But the earth is attracted by the sun as well as the moon; and, as regards absolute amount, the solar attraction is much greater than the lunar. That is, the earth as a whole is far more strongly attracted by the sun than the moon ; and if the tides were formed by the mere attraction of the sun and the moon, the solar tide would be much greater than the lunar. But the moon's tide-producing power is 2i times greater than the sun's—the tide being produced not by the attraction of the sun or moon on the earth as a whole, but by the difference of their attraction on different parts of the earth. The total attraction of the sun is vastly greater than that of the moon, but the difference in the attraction of the moon on different parts of the earth is greater than the difference in the sun's attraction. Independent of the moon, the sun would produce a tide 2 times less than the lunar tide ; and as both the sun and the moon act on the earth at the same time, but in con tinually varying directions—the solar tide is undistinguish able from and merged in the lunar. When the sun and the moon are on the same or opposite sides of the earth, so that their attractive power is exerted in the same line, the solar and lunar tides are combined, thus producing the greatest possible " flow " of the water, known as spring-tides. Spring tides are formed both at new and full moon. But when the
moon is in her first or last quarter, that is, 7 or 21 days old, her attraction is diametrically opposed to that of the sun. Consequently, the solar tide neutralises the lunar to some extent, and thus the lowest possible tides are formed, known as neap-tides. Spring-tides are thus the sum, and neap-tides the difference, of the solar and lunar tides.
The rotation of the earth on its axis brings every meridian successively under the sun and the moon. But while the sun is relatively to the earth immovable, passing over every meridian regularly in 24 hours, the moon travels east ward, in her monthly circuit, round the earth. The earth, therefore, must make somewhat more than a complete rotation before the moon is again over a given meridian. Thus, while the solar day is 24 hours, the lunar day is 24 hours 54 minutes, and as the moon is the principal agent in producing the tide, the flow and ebb will not occupy exactly six hours each time, but high-water will occur about 50 minutes later each day than on the previous day. Another fact which must not be overlooked is, that high-water does not occur even in the open ocean at the exact time of the moon's passage over a given meridian, but always about two hours after. Similarly both spring-tides and neap-tides do not occur exactly when the moon changes, or at full-moon, but a day or two later.