READING TEST 1 PART 3

PASSAGE 3 Questions 21-30

Factors that influence tides

The force that generates tides results from the interaction of two forces: the centrifugal force produced by the revolution of the earth around the center of gravity of the earthmoon system; and the gravitational attraction of the moon acting upon the earths waters. Although the moon is only 238,852 miles from the earth, compared with the suns much greater distance of 92,956,000 miles, the moons closer distance outranks its much smaller mass, and thus the moons tide-raising force is more than twice that of the sun.

The tide-generating forces of the moon and sun cause a maximum accumulation of the waters of the oceans at two opposite positions on the earths surface. At the same time, compensating amounts of water are drawn from all points 90 degrees away from these tidal bulges. As the earth rotates, a sequence of two high tides occur on an average of 12.4 hours apart. High tide at any given location occurs when the moon is overhead and low tide when it is at either horizon.

The highest and lowest levels of high tide, called spring tide and neap tide, each occur twice in every lunar month of about 27.5 days. A spring tide occurs at the new moon and at the full moon, when the moon and earth are lined up with the sun, and thus the moons pull is reinforced by the suns pull. At spring tide, the difference between high and low tides is the greatest. A neap tide, the lowest level of high tide, occurs when the sun-to-earth direction is at right angles to the moon-to-earth direction. When this happens, the gravitational forces of the moon and sun counteract each other; thus, the moons pull is at minimum strength, and the difference between high and low tides is the least. Spring and neap tides at any given location have a range of about 20 percent more or less, respectively, than the average high tide.

The vertical range of tides the difference between high and low varies according to the size, surface shape, and bottom topography of the basin in which tidal movement occurs. In the open water of the central Pacific, the range is no more than about a foot; in the relatively small, shallow North Sea, it is about 12 feet. Along the narrow channel of the Bay of Fundy in Nova Scotia, the difference between high and low tides may reach 45 feet under spring tide conditions the worlds widest tidal range. At New Orleans, which is at the mouth of the Mississippi River, the periodic rise and fall of the tides varies with the rivers stage, being about ten inches at low stage and zero at high. In every case, actual high or low tide can vary considerably from average.