Tides in the oceans cannot move freely because of the continents. They actually flow in a complex manner as shown below. Most often they circulate around nodes, very much the way a wave rotates around a cup as you oscillate it. In narrow, parallel-sided bodies of water the tides move as waves, for example in the South Atlantic. The map below shows the position of high tide crests at three-hour intervals.
The animation above shows how tides move in the real oceans (light=high and dark = low, though it really makes no difference). In oceans with parallel coasts, like the South Atlantic, or between Antarctica and Australia, the tides move like broad waves. Elsewhere they revolve around points called nodes like spokes of a wheel.
In narrow confined bodies of water, the tide crest takes time to move up the bay. Shown here is the tide crest in the Bay of Fundy. The numbers indicate hours after high tide in Boston.
Shown above is the height of the tide. The tide is funneled into an increasingly narrow space with nowhere to go.
|In small confined bodies of water, tides rotate like the crest in a gently swirled glass of water. Such tides are called amphidromic.|
Above is the amphidromic tide in the Gulf of St. Lawrence. Each incoming tide from the open Atlantic pumps energy into the revolving tide in the Gulf.
On December 16, 1773, American colonists, frustrated by British tax policies, planned a dramatic protest. The merchant ships Dartmouth, Endeavour, and Beaver were in port with a cargo of tea, on which the colonists refused to pay tax. Negotiations with British authorities had broken down that afternoon, and the cargo was due to be seized at midnight if the tax was not paid. A group of colonists, dressed as Indians (a disguise which fooled nobody) boarded the ships about 6 P.M. and threw the tea overboard in the famous Boston Tea Party. But what happened to the tea after that?
|Ideally, the tea should have been thrown overboard just after high tide, so the ebbing tide could carry it out to sea. In reality, because the colonists had only a few hours to stage their protest, they could not plan for the tides. The Boston Tea Party fell not just at low tide, but an exceptionally low tide at that. The incoming tide simply washed the tea ashore, and work parties with shovels cleaned it off the waterfront and threw it back into the harbor next day (pollution was not a concern for most people in those days!). By that time it was, as the character in Mary Poppins puts it, "undrinkable even by Americans."|
|The effect of tides on the Boston Tea Party is a humorous footnote to history. The effect of tides on the battle for Tarawa during World War II was anything but. Tarawa was one of the easternmost Pacific islands held by the Japanese, and the assault was the first amphibious assault in the Pacific during World War II. Sketchy tide data for the island suggested a tidal range of about seven feet. However, there were puzzling rumors of periods when the tides on Tarawa almost ceased, a condition local mariners called a Adodging tide@.|
|Tarawa is a type example of an atoll,
a flat-topped submarine mountain capped by coral. Most
atolls, like Tarawa, have a wide shallow lagoon ringed by
low coral islands.
The only island of consequence at Tarawa was one with an airfield. The plan was for Allied ships to stand offshore in deep water and send landing craft into the lagoon. The landing craft would go as far in as possible and discharge troops.
The invasion was set for November 20, 1943 when tide conditions were expected to be favorable. At low tide in the early morning, the bombardment would begin. As the tide rose and water levels in the lagoon reached 1.5 meters (five feet), landing craft would head ashore and by noon, at high tide, heavier craft could come ashore bringing tanks and supplies.
This isn't the sort of thing you can call off and reschedule if things go wrong, as they did. Once an attack is under way, the enemy knows your intentions. Any delay merely gives the enemy time to reinforce or escape.
|Unfortunately, the rumors of almost-tideless
periods at Tarawa were true. November 20 was near last-quarter
moon, resulting in a neap tide. Military planners knew
about the risks of neap tide but did not realize the moon
was unusually far from earth as well, weakening its tidal
effects even more. Also, the Earth was only seven weeks
from perihelion, meaning solar tides were unusually
strong as well.
The top diagram shows what planners expected. The bottom one shows what actually happened.
Landing craft hit bottom hundreds of meters offshore and the Marines had to wade ashore under heavy fire. Once ashore, they had to fight without assistance, because supply ships could not come in. For 48 hours, the tidal range was only 60 centimeters (two feet), and it was four days before the tidal range increased to normal. 1027 Marines were killed and 2292 wounded in the battle.
D. W. Olson, The Tide at Tarawa, Sky and Telescope V. 74, no. 5. p.526, November1987
Donald W. Olson and Russell L. Doescher, "The Boston Tea Party," Sky and Telescope, vol. 86 (Dec. 1993), 83–86
Created 21 May 1997, Last Update 14 December 2009
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