Tides result primarily from the Earth's reaction to the
Moon's gravitational attraction. Since the water in the oceans
is more mobile that the rocks of the Earth's surface, the Moon's
gravity pulls a bulge in the oceans that is directed towards the
Moon. The water on the Earth facing the Moon is more strongly
attracted than the center of the Earth or the water on the side
of the Earth opposite the Moon. This difference in the amount of
gravitational attraction results in a tidal force which produces
two rising bulges in the ocean waters. Land masses pass through
each of the two tidal bulges as the Earth rotates. The land
moves toward the water which is deeper than normal. The water,
in turn, moves up on the land to produce a rising tide along the
coastal regions. When the land rotates past the bulge, the water
recedes and the tide ebbs. Successive tides occur about 50
minutes later each day because of the Moon's orbital motion.
This means that there is approximately 6 hours and 13 minutes
between the highest and lowest tide levels. During Full and New
Moon phases, the highest tides (called spring tides) occur
because the Moon and Sun are acting together to produce a greater
gravitational pull. During the First and Third Quarter phases,
the Moon and Sun are at right angles to each other. This means
that their gravitational forces tend to counteract one another.
At this time the lowest of the high tides (or neap tides) occur.
Figure 2 illustrates with vectors the reason for two high tides
occurring on opposite sides of the Earth.
The tides create forces on the Earth and Moon that slow
their rotation, a phenomena known as tidal braking. As the Earth
spins, friction between the oceans and the solid Earth below
drags the tidal bulge ahead of a line drawn between the Earth and
Moon centers. The Moon's gravity tries to pull the bulge back in
alignment with the Moon's center. This drag is transmitted
through the oceans to the Earth slowing its rate of rotation the
same way a brake works to slow a turning wheel. Tidal braking is
causing the Earth's period of rotation to lengthen by 0.002
seconds every 100 years.
Questions:
1. Explain how the tides in Earth's oceans are produced. How many tides occur each 24-hour period?
2. Define spring tides and neap tides.
3. Explain what happens to the water and solid portions of the Earth as the Moon's gravity attracts them?
4. What effects do tides have upon the Earth?
