Tides result from the regular rise and fall of the water level in the oceans. This phenomenon is caused by the gravitational pull of the Moon and Sun on the Earth and by the centrifugal force generated by the rotation of the Moon-Earth sys­tem as a whole. These two forces act in opposite directions.

The bulge of the water surface caused by the gravitational pull of the Sun and Moon is repeated on the opposite side of the Earth, where the bulge is due to the centrifugal force. The Sun, although much more massive than the Moon, is also much farther from the Earth. As a result, the Sun's effect on the Earth's tides is only half that of the Moon.

Tides occur every day, and their timing and posi­tion depend on the Earth's rotation on its axis related to the position of the Moon. If the Moon were stationary, each point on the Earth would have two high and two low tides every day. But because the Moon orbits the Earth every 28 days and because the Earth takes just more than a day (25.02 hours) to rotate once in relation to the Moon, the positions of the high tides on the Earth change. At any one location they occur about an hour later each day.

When the Moon and Sun are aligned on the same side of the Earth, the tide caused by their combined action is higher than-normal and is called a spring tide. This event recurs every 14 days, at new Moon or full Moon. When the Sun and Moon form a right-angle with the Earth, the gravitational pulls of the Moon and Sun are in opposition and result in weaker, intermediate tides called neap tides. These also take place every two weeks, at the first and third quarters of the Moon.

Because tides depend on the rotation of the Earth and that of the Moon and the Earth around their common center of mass, the time of high and low tide at any point on the Earth can be pre­dicted. The solar tide has a period of 12.41 hours (because of the monthly rotation of the Moon around the Earth). The Moon's closest approach to the Earth is at perigee, and the farthest point of its orbit is at apogee, the difference between the two being about 24,000km. When perigee coin­cides with the spring tides, they rise about 20 per cent higher than their normal level.

Other variations in tidal height and occurrence can arise depending on the size, depth and shape of the ocean, the shape of the coastline, the lati­tude and the angles of the Moon and Sun relative to the equator.

Spring tides have a larger tidal range than do neap tides. Most parts of the ocean have two high and low tides a day, but a few areas have only one high and one low tide. In some areas of the Pacific Ocean the high tides differ in height from each other, and so do the low tides.

Tidal Range and Energy

Rance tidal power plant in Brittany, FranceThe tidal range in the open ocean is usually quite small - only a few tens of centimeters in amplitude - but towards the coasts waves increase in height and velocity. The English Channel, for example, has a daily tidal range of about two meters. In narrow bays the tidal range can be grea­ter.

The largest tidal range is in the Bay of Fundy, on the eastern coast of Canada, where it can reach 18m. Estuaries or seas that are almost enclosed, such as the Mediterranean Sea, often have their own tides that are small in range. When these cycles coincide with the Earth's tidal period, how­ever, such relatively small bodies of water can experience large tidal ranges.

Attempts have been made in areas with large tidal ranges to use the regular rise and fall of water to generate electricity, such as the Rance tidal power plant in Brittany, France, where the average height range of the tide is 8.5m.

Energy from tides was used even as early as 1650 in New England, when tidal power was harnessed to run grist mills. Today, tidal energy is converted into electrical power by making the tidal waters drive reversible turbines. These machines extract the potential energy from the water that flows in and then ebbs away.

Tidal Bores

When an incoming mass of water flows into a nar­rowing channel such as a bay or an estuary, the funneling process causes the water to flow much quicker than it would were it allowed to spread.

The water at the front of the tide tends to slow down because of the narrowing but the water behind it rushes up at normal speed, so that the water increases in height and velocity, resulting in a wall of water which surges up the channel.

This happens particularly in Asia, the best-known example being that of the Qiantang River estuary in northern China, where the tidal bore reaches a height of 7.6m and sometimes 9m and moves at about 23km/h. Tidal bores occur mainly during spring tides.

Tidal Currents

Changing tides can also create tidal currents, par­ticularly in areas that are almost enclosed. The typical speed of these currents in most channels and straits is about 1 lkm/h, but in some regions it may reach 18km/h, such as the Straits of Georgia, which lie between Vancouver Island and British Columbia.

Tidal currents are most power­ful in deep waters and weakest in shallow areas. They change position and direction with the Earth's rotation, just as the tides do. These cur­rents sometimes run at right angles to the tide, in the opposite direction or even in the same direc­tion.

Local climate can also affect the level of coastal waters to an even greater degree than the tides. The surging water is then known as a meteorolo­gical tide. A storm surge, for example, is the result of a climatic effect on water.