Moon

Moon

The Moon is a most remarkable object, even within the limitations of naked-eye observation. It appears to alter its shape in a regular monthly rhythm. At the same time, moves fairly swiftly against the background of stars. These changes, as the ancient Greeks realized, are because it shines by reflected sunlight. The moon also travels around the earth. It is our nearest neighbor in space, and earth’s only natural satellite.

Relative to the stars, the moon orbits the earth once in about 27 ½ days at an average distance of 238, 856 miles. Since its orbit is an ellipse, the distance varies between 221, 463 and 252, 710 miles. Its diameter, 2,160 miles, makes it one of the largest satellites in the solar system, only two-thirds smaller than the planet Mercury. Compared with the earth’s diameter of 7,926 miles, the moon is more like a planet than a satellite. Indeed, some astronomers think that it once traveled as an independent body about the Sun and then was captured by the Earth.

The Moon, photographed with the 36-inch refractor of the Lick Observatory

A Barren World

The telescope resolves the patchy appearance of the moon’s face into a scene of savage grandeur. The dark areas, once thought to be seas, are great plains. The bright areas stand out as a hodgepodge of mountains and circular formations usually, although loosely, called “craters”. Under a low sun the mountains and edges of the craters cast long, black shadows or exquisite sharpness. No twilight effects or half-lights soften the harshness of the scene. We see no clouds, rivers, seas, and no forests or grassy plains. The whole moonscape, bathed in brilliant sunshine, is apparently both airless and waterless. Nor can there be any sound, for sound waves cannot travel in a vacuum. The world of the moon lies silent, without air, water, or life. It is tortured by day by the grilling sun, and by night by the intense cold of interplanetary space.

As the moon orbits the earth it rotates once on its axis and therefore always shows the same face to the earth. Speculations about the nature of all the moon’s surface features ended when Soviet and American spacecraft took photographs of the far side of the Moon. They showed that the two sides are similar, although on the far side the plains or “maria” are remarkably few in number.

The Moon in Close-Up

Spacecraft, commencing with the U.S. Ranger 7, launched in 1964, also have taken great numbers of close-up pictures of the near side. They show that the moon’s surface is literally peppered with small depressions. Even a conspicuous crater like Copernicus has ramparts and mountain peaks which are remarkably smooth.

The results obtained by the US spacecraft indicate that the moon’s surface is similar in hardness and texture to well compacted soil on the earth. We know that certain areas are strewn with rocks and boulders. The surface in general is not rough and rugged. Some areas look particularly suitable for a manned landing and exploration of foot or in some special form of lunar vehicle. We also know that the surface as a whole reflects only about 7 percent of the light it receives. This means that it is not white or silvery but largely grayish-brown in colour. But the problem of how the craters and depressions were formed is still unresolved. According to one popular view, they are collision scars produced by impacting meteorites. Another view is that they are the result of volcanic activity or, more generally, of forces that acted outwards from within the moon.

The interior of the crater Copernicus, with mountains rising about 1,000 feet from the floor. A high resolution photograph, taken by Lunar Orbiter II

Extremes in Temperature

Since the moon rotates once as it orbits the earth, day and night on the moon each last about 14 days. At and near the equator, when the sun is high in the sky, the surface gets almost as hot as boiling water. Yet during the long lunar night the absorbed heat leaks into space so effectively that the temperature falls almost to that of liquid air. Studies of the moon using radio telescopes indicate that the great range of temperature is restricted to the surface layers. Several feet down the temperature seems to be fairly constant at about -40 degrees centigrade. This suggests that the moon is a comparatively cold body. That also suggests that its surface is an extremely poor conductor but good insulator of heat.

Since the moon has no appreciable atmosphere the lunar sky, like that in the planetarium, is of the deepest black. From the moon, the Milky Way and myriads of stars can be seen in daytime. But by far the most impressive sight in the earth, shining with the radiance of 50 full moons and appearing about four times as broad as the moon appears to as.

The Earth Seen From the Moon. Photograph of the Earth transmitted from the vicinity of the Moon by Lunar Orbiter 1. The Earth is shown in the upper part of the photograph, and the surface of the Moon is below

Earth in the Sky

Seen from the moon, the earth goes through a cycle of phases once every month. It first appears as a thin crescent in the lunar daytime. Then it waxes to reach full earth in the lunar night. Finally it wanes to become a crescent again. When it is full or nearly full, it lights up the dark part of the moon to produce the earthshine or the effect known as “the old moon in the new moon’s arms”. Occasionally it covers the sun to bring about a total solar eclipse. Its dark disk is then rimmed with a bright halo, emphasizing the red shades of sunset and providing striking evidence of the presence of the earth’s atmosphere.

Contrary to the dreams of astrologers, the moon has no effect whatever on the brains and minds of human beings. But it does raise tides both in the waters of the earth and within the earth itself. The waters directly beneath the moon heap up to form a slight bulge. Those on the opposite side, or side facing away from the moon, do the same. The two bulges stay in line with the moon as it revolves about the earth. Ad the earth rotates beneath them to produce a succession of alternate high and low tides. Without the tides, many estuaries and ports would be useless. Many of our shores would become permanent rubbish heaps.

The End of the Moon

The tides produced in the body of the earth are much smaller than those produced in the seas and oceans. They distort the earth’s crust, causing many places to fall and rise several inches a day. They also, along with the friction produced by tides in shallow seas, cause the earth’s rotation to slow down by a tiny fraction of a second a century. The day is therefore getting longer, and after several thousand million years from now the day will contain about 1,400 hours.

The earth exerts a similar braking action on the moon. The main effect of this action is to increase the moon’s distance and its period of revolution about the earth. When the moon has moved out to a distance of about 340,000 miles, its period of revolution also will equal about 1,400 hours. In other words, the length of the day will equal that of a lunar month. After that, for physical reasons, the moon will slowly draw closer to the earth. In the end it will probably break up to form rings of debris similar to the rings of Saturn.

The Sun photographed at sunspot maximum, December 21, 1957