Water And Oceanography

Characteristics of Tides



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When King Canute, the medieval Danish King who ruled England, sat on his throne on the beach trying to halt the incoming tide it was clear that he didn’t really know his physics. He also must have felt like something of an idiot. The truth is humans have no control of the tides, even deluded kings.

Canute didn’t know that the constant rise and fall of the seas and oceans are the response of oceans to the gravitational pull of the moon and the sun. When the tide is high and rises, the flood tide, water creeps up the shore. And when the tide retreats or falls, ebb tides, the shores are uncovered. Canute, if he had known this, would have had the good sense to out least put his throne out on the beach when the tide was retreating. Sadly he didn’t. He could have looked really clever if he had come back later as the tide was coming in and commanded it to return.

The bearded (most King’s had beards then) Danish King, if he had done his homework and studied the seas for some time, might have been able to learn how the tides were interconnected to the locality of the two main objects in the sky - the sun and moon.

Canute probably would not have been able to work out the third main factor on the tides: the Coriolis Effect. The Coriolis Effect is caused by the rotation of the Earth and is the effect seen in the deflection of winds and oceans currents to the right of their original path in the Northern Hemisphere and to the left in the Southern Hemisphere but the other two factors he should have noticed.

The sun is, of course, thousands of times larger than the moon; however, due to its close proximity to the Earth the gravitational pull of the moon is twice as strong. If Canute had known this he would have seen that the locations of the hide tide follow the path of the moon instead of the sun. The characteristic monthly sequence of the tides is based upon the moon’s revolution around planet Earth. A lunar month is 29.5 days during which it travels 12.2 degrees each day around the Earth. A lunar day is 24 hours and 50 minutes long this is the time taken for the moon to be over the same longitude at which it started.

Canute should have noticed that during the year there are times when the daily tides are higher and lower than at other times. This is when the sun, the moon and the Earth are in line. At this time the effect of the pull on the oceans is magnified to form the high and low spring tides. Spring tides form during full moons and new moons. When the moon is at its first and third quarter the sun, moon and Earth form a right angle to another resulting in the neap tides. Neap tides have a smaller range between the high and low tides during the lunar day. Most shorelines have a tidal range between three to six feet and two high and low tides a day –called semidiurnal tides.

It is, however, more than just the celestial bodies that affect the tides. The land underneath the water naturally plays a part as well as the shape of the coastline against which the tides are lapping. These topographical discrepancies effect the elevation and timing of the tides.

A little bit more study on King Canute’s part and a little less banqueting and he really could have looked extremely clever as the tides are predictable if you just know what things to look for.

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