Some Facts About the Moon

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Nature Magazine, November 1948

Among all the satellites of the solar system our moon is unique because of its great size relative to that of the earth around which it revolves. The two largest satellites of Jupiter have diameters only about one-twentieth that of Jupiter, but the diameter of the moon is about one-fourth that of the earth. Seen from Venus at the time that planet is nearest the earth, and between it and the sun, the earth-moon system forms a beautiful double star in which the earth appears two or three times more brilliant than Venus does to us, and the moon appears as bright as Jupiter. As the moon revolves around the earth the distance between the two bodies, seen from Venus, is continually changing and can never be greater than about one-half degree, or the apparent diameter of the moon viewed from the earth.

One of the most remarkable facts about the moon is that only six-tenths of its surface, approximately, is seen from the earth. The remainder of the lunar surface has never been seen from our planet. This is due to the fact that the moon turns once on its axis in the same time that it completes one revolution around the earth. As a result, the same face is always turned toward the earth. There are, however, "librations," or apparent swayings of the moon, arising from various causes, as a result of which we see first around one edge and then around another edge; so at one time or another about six-tenths of the entire surface comes into view. How the other four-tenths of the surface appears we can only surmise but it is probably very similar in its general features to the side turned toward us.

For the small telescope the moon is the most interesting to observe of all the heavenly bodies. Even if one has only binoculars it is possible to distinguish some of the most remarkable surface features of the moon.

The most characteristic features of the lunar surface are the craters, which vary in size from the tiny craterlets, which appear as mere pinpricks on the surface less than a mile in diameter, to the great walled plains, a hundred miles or more across. The typical crater averages from ten to sixty miles in diameter, and usually has a central peak, or group of several peaks, rising from a floor depressed a mile or more below the surrounding terraced walls or ramparts. Small crater-pits often are found on these encircling walls, and, in some instances, light-colored rays extend in all directions from them. Two of the most interesting ray-systems are those associated with two of the most conspicuous and beautiful craters, Tycho and Copernicus. The rays connected with Copernicus are feathery and intricate in pattern, while those that emanate from Tycho, and appear to originate in the small craterlets on the rim, extend in long, straight lines like meridians from a pole to distances of hundreds of miles. The most remarkable of these extends centrally across the surface of the moon nearly to the northwest limb. The origin of these enigmatical markings has never as yet been satisfactorily explained. They appear to lie always at the same level as the surface over which they pass.

The large, dark, smooth areas that are so conspicuously visible on the face of the moon, misnamed seas, or maria, since no water exists on the moon, are found most extensively in the northern hemisphere of the moon, although the Mare Nubium, Sea of Clouds, fills a large part of the south-eastern portion of the surface. The roughest and most spectacular appearing portion of the lunar surface lies in the high southern plateau region, where craters are piled upon craters, and there are huge walled plains in the wildest confusion. Almost every type of lunar marking is to be found in this region – deep, narrow lunar valleys, or rills as they are called; cracks or clefts in the surface appearing only as thin lines even in large telescopes, and countless lofty mountain peaks. The Leibnitz Mountains, on the southwestern limb, and the Doerfel Mountains, on the southeastern limb, are the loftiest of the lunar mountain ranges, with some peaks exceeding 27,000 feet in height. As they appear on the southern limb of the moon they cause the extreme roughness of the edge and produce the remarkable "Baily's Beads" caused by light shining through the lunar valleys on the edge of the moon at time of a total eclipse of the sun.

Baily's Beads will occur, of course, at any point on the rough edge of the moon where lofty mountain peaks and deep valleys appear in contour. They are particularly remarkable when seen along this roughest portion of the lunar surface. The most remarkable mountain ranges on the moon, the most massive and longest, lie in the northern hemisphere, where the lunar Alps, Caucasus, and The Apennines are located.

The latter is considered to be the most magnificent range of mountains on the lunar surface. It extends a distance of five hundred miles or more in one continuous curve along the edge of the Mare Imbrium, and has more than 3000 peaks, many rising to heights of three miles or more. The range ends in a conspicuous ring-plain, called Eratosthenes, about 40 miles in diameter. It has a two-peaked, central mountain rising from a floor depressed 8000 feet below the level of the Mare Imbrium. The northern extremity of the lunar Apennines terminates in a lofty mountain peak, Mount Hadley, which rises to a height of more than 15,000 feet. Although the mountains of the moon have been named chiefly for terrestrial mountains, and the seas bear such fanciful names as Sea of Serenity, Sea of Tranquility, Sea of Clouds, etc., the craters bear the names, with few exceptions, of great philosophers and noted astronomers. A visitor to the moon could be provided with a map of a world as well charted as that from which he came, with the exception of the mysterious four-tenths of the surface awaiting his exploration.

No body in the solar system has given the astronomer more work and more problems to solve than our own satellite. Quite aside from the observation, naming and mapping of the principal features, which has been, with the determination of the size and elevation of mountains and their peaks as well as the measurements of craters in great numbers, a task of no mean size, there has been also the problem of the cause and origin of the different types of lunar features. Many scientific articles and books have been written upon the subject of the moon and the problems it presents. Of all the pages of astronomical data and information published in The American Ephemeris and Nautical Almanac, more than four-tenths of the total number are devoted to data and information concerning the moon. The same is true, approximately, of national almanacs of other countries. The position of the moon is published for every hour. Pages are devoted to eclipses of the sun, caused by the moon, and eclipses of the moon in the earth shadow. These and many other pages, giving data on occultations of stars by the moon, are of great importance in many scientific investigations now being carried on. When a star disappears behind the moon, reappearing a number of minutes or an hour or more later, it is said to be occulted by the moon. Observations of a large number of these occultations are made by observers all over the world, so that the moon's position may be determined as accurately as possible. Eclipses of the sun are valuable for the same purpose, as well as for other scientific purposes of considerable value and importance.

One of the greatest of the mathematical problems that have engaged the attention of men skilled in celestial mechanics has been the problem of the moon's motion, which has not yet been completely solved. There are corrections of the moon's position to be made continually to bring agreement between theory and observation.

Other information furnished, concerning the moon, for those who need it has to do with the tides, which are caused by the action of both sun and moon, although this is not supplied by the Almanac Offices. They do, however, publish much necessary information for observations of the surface features of the moon. Data on time of moonrise and moonset and on the phases of the moon are also published and widely used. One might conclude, then, that our satellite is quite a troublesome as well as important little body. One should add that it also gives us more pleasure and satisfaction than any other body in the heavens, with the exception of the all-important sun. It would be a dreary world, indeed, without the friendly light of the moon, especially in winter in high latitudes, where the long nights are brightened by its presence for many long hours.

On November 1 there will be a total eclipse of the sun visible in its total phase within a narrow strip extending from central Africa across the Indian and Antarctic Oceans, and in its partial phases over all of Africa, except the western part, and over most of Australia and New Zealand. The longest duration of the total eclipse will be less than two minutes.

The planet Mercury will be in fine position for observation in the eastern sky before sunrise until after the middle of the month. It will be farthest west of the sun on November 4. Venus is still a fine object in the morning sky before sunrise, although it is now drawing closer to the horizon. Mars is still in the evening sky, but is getting too low for good observation. Jupiter is also in the western sky in the evening in Sagittarius and sets a few hours after sunset. Saturn is in Leo and will be in quadrature, ninety degrees west of the sun, on November 28. It then rises about midnight and is on the meridian near sunrise.

The Leonid meteors should make their annual appearance on November 13-15 and be seen in largest numbers in the early morning hours on those dates.