Page:The moon (1917).djvu/8

Rh miles. This leads to the figures 2160 miles, a little more than one-fourth the diameter of the Earth.

Several of the satellites of Jupiter and of Saturn are fully as large as or even larger than our Moon, but the planets themselves are so much larger than the Earth that the contrast between planet and satellite is very much greater. Our Moon, in fact, ought really to be called the Earth's companion rather than its satellite. Viewed from Venus or from Mars it would easily be seen without the telescope, forming with the Earth a beautiful double star.

It is its nearness to us, however, rather than its size that makes the Moon the only body except the Sun which exercises a direct influence upon our lives here on the Earth. I am speaking now from the strictly utilitarian point of view. Planets and stars could be blotted out and in one sense our lives would go on without the slightest inconvenience, tho our intellectual and spiritual loss would be immeasurable. But let the Moon be annihilated! Immediately the effect would be felt in every shipping port in the world. The ships in dock could not get out; the ships outside could not get in; and the maritime commerce of the world would be thrown into dire confusion, for the Moon is the principal factor in producing the tides. The Sun also raises tides on the Earth but its effect is only half that of the Moon.

We cannot enter now upon the story of the tides; that would make a lecture in itself. But I want to take up one point very briefly. If the Moon raises tides upon the Earth, then the Earth must likewise exercise a tidal strain upon the Moon and because the Earth's mass is so much the greater of the two, this strain must be about 20 times that exerted by the Moon upon the Earth. We think of the tides as a phenomenon connected with the ocean, but a moment's reflection will make it clear that the pull of the Moon is just as strong upon the solid crust of the continents. The waters of the ocean are freer to move, that is all. Now it can be shown that when a body rotates upon its axis in the same direction as its motion in its orbit, and the rotation time is shorter than the revolution period such a tidal force acts as a brake to slow up the rotational motion until the two periods are equal. It is thought by most astronomers that the Moon originally rotated much faster than it does now and that the cumulative effect of the Earth's tidal action upon it thru the ages is