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Lecture

A basic definition of tides

Tide pools
Organisms living in tidal pools, such as this one near Capitola, California, must adapt to the alternating wet and dry conditions they experience due to tides. (Source: Steve Lonhart, SIMON/MBNMS)

Anyone who has been to the beach is familiar with the daily rising and falling of the water level – tides. Tides are defined as periodic, short-term changes in the height of the ocean surface caused by the moon and sun’s gravitational forces and the rotation of the earth. Tides can be considered waves – the largest on Earths surface! But tides are considered forced waves (ocean waves are free waves) because they are never free from the forces that cause them.

In this module, we will examine both the equilibrium theory of tides and the dynamic theory of tides. The equilibrium theory of tides deals with the position and attraction of the Earth, moon and sun to explain why tides occur. This theory would best describe tides on a completely water-covered Earth – so this is an idealized model for tide formation. The dynamic theory of tides takes into account the effect of continents, shallow water, and partially enclosed ocean basins on tide formation. This is a more realistic model of tides. We will start with the equilibrium theory because this best helps us understand tides without any complicating factors.

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