The revolutions of the moon have been a measurement of time for ages, and her varying appearances during lunation are always observed with interest. The illustration (fig. 567) will assist us materially. The sun’s rays fall in a parallel direction upon the earth and moon, and let us suppose that S is the sun in the diagram and T the earth; C at the various points is the moon, the capital letters, A, B, C, etc., indicating the planet as she appears from the sun, and the small letters show how she appears to us from the earth.
Let us suppose that the sun, earth, and moon are in conjunction—or in a direct line. The phases, C and G, are the moon’s “quadratures.” At A we see the sun shining on the moon, but we only have the dark side. It is then “new” moon; but by degrees, as she goes round in her orbit, we perceive a small crescent-shaped portion, lighted up by the sun at B and b. At c´ we have the first quarter or half-moon. When she is in opposition she is at full moon, and so on to the last quarter and conjunction again.
Fig. 568.—Crescent Moon.
The moon’s phases may be easily shown by means of a medium-sized lamp to represent the earth, a smaller one to serve as moon, and a light to act as sun all at the same height. Colour the “lunar” globe white, and if we move it about the “earth” globe, we shall see the various phases of the moon in the sharp shadows.
The Tides.
The ebb and flow of our tidal waters depend upon the moon to a great extent. The phenomenon is so common, that we need only refer to it, but the cause of the tides may be stated. Twice every day we have the tides twelve hours apart, and the flow and ebb are merely examples of the attraction of gravitation, which is exercised upon all bodies, either liquid or solid. The tides are highest at the equator and lowest at the poles, because the tropics are more exposed to the influence of the lunar attraction.
Fig. 569.—Moon’s attraction.
By the small diagram (fig. 569) we shall be able to see in a moment how the moon acts. The moon being nearer to the earth at b, the water will be naturally attracted to the ball, m, and cause high water (a); and a similar effect will be produced opposite, because the earth is attracted, so the waves are higher than the ground which has been attracted away from the water, and the waters will flow in and cause a high tide at d, but not so high a tide as at the opposite point, a. It can then be understood that there will be low water at the other two sides, e and f, because the water has been taken away, so to speak, for the high tides at a and d. We shall learn more of this under Physical Geography.