The reason for earth's tides, and
why high and low tides occur at regular intervals, was a subject of interest
to thinkers for thousands of years, but not the good old Greeks. The Greeks
live on the shores of the Mediterranean Sea. That sea happens to be relatively
tideless because it is so nearly land-locked small body of water.
About 325 BC, however, a Greek explorer,
Pytheas of Masslia, ventured out of the Mediterranean and into the Atlantic.
There he came across good pronounced tides, with two periods of high water
each day and two periods of low water in between. Each month there were
two periods of particularly large range between high and low tides (spring
tides) and, in between, two periods of particularly small range (neap tides).
What's more, the monthly variations matched the phases of the moon. The
spring tides came at first quarter and third quarter. Pytheas suggested,
therefore, that the tides were caused by the moon. But, his suggestion
lay fallow for two thousand years.
The main factor that spoiled the
moon-tide connection for thoughtful scholars was the fact that there were
two tides a day. For instance, suppose there is a high tide when the moon
is high in the sky. That would make sense. The moon might well be drawing
the water to itself by some mysterious force. If the water heaped up under
a high moon, a point on the rotating earth, passing through the heap, would
experience a high tide followed by a low tide. But a little over twelve
hours later, there would be another high tide and then the moon would be
nowhere in the sky. In fact, it would be on the other side of the globe.
If the moon were exerting an attractional force, the water on one's side
of the globe should be pulled downward in the direction of his feet. There
should be a hollow in the ocean, not a heap.
Could it be that the moon exerted
an attraction on the side of the earth nearest itself and a repulsion on
the side opposite? Then there would be a heap on both sides and it would
explain the two high tides each day.
The notion that the moon would pull
in some places and push in other places must have been very hard to accept,
and most scholars didn't try. So the moon's influence on the tides was
put down to astrological superstition by the astronomers of early modern
times.
It was only in the 17th century
that a clear explanation was given -- by Newton. Newton argued that the
gravitational attraction of the moon causes tides. This force has only
a slight effect on the solid land mass (and it is detectable), but the
great mass of water that make up the oceans is free to move. Because the
gravitational force decreases with the square of the distance, the moon
pulls harder on that part of the earth closest to it. The water in the
oceans is puled toward the moon, resulting in a high tide on the side of
the earth facing the moon. Simultaneously a high tide occurs on the opposite
side of the earth. Why this second high tide? Because the pull of the noon
on the far side of the earth is less (since it is farther from the moon)
than the pull on the central (nearer) parts of earth, (we may visualize
this as the earth being pulled away from the water on the far side, just
as the water on the side of the earth near the moon is pulled away from
the earth a bit) the ocean water thus tends to collect on the sides closest
and farthest from the moon, and is taken from the region between, where
these will be low tides. Since the moon moves relative to the earth so
that it returns to the same position overhead after about 25 hours, there
are 2 high and 2 low tides at any point every 25 hours (approximately 2
of each per day). Because the high tides stay more or less in line with
the moon, it is as if the solid earth moved beneath the tidal bulges.
High tides do not necessarily correspond
precisely to the time at which the moon is directly overhead since other
forces are also acting; among the complicating factors are the rotation
of the earth, friction between the solid earth and the moving water, and
the gravitational pull of the sun. These factors also affect the height
of the tides. For example, the highest and lowest tides occur when the
sun and moon are lined up so both are pulling in the same direction (called
spring tides); when the sun and moon are at right angles, the tides are
smallest (neap tides).
But why spring tides and neap tides
and what is the connection between tides and the phases of the moon? For
that we have to bring in the sun. It too, exerts a gravitational influence
on the earth. The gravitational pull of two separate heavenly bodies on
the earth varies directly with the mass of the bodies in question and inversely
with the square of their distance from the earth. It turns out that the
sun's gravitational pull on the earth is 176 times that of the moon. However,
the moon produces the major tidal effect. Why is that? The answer is that
it is not the gravitational pull itself that produces the tides, but the
difference in that pull upon different parts of the earth. The difference
in gravitational pull over the earth's width decreases rapidly as the body
under consideration is moved farther off, since, as the total distance
increases, the distance represented by the width of the earth makes up
a smaller and smaller part of the total. So, even though the gravitational
pull of the moon is only a small fraction of that of the sun, due to the
much closer distance, the tide-producing effect of the moon is about 2.2
times that of the sun.
The tides, in a way, affect time.
At least, it is the tides that make our day twenty-four hours long. As
the tidal bulge travels about the earth, it scrapes against shallow sea
bottoms and the energy of earth's rotation is dissipated as frictional
heat. This dissipational energy is enough to be slowing the earth's rotation
and lengthening the day by one second every one hundred thousand years.
This isn't much on the human time scale, but if the earth has been in existence
for five billion years and this rate of day-lengthening has been constant
throughout, the day has lengthened a total of nearly fourteen hours. When
the earth was created, it must have been rotating on its axis in only ten
hours (or probably less).
As the earth's rate of rotation
slows down, it loses angular momentum as well, but this angular momentum
cannot be dissipated as heat. It must be retained, as angular momentum,
elsewhere in the moon-earth system. What the earth loses, the moon must
gain and it can do this by receding from the earth. The effect of the tides,
then, is to slow the earth's rotation and to increase the distance of the
moon.
This slowing down of the rotation
due to tidal effect would also explain why the moon faces the earth on
one side. Even if there were no oceans on the earth, there would still
be tidal friction, for the solid substance of the earth does yield a bit
to the differential pull of the moon. This bulge of solid material traveling
around the earth also contributes to internal friction and to the slowing
of the earth's rotation. We can see this at work on the moon, which has
no oceans. Just as the moon produces tides on the earth, so the earth produces
tides on the moon. The effect of earth-on-moon is 32.5 times as much as
moon-on-earth. With this much tidal effect, plus the much less original
rotational energy, the moon has dissipated its rotational energy to the
point where the tidal bulge is frozen into the moon, and it now faces one
side only toward the earth.
There is a limit to how much the
earth's rotation will be slowed. Eventually, the earth will rotate about
its axis so slowly that one side will always face the moon as the moon
turns in its orbit. When that happens, the tidal bulges will be frozen
into place immediately under the moon and on earth's opposite side and
will no longer travel about the earth. Of course, the tidal bulges of the
sun will still be moving about the earth some seven times a year (a day
will be about fifty times longer than today) and this will have further
effects on the earth-moon system. (There is a planet-moon system that has
reached that point. The planet Pluto and its moon Charon both have their
faces frozen toward each other.)
We can also suspect that the tidal
effect of the sun upon the planets would slow down the rotations of the
planets. The closer the planet to the sun, the larger the tidal effect
and the slower the planet's rotation. With a bit of wisdom of hindsight,
one might suspect that the rotations of Venus and Mercury (which are very
close to the sun) are very slow and these indeed were true. Mercury's rotation
is one and half times in one of its year and Venus's rotation is slightly
longer than its year.
The tidal effect can also be used
to explain the beautiful rings of the gas giant planets but that is another
story. So that's all, folks! .
Duc Ta Vo, Ph.D.
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Copyright ©
1996 by VACETS and Duc Ta Vo
