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Tidal Energy Chapter | 3 49

FIG. 3.3 Net forces in an Earth-Moon system. See text for explanation.

FIG. 3.4 Vectors on the Earth’s surface indicate the difference between the gravitational force the
Moon exerts at a given point on the Earth’s surface, and the force it would exert at the Earth’s centre.
These resultant force vectors move water towards the Earth-Moon orbital plane, creating two bulges
on opposite sides of the Earth.

the Earth rotates within this ellipsoid, two high waters (and two low waters) are
experienced per day. This is known as the equilibrium tidal model. However,
there are three main problems with this equilibrium tidal model:

● the tidal wave cannot move fast enough to keep up with the Earth’s spin and
maintain ‘equilibrium’;
● the equilibrium tide does not account for continents; and
● the equilibrium tide does not account for Earth’s rotation.
As can be seen in Fig. 3.4, the equilibrium tide is actually a wave that
is stretched around the circumference of the Earth. The wave has two crests:
one directly under the Moon, and the other on the opposite side of the Earth.
Between these two crests are two troughs. As the Earth spins, the equilibrium
tidal model requires the wave to maintain itself with one crest directly under the
moon. Tidal waves behave as shallow water waves, with speed c given as

c = gh (3.2)