Brunswick County- A View from the Bridge
Originally published June 1993 We who live by the shore and travel about in boats also live by the tides. But people who live only a bit inland give nary a thought to this incessant rise and fall of the ocean water that so effects the lives of us who live and work by the sea. From the beginning of time it has been important to seamen, fisherman, and merchant interests along the coast to know exactly the state of the tide and to be able to predict the tides days and even weeks ahead. The scholars who wrote down the times of the tides were like seers or prophets in the early days and the formulas they used were closely guarded family secrets. Today such people are call Tidal Mathematicians and, of course, the computer does all the hard work. When one of these ultra smart people with her neat little tailored suit walked into the Shamrock the other day I thought that perhaps for the price of a couple of beers I could get the straight poop on the tides. Besides, she wasn't to hard to look at even if her name was Henrietta. I was greatly relieved when I found that she preferred to be called Muffy. Something left over from college days she said. Everyone knows that the tides are caused by the moon's gravity pulling up on the water making a bulge which travels around the world as the earth spins on its axis under the moon. Now, for a lot of complicated reasons, and everything about the tides is complicated, there are actually two bulges of water; one under the moon and the other on the opposite side of the earth from the moon. Suffice it to say that if there weren't two bulges the earth could be out of balance and we would have wobbled out of orbit a long time ago. So now we have two bulges of water with two low spots in between going around the world giving us two high tides and two low tides every day. But, everyday is really every lunar day which is 24 hours and 50 minutes long so the tides tomorrow are going to be about an hour later than they are today. Actually, this isn't what my new friend said at all. She said that the tides were caused by the moon's horizontal traction force along the surface of equilibrium...vector sum of the forces...line intersecting the surface tangent... My version is sort of a loose translation of the Muffy's mambo jambo. Capt'n Jack has a rule about never using the term "arc cosine". Just as the moon goes around the earth every lunar day, the sun goes around every solar day, that's the one that is only 24 hours long, and also causes a tide. The sun's tide is only about 40% as strong as the moon's and on a different schedule (by 50 minutes a day) so the extra tide either adds to or subtracts from the regular moon tide which accounts for the difference in tide heights throughout the month. At new moon and full moon, the sun and moon are exactly lined up so the two tides fully add up making extra high tides called spring tides. At the quarter moons the tides are extra low and called the neap tides. Listening to Muffy all this sounds perfectly simple and straight forward; except for the part about the hyperbolic intersections. But one thing has always bothered me. If, as we have established for reasons of neatness and safety, there are two high tides going around the world every day; how come some places have just one tide a day? Here on the eastern seaboard we are used to the apparently normal tide pattern, but down on the Gulf Coast they have just one diurnal tide. Many places in the world have diurnal tides. People in California think they have just one daily tide but in fact they have two; one is just much smaller than the other. And there, according to Muffy, is the whole answer. Everybody has two tides a day but one is large and the other is small. At Biloxi, Mississippi, the second daily tide is virtually zero so to them there is only one tide a day, and rightly so. Any why, you may ask as I did, is one tide so much bigger than the next? Muffy says the diurnal high-water inequality happens when the interference vector... harmonic periods...dynamic equilibrium... The gist of it has something to do with the outgoing tidal stream over riding the incoming tide and canceling it out one time and reinforcing it the next. This phenomenon is caused primarily by land forms which shape and direct the tidal flow. This land form effect can also cancel out the tides altogether in certain places in the Carribbean and Mediterranean or amplify them to the extreme such as in Nova Scotia where tides run as high as 50 feet. Enough about the tides, it was one of those beautiful warm afternoons so I asked Muffy if she wanted to dig some clams at low tide, which she predicted was about 43 minutes and 15 seconds away. She said that she would love to for awhile, but then she had to go to a meeting of the astrophysics club in Wilmington. Someday, though, for the price of a couple more beers she would come back and tell me why the Carolina sky is so blue. I can hardly wait, really.