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[sci.astro] Time (Astronomy Frequently Asked Questions) (3/9)
Section - C.13 Why are there two tides a day and not just one?

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	Paul Zander <paulz@sc.hp.com>

An easy way to think of the Moon's effect on the Earth is the
following.  The Moon exerts a gravitational force on the Earth.  The
strength of the gravitational force decreases with increasing
distance.  So, because the surface of the ocean is closer to the Moon
than the sea floor, the surface water is attracted more strongly to
the Moon.  That's the tide that occurs (nearly) under the Moon.
 
What's happening on the other side of the Earth?  On the other side of
the Earth from the Moon, the sea floor is being pulled more strongly
toward the Moon than the surface water.  In essence, the surface water
is being left behind.  Voila, another bulge in the surface water and
another tide.
 
In principle, there should be two tides of equal height in a day.  In
practice, many parts of the earth do not experience two tides of equal
height in a day.

First, because the Moon's orbit is at an angle to the Earth's equator,
one tidal bulge may be in the northern hemisphere, while the other is
in the southern hemisphere.

Except around Antarctica, the shape of the Earth's continents prevent
the tidal bulges from simply following the moon.  Each ocean basin has
its own individual pattern for the tidal flow.  In the South Atlantic
Ocean, the tides travel from south to north, taking about 12 hours to
go from the tip of Africa to the equator.

In the North Atlantic, the tides travel in a counter-clockwise
direction going around once in about 12 hours.  The effect is similar
to water sloshing around in a bowl.  Because the two tides are roughly
equal, they are called semidaily or semidiurnal.

In some parts of the Gulf of Mexico, there is only one high tide and
one low tide a day.  These are called daily or diurnal tides.  In much
of the Pacific Ocean, there are two high tides and two low tides each
day, but they are of unequal height.  These are called mixed tides.

The traditional way to predict tides has been to collect data for
several years to have enough combinations of positions of the moon and
sun to allow accurate extrapolation.  More recently, computer models
have been made taking into account detailed shapes of the ocean
bottoms and coastlines.

Even the best predictions can have difficulties.  The extremely heavy
snow fall during the winter of 1994--95 in California and the
associated run-off as it melted were not part of the model for San
Francisco Bay.  Sail boat races scheduled to take advantage of tidal
currents coming into the Golden Gate found the current was still going
out!

Ref: Oceanography, A View of the Earth, M. Grant Gross, Prentice Hall,
Englewood Cliffs, New Jersey, 1972.

For even more details, see
<URL:ftp://d11t.geo.tudelft.nl/pub/ejo/tides> and
<URL:http://www.co-ops.nos.noaa.gov/restles1.html>.

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Top Document: [sci.astro] Time (Astronomy Frequently Asked Questions) (3/9)
Previous Document: C.12 What is the time delivered by a GPS receiver?
Next Document: Copyright

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