FIG. 21.—THE TIDES.
A, Spring Tide (New Moon); B, Neap Tide.
The most conspicuous service which our satellite performs for us is that of raising the tides. The complete statement of the manner in which she does this would be too long for our pages; but the general outline of it will be seen from the accompanying rough diagram (Fig. 21), which, it must be remembered, makes no attempt at representing the scale either of the bodies concerned or of their distances from one another, but simply pictures their relations to one another at the times of spring and neap tides. The moon (M in Fig. 21, A) attracts the whole earth towards it. Its attraction is greatest at the point nearest to it, and therefore the water on the moonward side is drawn up, as it were, into a heap, making high tide on that side of the earth. But there is also high tide at the opposite side, the reason being that the solid body of the earth, which is nearer to the moon than the water on the further side, is more strongly attracted, and so leaves the water behind it. Thus there are high tides at the two opposite sides of the earth which lie in a straight line with the moon, and corresponding low tides at the intermediate positions. Tides are also produced by the attraction of the sun, but his vastly greater distance causes his tide-producing power to be much less than that of the moon. His influence is seen in the difference between spring and neap tides. Spring tides occur at new or full moon (Fig. 21, A, case of new moon). At these two periods the sun, moon, and earth, are all in one straight line, and the pull of the sun is therefore added to that of the moon to produce a spring tide. At the first and third quarters the sun and moon are at right angles to one another; their respective pulls therefore, to some extent, neutralize each other, and in consequence we have neap tide at these seasons.
The Moon, April 5, 1900. Paris Observatory.
No one can fail to notice the beautiful set of phases through which the moon passes every month. A little after the almanac has announced 'new moon,' she begins to appear as a thin crescent low down in the West, and setting shortly after the sun. Night by night we can watch her moving eastward among the stars, and showing more and more of her illuminated surface, until at first quarter half of her disc is bright. The reader must distinguish this real eastward movement from the apparent east to west movement due to the daily rotation of the earth. Its reality can readily be seen by noting the position of the moon relatively to any bright star. It will be observed that if she is a little west of the star on one night, she will have moved to a position a little east of it by the next. Still moving farther East, she reaches full, and is opposite to the sun, rising when he sets, and setting when he rises. After full, her light begins to wane, till at third quarter the opposite half of her disc is bright, and she is seen high in the heavens in the early morning, a pale ghost of her evening glories. Gradually she draws nearer to the sun, thinning down to the crescent shape again until she is lost once more in his radiance, only to re-emerge and begin again the same cycle of change.
The time which the moon actually takes to complete her journey round the earth is twenty-seven days, seven hours, and forty-three minutes; and if the earth were fixed in space, this period, which is called the sidereal month, would be the actual time from new moon to new moon. While the moon has been making her revolution, however, the earth has also been moving onwards in its journey round the sun, so that the moon has a little further to travel in order to reach the 'new moon' position again, and the time between two new moons amounts to twenty-nine days, twelve hours, forty-four minutes. This period is called a lunar month, and is also the synodic period of our satellite, a term which signifies generally the period occupied by any planet or satellite in getting back to the same position with respect to the sun, as observed from the earth.