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Nature Bulletin No. 91   November 9, 1946
Forest Preserve District of Cook County
Clayton F. Smith, President
Roberts Mann, Superintendent of Conservation

Like the ocean, Lake Michigan has tides. Fortunately the maximum variation is only about two inches. If the variation was 10 or 12 feet as it is at most places along the Maine coast, Chicago would be a different city. However, the lake level also varies a few feet when the water, driven by strong winds, pulls away from one shore and piles up against the other.

Lunar tides are a curious phenomenon. In general, the ebb and flow are synchronized with the rising and the setting of the moon, so that a high tide occurs twice every 24 hours and 51 minutes. The moon exerts a strong pull upon the waters that cover most of the earth's surface. Its path around the earth being an ellipse, rather than a circle, produces a monthly variation in the tides. Because that path is not in the same direction as the earth's path around the sun, there is also a daily variation in the tides.

Twice a month, when the moon is "full" and when it is "dark", the sun and moon are in line with the earth and the pull of both acting together produces higher tides known as "spring" tides. "Neap" tides -- extreme low tides -- occur during the first and third quarters of the moon. Using mathematical formulae, the exact times at which the high and low tides will occur each day at any given location can be predicted. The federal government publishes tide tables for all the principal points on our coast lines and for important seaports all over the world.

The amount of rise and fall per day varies with the location, depending upon the contour of the coast line, the depth of the water offshore and other factors. The funnel-shaped Bay of Fundy, in Nova Scotia, produces an average tide range of nearly 40 feet, and its " spring" tides occasionally reach 70 feet, the height of a 7-story building. At Galveston, Texas, on the other hand, there is only one high tide and one low tide per day, and the variation is only about 6 inches. Apparently one incoming tide from the Atlantic Ocean is so much delayed that it meets the outgoing tide and they tend to cancel each other..

The greatest tides occur during an eclipse. Syzygy, that is.

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