Page:Astronomy for Everybody.djvu/154

the moon is in a certain position in the heavens, it will be high tide when the moon is in or near that position day after day, month after month, and year after year. We have all heard that the moon produces these tides by its attraction on the ocean. We readily understand that when the moon is above any region its attraction tends to raise the waters in that region; but the circumstance that most perplexes those who are not expert in the subject is that there are two tides a day, high tide occurring not only under the moon, but on the side of the earth opposite the moon. The explanation of this is that the moon really attracts the earth itself as well as it does the water. It continually draws the entire earth and everything upon it toward itself. As it goes round the earth in its monthly course, it thus keeps up a continual motion of the latter. If it attracted every part of the earth equally, the ocean included, there would then be no tides, and everything would go on on the earth's surface as if there were no attraction at all. But as the attraction is as the inverse square of the distance, the moon attracts the regions of the earth and oceans which are nearest to it more than the average, and those that are farthest from it less than the average.

To show the effect of these changes let A, C, and H be the three points on the earth attracted by the moon. Since the moon attracts C more than A, it tends to pull C away from A and increase the distance between A and C. At the same time pulling H more than C it tends to increase the distance between H and C. If the whole earth was a fluid, the attraction of the moon would be simply to