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The Solar System

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Pages: 166

Book format: An electronic version of a printed book that can be read on a computer or handheld device designed specifically for this purpose.

Publisher: Date:8/4/2009 - General Books LLC

By: Charles Lane Poor

Excerpt from book: CHAPTER III TIDES AND TIDAL EVOLUTION THE tides are the regular periodic changes in the " level of the sea caused by the attractions of the sun and moon. Twice in each twenty-four hours the water rises and covers rocks and shoals along the coast, and just as regularly twice each day the water sinks back, leaving the reefs and beaches again bare. In this periodic ebb and flow of the tide there are each day moments when the sea-level is highest and moments when it is lowest. These moments are the times of " high " and " low " water respectively and the positions of the sea-level at these instants mark the heights of the high and of the low water. This periodic rise and fall of the water is accompanied by strong currents, which sweep up and down the coast. In deep bays and narrow arms of the sea these tidal currents are often very powerful and become a menace to navigation. These currents are often loosely referred to as " tides." The average interval from one high water to the next is not exactly twelve hours, but more nearly twelve hours and twenty-five minutes, so that the time of the corresponding high water is apparentlydelayed some fifty-one minutes each day. This average retardation of the tides is identical with that of the moon, the interval hetween two successive passages of the moon over the meridian being twenty-four hours and fifty-one minutes. The time of high water does not exactly coincide with the time of meridian passage of the moon, but follows after it by a certain number of hours and minutes: an interval which varies for each place of observation. Even for a single station, however, this difference of time between the moon's passage and high water is not absolutely constant, but fluctuates a few minutes either way during each month. The average interval,...  

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