Why are there two high tides per day?
The sea's daily pattern of two tides is caused by a combination of the the Earth's rotation and the Moon's gravitational pull.
Asked by: Richard Hill, by email
The daily pattern of two high tides is a familiar feature of Britain's seaside resorts, but its cause is surprisingly subtle. Roughly speaking, it's the result of the level of the sea rising as the Earth's rotation brings it into two positions: first, directly facing the Moon, and second, facing away from it. In the first position, the rise in sea level is principally due to the Moon's gravitational pull.
The cause of the second bulge in the level of the sea, when it's on the opposite side of the Earth, is more subtle. In essence, it's the result of the fact that the Moon is relatively massive compared to the Earth, thus causing both of them to spin round each other like two gigantic dumb-bells. This rotation generates a centrifugal force, which on the Earth is strongest at locations facing away from the Moon. This in turn causes the sea level in these locations to rise up, forming the second high tide during the course of a day.
Working out precisely what happens at any given coastal location involves further complications. The Sun also plays an important role - as does local geography, which can lead to some areas getting just one high tide each day, or three, or even none at all.
Robert is a science writer and visiting professor of science at Aston University.