In the window of the harbour office there is a sheet of paper that predicts the sea for a whole year. It lists, for every day, the times of the two high waters and the two low waters, and how high and how low each one will be, to the centimetre. It was printed before the year began. It is, as far as anyone can tell, correct.
I find this one of the most astonishing documents a town keeps, and almost nobody stops to read it.
How the sea is predicted
A tide is not weather. It is arithmetic. The water rises and falls because the moon and the sun pull on it, and because those pulls repeat in cycles that can be measured: the main one, driven by the moon, takes a little over twelve hours and twenty-five minutes, which is why high water arrives about fifty minutes later each day.1 Add the sun’s smaller cycle, the slow wobble of the moon’s orbit, and the shape of the particular coast, and you can work out the sea years ahead. The office does not observe tomorrow’s tide. It looks it up.
Here the tides are small. On an ordinary day the difference between high and low water is less than the height of a kitchen table, and visitors from the ocean coasts laugh at it. But small is not the same as unimportant. That table’s worth of water decides whether the old steps by the fish market are dry, whether a boat with a deep keel can leave its mooring, whether the children can walk out to the rock with the iron ring on it.
What it does not say
The table has one great silence, and the people who live by it learn it early. It predicts the sea that the moon makes. It cannot predict the sea that the wind makes. A strong wind from the south piles water against the shore and the high tide arrives higher than printed; a cold wind from the north-east, the one that comes down from the plateau in winter, can push the sea away and leave the harbour lower than anyone expected.2
So the table is read with an eye on the sky. The fishermen I asked do not trust it and do not ignore it. They treat it the way you treat a very good friend’s directions: correct about the roads, wrong about the traffic.
I think that is the right way to read most printed things that claim to know the future. Read the arithmetic. Then look out of the window.
Notes
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Tidal scientists call these regular cycles constituents, and the main lunar one has the pleasingly dry name M2. A tide prediction adds together dozens of them, each with its own speed and size for one particular harbour.
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On those days the harbour office tapes a handwritten note to the corner of the table. It says, every time, the same three words, which I will not translate because they are not polite.
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