The Heavens Above: A Popular Handbook of Astronomy

by J. A. Gillet; W. J. Rolfe

Fig. 84.

71. Cause of the Change of Seasons.—Variety in the length of day and night, and diversity in the seasons, depend upon the obliquity of the ecliptic. Were there no obliquity of the ecliptic, there would be no inequality in the length of day and night, and but slight diversity of seasons. The greater the obliquity of the ecliptic, the greater would be the variation in the length of the days and nights, and the more extreme the changes of the seasons.

Tides.

72. Tides.—The alternate rise and fall of the surface of the sea twice in the course of a lunar day, or of twenty-four hours and fifty-one minutes, is known as the tides. When the water is rising, it is said to be flood tide; and when it is falling, ebb tide. When the water is at its greatest height, it is said to be high water; and when at its least height, low water.

73. Cause of the Tides.—It has been known to seafaring nations from a remote antiquity that there is a singular connection between the ebb and flow of the tides and the diurnal motion of the moon.

Fig. 85.

This tidal movement in seeming obedience to the moon was a mystery until the study of the law of gravitation showed it to be due to the attraction of the moon on the waters of the ocean. The reason why there are two tides a day will appear from Fig. 85. Let M be the moon, E the earth, and EM the line joining their centres. Now, strictly speaking, the moon does not revolve around the earth any more than the earth around the moon; but the centre of each body moves around the common centre of gravity of the two bodies. The earth being eighty times as heavy as the moon, this centre is situated within the former, about three-quarters of the way from its centre to its surface, at the point G. The body of the earth itself being solid, every part of it, in consequence of the moon's attraction, may be considered as describing a circle once in a month, with a radius equal to EG. The centrifugal force caused by this rotation is just balanced by the mean attraction of the moon upon the earth. If this attraction were the same on every part of the earth, there would be everywhere an exact balance between it and the centrifugal force. But as we pass from E to D the attraction of the moon diminishes, owing to the increased distance: hence at D the centrifugal force predominates, and the water therefore tends to move away from the centre E. As we pass from E towards C, the attraction of the moon increases, and therefore exceeds the centrifugal force: consequently at C there is a tendency to draw the water towards the moon, but still away from the centre E. At A and B the attraction of the moon increases the gravity of the water, owing to the convergence of the lines BM and AM, along which it acts: hence the action of the moon tends to make the waters rise at D and C, and to fall at A and B, causing two tides to each apparent diurnal revolution of the moon.

74. The Lagging of the Tides.—If the waters everywhere yielded immediately to the attractive force of the moon, it would always be high water when the moon was on the meridian, low water when she was rising or setting, and high water again when she was on the meridian below the horizon. But, owing to the inertia of the water, some time is necessary for so slight a force to set it in motion; and, once in motion, it continues so after the force has ceased, and until it has acted some time in the opposite direction. Therefore, if the motion of the water were unimpeded, it would not be high water until some hours after the moon had passed the meridian. The free motion of the water is also impeded by the islands and continents. These deflect the tidal wave from its course in such a way that it may, in some cases, be many hours, or even a whole day, behind its time. Sometimes two waves meet each other, and raise a very high tide. In some places the tides run up a long bay, where the motion of a large mass of water will cause an enormous tide to be raised. In the Bay of Fundy both of these causes are combined. A tidal wave coming up the Atlantic coast meets the ocean wave from the east, and, entering the bay with their combined force, they raise the water at the head of it to the height of sixty or seventy feet.

75. Spring-Tides and Neap-Tides.—The sun produces a tide as well as the moon; but the tide-producing force of the sun is only about four-tenths of that of the moon. At new and full moon the two bodies unite their forces, the ebb and flow become greater than the average, and we have the spring-tides. When the moon is in her first or third quarter, the two forces act against each other; the tide-producing force is the difference of the two; the ebb and flow are less than the average; and we have the neap-tides.