The Moon will be full tomorrow night, and people who live on the coast know what that means: some of the highest tides of the month. And in fact, people who live along the shores of big lakes will see big tides, too — relatively speaking that is: up to an inch or two on Lake Michigan, and a bit less on the other Great Lakes.
Ocean tides are caused by the gravitational pull of the Moon and Sun. Their gravity pulls a little more strongly on the side of Earth that faces them than on the side that faces away from them, producing “bulges” in the oceans. The bulges sweep all the way around the planet, following the Moon as Earth turns on its axis. When they hit land, they cause the water level to rise — producing high tides.
The tides are strongest when the Moon and Sun are pulling along the same line — either at new Moon, when the Moon and Sun are on the same side of Earth, or at full Moon, when they’re on opposite sides of the planet.
The Moon and Sun also pull on lakes. But because these bodies are smaller, and they’re hemmed in by land, they can’t produce the same kinds of waves that oceans do — tidal waves that travel all the way around the planet.
Even so, bodies like the Great Lakes do have their own tides. But they’re so small that they’re hard to detect — wind or other local effects easily drown them out.
Tomorrow night’s full Moon, by the way, is the most famous full Moon of them all: the Harvest Moon. More about that tomorrow.
Script by Damond Benningfield, Copyright 2011
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