Tides are the regular and predictable rise and fall of the sea caused by the gravitational attraction and rotation of the earth, moon and sun system. Tides are normally used to refer to the vertical change in sea level, whereas the term tidal currents are used for the horizontal movement of water.
Tides are caused by the combined effects of gravity and the revolution of the Earth-moon system about its common centre of mass, known as the barycenter. At this point (which lies within the solid Earth) the gravitational attraction between Earth and moon exactly balances the forces required to maintain the moon’s orbit (Figure 1). Elsewhere the two forces are not in balance and give rise to the so-called tide generating force. This explains why there are two tides per day at most places around the globe – described as semidiurnal tides. Some parts of the world’s oceans do not respond strongly to these forces and have only one tide per day – described as diurnal tides. The gravitational pull of the Sun acts in a similar way but because it is so much further away the solar tide generating force is only 0.46 that of the moon. Every fortnight when the moon is full or new the solar and lunar tides combine give higher than usual tidal ranges. These are called spring tides (it has nothing to do with the season). At the moon’s quarter phases the lunar and solar forces cancel out to some extent giving smaller tides called neap tides.
Figure 1: The magnitude and direction of the tide-generating force on Earth (looking side on to the Earth), created by the balance between the centrifugal force and the gravitational force.