Electrical Aptitude Practice Test 6

The Rhythm of the Tides

¹Along most coastlines, the sea creeps up the shore and slips back again in a steady rhythm. These rises and falls, called tides, are caused mainly by the gravitational pull of the moon and, to a lesser degree, by the pull of the sun. As Earth rotates, any given point on its surface passes through two tidal "bulges" of ocean water, so most coastal areas have two high tides and two low tides in what is called a lunar day.

²A lunar day is not the same as an ordinary 24-hour solar day. Because the moon travels around Earth in the same direction that Earth spins, a spot on Earth needs about 50 extra minutes to come back under the moon. A lunar day therefore lasts 24 hours and 50 minutes. High tides on such a coast arrive 12 hours and 25 minutes apart, and the water takes about 6 hours and 12 minutes to fall from high tide to low tide. For the same reason, the tides at a given beach come about 50 minutes later each day.

³Not every coast follows the same pattern. Many places on the eastern coast of North America have semidiurnal tides: two high and two low tides of about equal size each lunar day. Much of the western coast has mixed semidiurnal tides, in which the two highs and the two lows differ in size. Many areas in the Gulf of Mexico have diurnal tides, with only one high and one low tide each lunar day.

⁴The sun changes the size of the tides more than their timing. Solar tides are only about half as large as lunar tides. At the new moon and the full moon, the sun, moon and Earth line up, and the sun's pull adds to the moon's. The result is a spring tide, with extra-high high tides and very low low tides. About a week later, at the quarter moons, the sun and moon pull at right angles to each other. The sun's pull then partly cancels the moon's, producing moderate neap tides. Each lunar month has two sets of spring tides and two sets of neap tides.

⁵Distance matters as well. The moon's orbit is an oval, so its distance from Earth changes by about 31,000 miles during a month. At perigee, the moon's closest point, tidal ranges are above average; at apogee, about two weeks later, they are below average. Earth is also closest to the sun around January 2 and farthest around July 2, which slightly strengthens tides in January and weakens them in July.