The real world

The Equilibrium Tide developed from Newton’s theory of gravitation consists of two symmetrical tidal bulges, directly under and directly opposite the Moon or Sun. Semidiurnal tidal ranges would reach their maximum value of about 0.5 m at the equator. The individual high water bulges would track around the Earth, moving from east to west in steady progression. These characteristics are clearly not those of the observed tides.

The observed tides in the main oceans have mean ranges of about 0–1 m (amplitudes 0–0.5 m), but there are considerable variations. The times of tidal high water vary in a geographical pattern, for the daily solar and semidiurnal lunar tides, which bears no relationship to the simple ideas of a double bulge. The different tidal patterns generated by the global and local ocean responses to the tidal forcing are clear in Figure 5.1. The tides spread from the oceans onto the surrounding continental shelves, where much larger ranges are observed. In some shelf seas the spring tidal ranges may exceed 10 m: the Bay of Fundy, the Bristol Channel, the Baie de Mont Saint Michel and the Argentine Shelf are well-known examples of big tides. In the case of the northwest European shelf, tides approach from the Atlantic Ocean in a progression to the north and to the east, which is quite different from the Equilibrium hypothesis.