The main extraterrestrial influences on the world's tides are the moon and the sun. The relative importance of each at any time is dictated, inter alia, by its position relative to, and by its distance from, the Earth, and by the varying orbital characteristics of the three bodies. Each of these variables - and many more - has its own individual effect upon our tides, and the water at any place can be thought of as responding to the instantaneous sum of all these individual components.
When these known astronomical parameters are fed into a computer it is relatively easy to calculate what is known as the Equilibrium Tide, a theoretical model of the tides appropriate to a landless world, uniformly covered in deep water.
But the observed tides differ greatly from this Equilibrium Tide. Each ocean is, in a sense, an isolated bowl of water; and because of its dimensions, depth, and general shape, and the interfering walls of land around its edges, responds more readily to certain constituents of the tide-generating forces than it does to others. And in a similar way, every inlet of the sea has its individual character, depending on its shape.
The local pattern can be shown by what is called the "tidal curve", a graph on which the height of the water, ignoring waves, is plotted over a period of time. There are three main types: most tidal curves, including those for Irish coasts, show two equal high tides and two equal low tides per tidal day, semi-diurnal tides; a few ocean areas, such as parts of the Gulf of Mexico, have diurnal tides, with only one high tide and one low tide each day; and some parts of the world have two high tides and two low tides each day, but they are of quite different heights. These last are known as mixed tides.
In practice, to prepare predictive tide tables for a particular port, it is necessary first to install a tide gauge there, and to record the observed pattern of the rise and fall of water over an extended period. Careful examination of such a record allows it to be recognised as the rather complex combination of a large number of independent simple rhythmic undulations, each one corresponding to the known characteristics of one of the individual astronomical components.
When the relative strength of each astronomical component at a particular spot is empirically identified in this way, it can then be given the correct emphasis when constructing the mathematical equation from which a computer can calculate the tide at that place for any future time.