The Moon: A Popular Treatise

by Garrett Putman Serviss

The Moon and the Tides.

“The eclipse is not yet total,” I continued, glancing out of the door, “and we can finish our explanation before it becomes so. Have the kindness, then, to look at the diagram. Suppose E to be the center of the earth, and M the center of the moon. The protuberant portions of the earth C A D and D B C represent the waters of the ocean pulled away from the surface of the earth, if I may so describe it, by the moon’s attraction. You are probably aware that the attraction of gravitation varies with the distance of the attracting body. The distance from the center of the earth to the center of the moon is about 239,000 miles. But the earth being nearly 8,000 miles in diameter, the surface of the ocean at A is about 4,000 miles nearer to the moon than is the center of the earth E. It follows that the force of the moon’s attraction is greater at A than at E. If the water of the ocean were a fixed, solid part of the earth there would be no perceptible effect resulting from this difference in the amount of the moon’s attraction. But since the water is free to move, to a certain extent, it yields to the attraction, and is drawn up a little toward the moon. At the same time it is, in effect, drawn away from C and D. The consequence is the production of a tide on the side facing the moon.

“Now, for the other tide, produced at the same time on that side of the earth which is turned away from the moon. The point B is about 4,000 miles farther from the moon than E; consequently the moon’s attractive force is less at B than at E. From this it results that the body of the earth is more forcibly attracted by the moon than is the water at B. The earth therefore tends to move away from the water at that point, and another tidal protuberance is produced, with its highest part at B. I should add that while the water of the ocean is, to a certain degree, free to respond to these differences of attraction, the earth itself, being solid, can only move as a single body, and, mathematically, we may regard it as if its entire mass were concentrated at the center E. Please remember, however, that this explanation is only elementary, only intended as a graphic representation of the tides, and not as a mathematical demonstration of the way they are produced. Such a demonstration would only be suited to one of the technical books that you have not found as interesting as—some other branches of literature.

“There is just one other thing to which I must ask your attention, and then we shall return to the moon herself and the phenomena of the eclipse now in progress. You will notice in the diagram, that two arrows show the direction in which the earth is continually rotating on its axis, and that a dotted curve, terminating with an arrow point, indicates the course of the moon in her orbit surrounding the earth. The rotation of the earth is so much more rapid than the motion of the moon that the points A and B are carried out of the line drawn from the center of the moon to the center of the earth, in the direction of the arrows. From this it follows that the tides are never directly under the moon, or exactly opposite to her, but sweep in great waves round the globe. The tides produced by the attraction of the sun are only about two fifths as high as those caused by the moon. As I have already explained they are sometimes superposed upon the lunar tides—at New and at Full Moon—and sometimes they are situated at right angles to the lunar tides—at First and Last Quarters.”

“But the eclipse!” interrupted my friend, whose attention had evidently begun to wander. “I think the totality of which you spoke must be at hand, for notice how dark the park has become, and the fireflies are so brilliant under the trees.”

The total phase of the eclipse was, indeed, beginning, and we stepped out on the lawn before the door to watch it. The moon had now passed entirely within the earth’s shadow, but although her light was almost completely obscured as far as its power to illuminate the landscape was concerned, still the face of the moon was dimly visible, as if concealed behind a thick veil. Certain parts of it had a coppery color, producing a very weird effect.

“Dear me!” exclaimed my companion, “I did not think it would look like that! I naïvely supposed that one could not see the eclipsed moon at all, but that she either disappeared or was turned into a kind of black circle in the heavens. And what a strange color she has! Positively it fills me with awe.”

“It is very rare,” I said, “for the moon to become invisible during an eclipse. That can only occur when the earth is enveloped in clouds.”

“Indeed, and what have the clouds to do with it? If the solid body of the earth cannot cast a shadow dense enough to hide the moon, I should not expect things so evanescent as clouds to be more effective.”

“It is all owing to the earth’s atmosphere,” I replied. “If our globe were not surrounded with a shell of air the moon would always be totally invisible when eclipsed. But the atmosphere acts like a lens of glass inclosing the earth; that is to say, it refracts, or bends the rays of sunlight around the edge of the earth on all sides, and throws a portion of them even into the middle of the shadow, at the moon’s distance. It is these refracted rays which cause the singular illumination that you perceive on the moon. But when, as occurs only occasionally, all that part of the atmosphere which surrounds the earth along the edge visible from the moon is filled with clouds, the air can no longer transmit the refracted rays, and then, no light being sent into the shadow, a ‘dark eclipse,’ as astronomers call it, results. An eclipse of the sun is a very different thing. That is caused not by a shadow but by the opaque globe of the moon passing between the earth and the solar orb. When this occurs the sun is completely hidden behind the moon, and only its corona, which projects beyond the moon on all sides, is visible.”