The moon

By the beginning of the twentieth century the physical features of the moon had become well known, although questions about its origin and history were still unsettled. Its size, weight and density had all been accurately determined. Its motions had been precisely observed, and its location could be predicted for millions of years to come.

The basic statistics of the moon have been well established for the last 75 years. The moon swings around the earth in a nearly circular orbit that is about 382,000 kilometres away. This is not a great distance; an active executive might travel that far in less than two years.

The moon is a sphere whose diameter is 3,500 kilometres — about equal to the distance between New York and El Paso, Texas, or between St. Louis and San Francisco. The surface area of the moon is about 38 million square kilometres-nearly that of North and South America combined.

Although the diameter of the moon is about one quarter that of the earth, the moon weighs only about one-eightieth as much as the earth. The force of gravity at the moon's surface is only one-sixth that of the earth. A fully suited astronaut weighing about 350 pounds on the earth weighs only about 60 pounds on the moon.

Out of these basic statistics emerges a fundamental difference between the earth and the moon. The moon's density is 3.35 grams per cubic centimetre whereas the density of the earth is 5.5. The fact that the earth is 60 per cent denser than the moon suggests that there is some basic difference in their chemical composition — a difference hard to explain in two bodies that are so close together in space,

The moon has no atmosphere. When stars pass behind the moon they disappear sharply and suddenly with none of the gradual dimming that would be produced if their light was passing through a lunar atmosphere. More recent studies have shown that natural radio sources in the sky are cut off in the same sudden way as the moon moves in front of them. These movements show that at the lunar surface there is more complete vacuum than can be produced in any terrestrial laboratory.

The absence of a lunar atmosphere is not surprising; the moon's gravity is too weak to hold an atmosphere like the earth's. If relatively light gases like oxygen, nitrogen and water vapour were ever present on the moon, their molecules must have escaped into space long ago.

This lack of atmosphere means that, unlike the earth, the surface of the moon has no protection from continuous bombardment by tiny meteorites and from scorching by lethal X-rays, gamma rays, and cosmic rays that emanate from the sun and the rest of the universe. Fortunately for us, this dangerous matter and energy is absorbed by our atmosphere before it reaches the surface of the earth.

The moon completes one orbit around the earth in 27.3 days. However, the earth also moves along its orbit around the sun while the moon is swinging around the earth. As a result, the angle of illumination of the moon by the sun changes slightly, and a longer period

passes before the moon returns to the same phase as seen from the earth. This latter period, the time between one full moon and the next, is 29.5 days, and it has long been the basis of the lunar calendar.

The moon is also locked in its orbit, and as it moves around the earth, it turns so slowly that it always keeps the same side facing toward the earth. The moon thus rotates once on its axis in the same time that it makes one trip around the earth. To keep one face turned always to the earth, the moon must turn its back on the sun during half its orbit.

As a result of these motions, the 29.5-day month is divided on the moon into a lunar «day» and a lunar «night», each about two weeks long. Because the moon has no insulating atmosphere, the «daytime» temperature in direct sunlight .is about 134°C, well above the boiling point of water. During the lunar «night» the temperature drops suddenly to about — 170°C much colder than the freezing point of carbon dioxide ("dry ice").