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).
The Earth tides present and display
substantial cyclical increases,
as a result of the different planes and periods of rotation of the
Earth,
Moon and the Sun Each 4-5 years the Moon is more near the one of the
Earth.
When this happens around the equinoxes, when the Earth is near the Sun,
take place the astronomical tides more discharges, whose differences
can
reach to the 20 centimeters respect to the higher tides average. The
last
ones occurred in the 1997-98 and 2002-03.
