The Sun and the Moon determine high and low tides. The water level rises and drops in a daily rhythm with the tides. The tides are not tangible on the high seas. But along the coasts and the river estuaries, high and low tide alternate periodically. The water rises for six hours and twelve minutes; it needs the same time for ebbing.
This is the same period as the time required for half an orbit of the Moon around the Earth. The difference of the water level between high and low water is called the tidal range. Along most coasts, the tide rises and ebbs twice a day in one lunar day. The main cause of the tides is the Moon.
It exerts a strong gravitational pull on the water in that half of the Earth that is turned towards it at any time. It increases above the normal level, i.e. the tide rises. Also, the attraction exerted by the Moon causes a rise of the tide on the side of the Earth that is turned away from the Moon. Both these areas together form about half of the surface of the Earth. The rest of the Earth then has low tide. The Sun also has an action on the tide.
However, the force that it exerts is only about 48% of that of the Moon. On the other hand, if the Sun, Moon and Earth are in a straight line with each other, the centrifugal_forces of the Moon and Sun add up, giving rise to a spring tide.
The high tide is then somewhat higher than normal, and the low tide is somewhat lower. If the Sun and Moon are at right angles to each other, the two forces cannot combine and there is a neap tide, which is lower than normal.
Especially around 21st June and 21st December, the Sun is situated far away from the line joining the Earth to the Moon. The spring tides are then particularly low. Conversely, on 21st September and 21st March, the tides are particularly high.
The energy of the tides is used in tidal power plants for generating electrical energy. An example of this is La Rance in the northwest of France. The tidal range, i.e. the difference in water level between ebb and flow, must be high enough to be able to use the tidal power.
In France, it is 12 to 14 meters. The inflow during high tide is controlled with dams. The inflowing water drives turbines and is stored behind the dams. At low tide, the water is released and can thus drive the turbines again. The highest tidal range is 21 metres in the Bay of Fundy in Canada.
