Visualising Earth's Ocean Currents
Footage: TedEd & Jennifer Verduin
The currents and tides affected the Ashmore's 102-day journey from Great Britain to New Zealand in 1882. If she sailed too far south, she ran the risk of icebergs, but that was where the strong winds blew. Here is a short history on mapping the tides and currents around the world.
Photo: Courtesy of Catolico
Tides It's been known since classical times the tides were related to the Moon's journey around the earth. But it took many years to predict their tides. Mediterranean tides had a maximum rise and fall of only three feet which meant their tidal streams were weak. However, along the north-western shores of Europe, the rise and fall could be in excess of thirty feet. Tidal streams in that area could be as fast as three or four knots, even eight knots in narrow channels.
'Working the tides' involved making headway against a light wind by drifting along with the tide, making the voyage faster and more profitable.
"The earliest surviving prediction tables were calculated by the monks of St. Albans for London Bridge in the thirteenth century, but because most seamen were then illiterate there developed in medieval times a method of calculating the time of high water for a given port from the age of the moon during its twenty-nine and one half day cycle around the earth. Because there were no sea-going clocks, and sundials were impractical on a ship, seamen expressed time as a factor of a compass bearing of the sun. An east sun was six o'clock, a south sun was noon and so on, the thirty-two points of a compass being equivalent to twenty-four hours, expressed in two lots of twelve hours. One compass point, therefore, was equivalent to forty-five minutes of time.
In consequence, when the time of high water, or 'full sea' as it was called, was noted at a port, seamen expressed the time as a compass bearing. Since the highest high waters were found to occur at full and new moons (spring tides), this became the establishment of the port." The Country Life Book of Nautical Terms Under Sail.
The earliest printed manuscript of tide tables was recorded in the nautical almanac of 1546 published by Breton, G. Brouscon. It wasn't until 1773 that Richard and George Holden published a commercial manuscript.
Sailing ship masters needed an understanding of the ocean's currents as their set and drift could affect their daily run by many miles. These ocean currents were recognized by explorers in the early sixteenth century on their voyages of discovery in the Indian and Atlantic oceans.
"It was thought at one time that the drift of water from east to west across the equatorial regions was caused, like the tides, by the moon and that the continents deflected this movement of water in other directions, thus setting up other currents.
William Bourne, writing in 1578 in The Treasure for Travellers, was able to give comparatively accurate descriptions of the currents to be found in the Atlantic Ocean, based almost entirely on the experiences of seamen. But the rate at which these currents flowed, their drift, was still a mystery.
The perfection of marine chronometers and the lunar-distance method of finding longitude in the second half of the eighteenth century enabled seamen to accurately fix their position at sea for the first time. This meant that they could compare their 'position by account' or 'dead reckoning position' with the position the ship was actually in by astronomical means. The difference between these two positions on an ocean passage was caused primarily by a current so that it was now possible to work out not only the set of a current but its drift as well." The Country Life Book of Nautical Terms Under Sail.
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