An Outdoor View: All about tides

Posted: Friday, September 10, 2010

With so much change and confusion in the world today, I find comfort in simple things I can count on, year after year. Things like the tides.

Tides are so predictable. Every year someone prints booklets containing the predicted tides for the coming year for all the important locations on earth. That's how I know that today, Sept. 10, low tide at Clam Gulch occurs at 11:39 a.m., and that it's a minus 2.0, a decent clam tide. This same prediction will repeat about 19 years from now, if you're wondering.

Tides are easily understood. According to "Chapman Piloting," tides are "The vertical rise and fall of ocean water, and waters affected by the ocean, caused by the gravitational forces of the moon and the sun."

"Chapman" says "tide" and "tidal current" are sometimes misused. "Tide" refers to the vertical change in water level, while current is the horizontal flow of water.

That means it's wrong to say, "The tide is coming in," and "The tide is running strong." It's only the water affected by the tide that comes in, and it's the tidal current that's strong, not the tide. Oh, well. Nothing is ever as simple as it seems.

Chapman defines tidal "range" as "The difference between high water level and the following low water, or vice versa." "Spring" tides have the greatest range. When the moon and sun are on the same side of Earth, they exert more gravitational force, causing the ocean to bulge. Spring tides occur just after a new or full moon. When the moon is at perigee -- closest to the earth -- tides have the greatest ranges. Spring tides are confusing because the name "spring" has nothing to do with the season.

"Neap" tides have the least difference between high and low water. They happen when the moon and sun are on opposite sides of the earth, which tends to cancel out some of their gravitational forces. When the moon is at apogee -- farthest from the earth -- it least affects tidal ranges.

"Chapman" says more than gravity is involved with creating tides. It seems that the earth's elliptical orbit around the sun causes a centrifugal force that "tries to pull it off into space," and causes the ocean to bulge even more. "Chapman" then -- a little too casually, I thought -- says the earth and the moon both revolve around a common point "situated deep inside the earth." Due to centrifugal force, both bodies want to "fly away" from this point, causing even more ocean bulge.

It's scary, all this stuff about bulging and flying off into space. The bumper sticker I once saw on a pickup truck in Soldotna explained tides best: "Gravity sucks."

Les Palmer lives in Sterling.

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