As you doubtless realize, the moon keeps the same side always toward the earth. As it swings through its monthly orbital cycle, the portion of the sunlit half turned toward us changes continually to produce the phases, but it is always the same physical hemisphere of the moon that faces us. Regardless of whether it is illuminated or dark, Mare Crisium is always near the eastern limb, Grimaldi is near the western limb, Ptolemy is just south of the center of the disk, etc. Had this not been so, the ancient concept of the Man in the Moon probably would not have developed, and the modern Soviet scientific feat of sending Lunik 3 to photograph the far side in October, 1959, would not have aroused so much interest.
Does the moon rotate on an axis as does the earth? Rotation is standard behavior for the bodies of the solar system and those of the outer universe as well. It is an effective way of storing energy, it provides stability, and we might even say it is an expected property of matter that accumulates in large quantities. All planets rotate, and their periods are precisely known, with the possible exception of our nearest planetary neighbor Venus. Galileo discovered that the sun rotates, and 300 years later Frank Schlesinger found that stars do likewise. So the moon should conform to the general pattern and rotate on an axis. "But it can't," is a frequent objection, "or we would see the other side." This seems to be a valid argument. When, as a bystander, we watch a merry-go-round carry its load of beaming children in musical circles, we certainly do see every part of its circumference. When it comes to observing the motions of the moon, however, we are not in the position of a bystander. We are involved in those motions as part of the earth-moon system, and our observation station is never more than a few thousand miles from the active focus of the moon's orbit. In effect, we are riding on the merry-go-round ourselves, and our point of view regarding what is taking place is quite biased. In order to view the moon's motions as a bystander, we would have to board a space ship and blast off into the far yonder. After trav eling several million miles we might settle down to observations of the earth and the moon to keep ourselves occupied during the many months a journey to another planet would take. Then, indeed, we would see all sides of the moon during a month.
The moon does rotate 011 an axis, an axis that is inclined 83',/» degrees to the plane in which its orbit lies. Its direction of rotation is the same as its direction of revolution around the earth (eastward), and the periods of rotation and revolution are exactly the same. That is why we never see the far side. For those who still may be unconvinced, the following lunar laboratory exercise is recommended: Get a can of peas or artichokes or anything else out of the kitchen. That will be the moon, and you will be the earth. Grasp the can by its base and hold it upright at arm's length so that the brand label is toward you. Now cause it to revolve in an orbit around you by turning yourself around in place as you stand. The can will make a complete revolution, but you will see
■ one side of it. Enlist the assistance of an unbiased observer who will serve as the bystander across the room. After you have revolved the can in the manner described through 360 degrees, ask the bystander if he saw all sides of it. If he is really unbiased, I believe he .will have to answer in the affirmative. As in the case of the moon, the can of peas will have rev olved in an orbit and rotated on an axis, both in the same direction and both in the same period.
Is it a coincidence that the moon's periods of rotation and revolution are identical? In the vast outer universe, which the scientist must explain in terms of natural law, is there room for what appear to be fortunate accidents? The answer to the second question is yes, and the moon provides an example. We have seen how enormous the sun is compared with the moon (56-foot sphere and a golf ball). In fact, the diameter of the sun is 400 times that of the moon. Yet it happens that the average distance of the sun from the earth is also about 400 times that of the moon. Consequently, both sun and moon appear the same size in the sky (diameter '/j degree), and we are permitted to enjoy perfect eclipses of the sun on those occasions when the new moon actually passes in front of the sun. If the moon's disk appeared considerably smaller, total solar eclipses would be impossible, and we would know much less than we do about the atmosphere of the sun. If the moon's disk appeared considerably larger, it would cut off not only the light of the sun's disk during eclipse but also much of the solar atmospheric light as well. The fact that it just fits is a coincidence unpredictable through natural law. The other planets of the solar system have a total of 42 satellites known to us, but none of them has such a relationship to the sun.
It is not a coincidence, however, that the moon rotates and revolves in identical periods. Long ago the moon undoubtedly turned much faster on its axis and showed all of its surface to the earth. At that time there were no people here to observe it and probably no living creatures of any kind. We know that the moon raises tides in the solid surface of the earth as well as on the oceans, and. of course, the earth does the same thing to the moon, it takes a staggering amount of energy to do all that work every day, and that energy has to come from some source. One source available was the rotational energy of the moon which was thus continually dissipated through tidal friction. Consequently, the rotational speed slowed down, and the lunar day grew longer. After many millions of years the rotational period lengthened until it was exactly equal to the period of revolution. Thereafter, no further change was required. This problem was investigated mathematically in the latter part of the nineteenth century by George Darwin, who predicted the same fate for the earth. The length of our day is increasing slowly because of loss of rotational energy through tidal friction, and the time will come when the earth will keep the same face toward the moon. Then some of the people will have the moon in their sky all the time, and other people will never see it unless they travel to the side of the earth that is toward it. But be not dismayed. You have plenty of time left to get acquainted with the moon. It will take a great many millions of years to bring about such a radical change.
Having gone to some length to convince you that the moon keeps the same side always toward the earth, I now am going to exercise again the professor's prerogative and point out that this is not the whole truth. If it were, we would have no possibility of seeing more than 50 per cent of the moon's surface. Actually we can do better, and through a series of observations we can see 59 per cent of the surface. This comes about through certain earth-moon relationships known by the ancient term librations. The most obvious is the libralion in latitude. We have seen that the moon's axis of rotation is not exactly perpendicular to the plane of its orbit around the earth since it is set at an angle of 831/2 degrees rather than 90 degrees to that plane. Even though the moon rotates rather slowly about that axis it still enjoys great rotational stability and keeps its axis at all times pointed in the same direction with respect to the stars. Consequently, on a certain date during each lunation (the moon's monthly cycle of phases), the lunar north pole is tipped about 6l/i degrees toward us, and we can observe the surface 6]A degrees beyond the north pole. Two weeks later, when the moon has traveled in its orbit around to the other side of the earth, the lunar south pole will be tipped toward us, and we will observe the surface 6</2. degrees beyond the south pole (Fig. 12).
The libration in latitude is a phenomenon analogous to the seasons on the earth. Since the earth's axis is inclined 66V£ degrees to the plane of its orbit around the sun, the sun "sees" 23% degrees beyond the north pole on June 22 each year and 23 >4 degrees beyond the south pole on December 21 each year. To illustrate this libration, stick a
MOON'S I FAR SIDE
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