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Amazing Facts


Tides result from the regular rise and fall of
the water level in
the oceans. This phenomenon is caused by the gravitational pull of the
Moon and Sun on the Earth and by the centrifugal force generated by the
rotation of the Moon-Earth sys­tem as a whole. These two forces act in
opposite directions.

The bulge of the water surface caused by the
gravitational pull of
the Sun and Moon is repeated on the opposite side of the Earth, where
the bulge is due to the centrifugal force. The Sun, although much more
massive than the Moon, is also much farther from the Earth. As a
result, the Sun's effect on the Earth's tides is only half that of the

Tides occur every day, and their timing and
posi­tion depend on the
Earth's rotation on its axis related to the position of the Moon. If
the Moon were stationary, each point on the Earth would have two high
and two low tides every day. But because the Moon orbits the Earth
every 28 days and because the Earth takes just more than a day (25.02
hours) to rotate once in relation to the Moon, the positions of the
high tides on the Earth change. At any one location they occur about an
hour later each day.

When the Moon and Sun are aligned on the same
side of the Earth, the
tide caused by their combined action is higher than-normal and is
called a spring tide. This event recurs every 14 days, at new Moon or
full Moon. When the Sun and Moon form a right-angle with the Earth, the
gravitational pulls of the Moon and Sun are in opposition and result in
weaker, intermediate tides called neap tides. These also take place
every two weeks, at the first and third quarters of the Moon.

Because tides depend on the rotation of the
Earth and that of the
Moon and the Earth around their common center of mass, the time of high
and low tide at any point on the Earth can be pre­dicted. The solar
tide has a period of 12.41 hours (because of the monthly rotation of
the Moon around the Earth). The Moon's closest approach to the Earth is
at perigee, and the farthest point of its orbit is at apogee, the
difference between the two being about 24,000km. When perigee
coin­cides with the spring tides, they rise about 20 per cent higher
than their normal level.

Related Tags: Ocean  Earth  Tides  
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