"Tides" refers to the alternating rise
and fall of sea level within a day. What
causes the sea level to change? It is
actually the gravitational attraction of the
sun and moon that cause waters of the
ocean to swell and recede at different
parts of the earth.
The Moon Tide
The earth and the moon are two great masses that have a significant gravitational pull
on each other. This is what keeps the moon in orbit around the earth, and it is also
what causes tides to occur in the ocean. Picture the earth with a uniform level of
water all around it. The moon’s gravity pulls on the earth, and pulls the water towards
it. The water moves up into a slight bulge on the side of the earth that faces the
moon. At the same time, there is a force pulling water out in the opposite direction of
the moon. To understand this force, you need to picture the earth and the moon as
one unit. Picture two unequal balls on the ends of a stick.
If you spin this stick around, you can
imagine the force that a particle might
feel if it were on the far end of either
the moon or the earth. It would feel a
force outward, away from the centre of
the spin. This is due to inertia. The
water on the far end of the earth, away
from the moon is always being pulled
out from the centre of the spinning
earth-moon unit.
The gravitational and inertial forces are constant,
always pulling water towards the moon and
directly away from the moon. The forces in
either direction are equal to each other. The
bodies of water that feel these forces change
constantly as the earth rotates within these
forces, but the force directions are always
toward and away from the moon.
Semidiurnal tides
As the earth turns upon its own axis in about 24 hours, a point on the earth moves
through areas with these different forces acting on it. In one rotation (one day), a
point on earth travels from an area of high tide (where there is a force pulling water
outward), through an area of low tide, through an area of high tide again (the
opposite pull), and through another area of low tide, before it returns to the point of
origin at high tide. This results in two high tides and two low tides in a day (called
semidiurnal tides).
The Tidal Day
The moon does not stay put, but rotates around the earth at a rate of about 12° a
day, or one rotation a month. The rotation is in the same direction as the earth’s spin,
so by the time the earth has done one rotation, the moon has shifted 12° further, and
it takes an extra 50 minutes for the moon to be in the same position relative to a
point on the earth. Therefore, the tidal cycle is not 24 hours long, but 24 hours and 50
minutes. Because of this, high and low tides are about 50 minutes later every day.
The Sun Tide
The tides are caused mainly by the gravitational attraction of the moon and the earth,
but there is also a gravitational attraction between the earth and the sun. The effect
of the sun upon the tides is not as significant as the moon’s effects. Basically, the
sun’s pull can heighten the moon’s effects or counteract them, depending on where
the moon is in relation to the sun.
In one month, the moon rotates around the earth. When the moon is between the sun
and the earth (at new moon), the sun’s gravitational pull is in the same direction as
the moon’s. During these days the high tides are higher and the low tides are lower
than they'd be with just the
moon’s pull alone. This is called
spring tide.
The same thing happens when the
moon is on the direct opposite
side of the sun (full moon). The
two gravitational forces work
together to make high high tides
and low low tides.
When the moon is in its first quarter or its last
quarter,
the sun’s gravitational pull is in perpendicular direction to
that of the moon. The sun pulls water away from the areas
of high tide to the areas of low tides, resulting in lower
high tides and higher low tides. These are called neap
tides.