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The rise and fall of the sea level in a regular cycle of 12 hours and 25 minutes, caused by the gravitational action of the Moon on the Earth’s water masses. The gravitational pull of the Moon (and to a lesser extent, the Sun) causes the water masses to curve into an ellipsoid shape, pointing towards the Moon. As the Moon orbits the Earth, the area of high tide shifts. The time for which sea level rises in a given location is called the flood tide. After the high tide, there is an ebb tide when the sea level drops to a state called low tide. The height of the tide is determined by the current position of the Sun and Moon, the position on the Earth’s surface, the shape of the seabed and the coastline. The highest difference between the sea level at high and low tide is in the Bay of Fundy, where it reaches up to 16 metres.