The variations in the level of the sea, the tides, are associated
with several phenomena; and the commonest predictable one is the astronomical
one, product of the force caused by the gravitational attraction of the Moon,
and in smaller degree, of the Sun. These phenomena are predictable with enough
exactitude since they depend on the position of the stars, which can indeed
be well known. For example, we can calculate to the hour and height of the high
tide for a day within 2000 years.
The tides in the great water bodies are more evident: the oceans, seas and
lakes. But also tides in the solid part of the Earth take place.
The Moon has an elliptical orbit around the Earth, so
once a the month it is closer (perigee) and once a month it is farther (apogee).
When it is closer, the Earth experiences a greater gravitational attraction
and, as a consequence, higher tides occur.
The system is complex, since the Sun, although has minor effect, also is
added to the final result. The distance between the Earth and the Sun also
vary and, therefore, the force intensifies in the equinoxes of March and
September, when the Earth is nearer to the Sun (perihelion) and diminishes
in the solstices of December and June, when they are farther (aphelion).
