Thursday, July 21, 2011
Tools of the Trade: Time and Tide
General understanding and documentation of tidal patterns has been around since man went out to sea. The Egyptians, who were leery of the vast ocean – as they knew the Mediterranean – but very familiar with their beloved Nile, left enormous amounts of documentation not only on Nile floods but on ebbs and flows in the Delta as well. Neither the Greeks nor the Romans were particularly forthcoming about tidal patterns and those master mariners, the Vikings, were not keen on writing down things they seemed to consider common knowledge. Both in the east, particularly China, and in the west, documents regarding tidal fluctuations began to show up around 1200 CE.
The earliest extant tidal table was produced in 1250 at St. Albans Abbey in Britain. The listings are meager, giving fluctuations for London and the Thames only but it was a start. In 1802 the New American Practical Navigator was published in Boston. Its author, Nathaniel Bowditch, connected tidal flux with the phases of the moon in an almanac style that would influence future documents. By the 1830s, both the British Almanac and the American Almanac included predictions for high and low tides in major ports along the Atlantic coast and, in the case of the American Almanac, the Gulf coast as well.
There are no certainties when one is dealing with the sea, however, and sailors had to know the variations possible in documented tidal fluxes. This meant having knowledge of the factors that could alter the regular tides as documented. First and foremost, one must be clear that any tidal table was no more that an educated guess. Seamen know that in places with vast discrepancies between levels of water at the two high and low tides in any given day, the tables could not be trusted. Keeping an eye on the barometer was also advisable; higher tides than predicted were likely to occur when the atmospheric pressure was low and vice versa.
Strong winds, even those originating far out to sea or well inland, could affect high tides in particular. Wind coming in off the ocean might make a high tide much higher, while wind coming off land – particularly strong, warm winds like a Santa Ana or Mistral – could decrease the depth of a high tide. Heavy storms have the same affect with the added irritation of altering tide times by up to an hour.
There were places in the world that have extreme tidal fluxes, and by the late 18th century most mariners knew where the major ones were. The Sea of Okhotsk between Russia’s mainland and the Kamchatka Peninsula and the English Channel, for instance, are renowned for their very high tides. The most extreme tidal fluxes in the world are on the Atlantic coast of modern Canada, particularly the Bay of Fundy where tidal runs can range in height as much as forty-five feet. Some experts estimate that the mid-tide flow of water here is equivalent to that of all the rivers and streams on Earth.
The wonder of the tides is just another reason to stand in awe of the sea. Don’t stand too long, though; there’s work to be done and no one likes a gawpus.
Header: Fishing Boats on the Bay of Fundy by William Bradford c 1860