CHAPTER II
Rotation, Distance, Orbit, Light, Libration, Gravitation
The earliest observers must have noticed that the Moon always presented the same face to the Earth. This is due to the fact that it rotates on its axis in precisely the same time that it revolves around us in its orbit. If these two periods, each of a little less than a month, did not agree with each other within a fraction of a second, we should sooner or later see what was on the other side of the Moon. Of course, this exact agreement cannot be due to chance. For it we must again thank, or rather perhaps blame, the tides. But in this case it is not our tides raised by the Moon, but the Moon's tides raised by the Earth, that did the work.
This simple observation, that the Moon always shows us the same face, will then, when properly interpreted, lead us to the interesting conclusion that the Moon itself was formerly partly fluid and in this fluid condition was affected by the great tides of that early period—tides not of water, but of molten rock, rising not merely a few feet every twelve hours, but as many miles in height, or perhaps even more. Such fearful tides would quickly retard the rotation even of a body of the size of the Moon, with the result that it would soon cease to rotate with regard to the tides—that is, it would present always the same face to the Earth. Under these circumstances, as we have just seen, its period of rotation on its axis and of revolution in its orbit would coincide. If referred to some particular direction, such as that of some star, we find that at the present day they would very nearly equal twenty-seven and a third of our days.
That the Moon's interior is now practically solid is almost certain, while the great tides that fixed its rotation must have ceased ages before the human race was born. Since that early time the period of revolution of the Moon in its orbit must have, as we have already seen, increased. How is it, then, that since the tides have ceased its periods of rotation and revolution still coincide? The answer is that the tides must have left their permanent mark upon our satellite, so that its surface is not spherical, but slightly elongated, the elongation lying in the direction of our Earth. Consequently, if the Moon's time of revolution begins to exceed that of its rotation, the force of the
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