appears as a half-circle again. The entire cycle is repeated each lunar month. The Moon is full when it is farther away from the Sun than the Earth; it is new when it is closer. When it is more than half-illuminated, it is said to be gibbous. The Moon is said to be waning when it progresses from full to new, and to be waxing as it proceeds again to full.

III

Surface of the Moon

Temperatures on the Moon’s surface are extreme, ranging from a maximum of 127° C (261° F) at lunar noon to a minimum of -173° C (-279° F) just before lunar dawn. Ancient observers of the Moon believed that the dark regions on its face were oceans, giving rise to the Latin name “mare” (“sea”), which is still used today; the brighter regions were likewise held to be continents. Modern observation and exploration of the Moon have yielded far more comprehensive and specific knowledge. Since the Renaissance, telescopes have revealed a wealth of lunar detail, and lunar spacecraft have contributed further to this knowledge. Features discernible on the surface of the Moon include craters, mountain ranges, plains or maria, faults, domes, rilles, and rays. The largest distinct crater, called Bailly, is about 295 km (183 mi) wide and about 3,960 m (13,000 ft) deep. The largest mare is Mare Imbrium (Sea of Rains), about 1,200 km (750 mi) wide. The highest mountains, in the Leibnitz and Doerfel ranges near the south pole of the Moon, have peaks up to 6,100 m (about 20,000 ft) in height, comparable to the Himalaya on Earth. Craters as small as 1.6 km (1 mi) across have been defined in telescopic observations. The origin of lunar craters was long debated; the latest evidence indicates that nearly all of them are impact craters, formed by explosive impacts of high-velocity meteoroids or small asteroids, mostly during the early part of lunar history, when the solar system still contained many such fragments. Some craters, rilles, and domes, however, display characteristics of indisputable volcanic origin.

IV

Origin of the Moon

Before the modern age of space exploration, scientists had three major theories for the origin of the Moon: fission from the Earth; formation in Earth orbit; and formation far from the Earth. Then, in 1975, having studied Moon rocks and close-up pictures of the Moon, scientists proposed what has come to be regarded as the most probable of the theories of formation, planetesimal impact.