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).
