At the coast, the tide is most easily observed as daily water level variations. At most places in the world, the tide is essentially semi-diurnal, i.e. having a period of approximately half a day. This means that two high waters and two low waters can be observed daily. The tidal range between high water and low water can be more than 10 metres depending on the location.
Newton was the first to explain the generation of the tide. After having identified the tide-generating forces, he assumed the ocean water to respond instantly to these forces. That is why his theory is called the equilibrium theory of tides. He also neglected the presence of continents and assumed the earth to be entirely covered by water. In the present section, we follow his theory to explain some well-known tidal phenomena. Subsequently, in Sect. 3.8 the effects of land masses and non-instantaneous response are discussed.