What is a tide?
In
marine navigation, the word "tide" is a common term used to mean the
rise and fall in sea level. But do you really understand how to tides
work? Tides are the the result of the gravitational pull that the moon
and the sun have on our planet. Other factors that influence tides
include the depth of water
at a particular place, the ocean floor topography and the configuration
of the coastline there. Anybody living near the sea observes, daily,
that there is ‘high water’ (the highest sea level for the day) and a
corresponding ‘low water’. This usually occurs twice a day, (called semi diurnal tides) but not always. Some places have only one high water and low water every day. (diurnal tide). Sometimes they are even mixed.
What causes tides?
The
earth's gravity holds the ocean water close to the surface. However,
the gravitational forces of the moon and sun also act on the earth's
water, drawing it towards them. The force of gravity of the Earth and
these external forces caused by the sun and moon cause the water to rise
and fall on the earth’s surface depending on the strength of these
forces at a particular time. (some are variable, as we will see later)
High tides are produced in the ocean waters by the "heaping" action
resulting from the horizontal flow of water toward two regions of the
earth representing positions of maximum attraction of combined lunar and
solar gravitational forces.
As
water levels rise in one place, water from other places on Earth falls
to compensate, causing low tides in those areas. As has been said
earlier, other factors also play a part. Also, the earth and moon’s
revolution around the sun is elliptical, although the sun is not at
the centre of the ellipse. At times, the earth is closer to the sun and
so the sun’s effect is greater. Similarly for the moon.
The difference in the height between consecutive high and low tides at a particular place is known as the range of tide.
The difference in the height between consecutive high and low tides at a particular place is known as the range of tide.
Images
Spring and Neap tides (see diagram)
The
earth and the moon are revolving around the sun. We can see, from the
diagram, that the attraction caused by the sun and moon is maximum when
they are in line , or on opposite sides of the earth- either at New Moon
or Full Moon. This is because they complement each other at those
times. This greater attraction obviously causes a higher range than
normal and the tides are then called spring tides
When the moon is not complementing the sun in the First and Third Quarter, the range is minimum. The tides are then called neap tides
When the moon is not complementing the sun in the First and Third Quarter, the range is minimum. The tides are then called neap tides
Prediction of tides
The
times of High water and low water are calculated using a mix of
astronomical and historical data for any location near the sea. These
are tabulated and published yearly around the world in publications such
as the British Admiralty Tide Tables, which all ships carry. Knowing
the times and heights of low and high water, (LW and HW)
is of great benefit to marine navigation, specially when sailing
through rivers and entering ports where water levels are shallow.
Knowledge of currents related to tides is also beneficial for safety.
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