Translations of Encyclopedia about Geology

The tide

The tide is regulated by the sun and by the moon. The tide is the daily rhythm of alternately rising and falling water. The oceans are globally connected, which means that the sea level is approximately the same all around the world. Lakes or inland seas may be situated above the sea level or below it, for example, the Dead Sea.

The tide changes the sea surface. In the high sea the tide is not noticeable, but on the coast and in river deltas the high tide and the low tide periodically alternate. The water rises for six hours and twelve minutes, then it needs the same length of time to recede. This period of time coincides with half of the time it takes the moon to circle the earth. The almost imperceptible difference in high tide versus low tide in the open sea increases when the water mass is compressed, for example, by ocean valleys. The tide in the Mediterranean is barely noticeable, because it is a mass of water surrounded by land.

The diffference in the water level between the maximum high tide and the maximum low tide is called high tide and low tide elevation. In most of the coastal areas there are two maximum high tides and low tides during the course of one lunar day. The lunar day is about 50 minutes longer than the earth day, which means that the high tide and the low tide shift slightly every day.

The force causing the tide is the moon. Its considerable gravitational force pulls the water on that side of the earth's hemisphere that is facing the moon. The water thus rises above its normal stage, and that is called high tide. The moon's gravity causes high tide on the side of the earth that is turned away from it. Both these regions represent approximately half of the earth's surface. In the meantime, the regions between them are experiencing the low tide

The sun also plays a role in the regulation of the tide, but its gravitational force impacting the earth is only 48% of the gravitational force of the moon. However, when the sun, the moon, and the earth are in conjunction, the gravitational forces of the sun and the moon join, resulting in a high high tide, when the high tide is more elevated and the low tide is lower. The opposite happens when the sun and the moon are at right angle to each other. Their gravitational forces do not join, and the result is a low high tide.

Around June 21st and December 21st, the sun is at the greatest distance from the connecting line moon-earth. That is when particularly low high tides occur. On the other hand, around September 21st and March 21st the high tides are very high.

The energy produced by the tide is utilised for generating electricity in plants utilising different tide water levels. One such electric plant is in La Rance, in the northwest of France. In order to be able to utilize tide waters, the difference in water levels must be very high. In France, it reaches between 12 and 14 metres. A series of dikes regulate the incoming tide water. The water passes through, activating the turbines, and is collected behind the dikes. During the low tide, the water is released and rushes back, activating the turbines again. The highest difference in the high tide and low tide water level is 21 metres. It occurs in the Bay of Fundy in Canada.

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