From: Ronald Wong (ronwong@inreach.com)
Date: Fri Mar 05 2004 - 00:17:54 PST
Message-Id: <l03102800bc69ebc28bb5@[209.209.19.181]> Date: Fri, 5 Mar 2004 00:17:54 -0800 From: Ronald Wong <ronwong@inreach.com> Subject: Re: tides
Not too long ago, Treena said:
>I was wondering if I am correct in my understanding that the tides are
>caused by the Earth itself bulging in response to the Moon's
>gravitational pull, rather than the water. My understanding is that the
>bulging Earth causes the relative water levels to change. confirmation
>anyone?
Treena:
When you said that "the bulging Earth causes the relative water levels to
change" - I was left with the impression that you might not know what
causes tides to occur in the first place.
Soooo...
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If there was a huge body of liquid water 8,000 miles in diameter 250,000
miles from the moon instead of the object that we actually have - the
planet Earth with it's oceans - we would find that it would have bulges
just like those that we associate with tides (at least for as long as all
the water stuck around).
Relative to the moon, there would be one bulge on the near side and another
on the far side.
To see why this would be so, start by dividing this sphere of water into
three regions relative to the moon: the near side, the far side, and the
region in between.
The closer something is to the moon, the greater the force of gravity
acting on it. So, the water on the near side feels a greater force acting
on it than the water in the "region in between". For the same reason, the
water in the "region in between" feels a greater force acting on it than
the water on the far side.
As a result of the unbalanced force on each of these three regions, the
near side has a greater acceleration towards the moon than the "region in
between" and the "region in between" has a greater acceleration towards
the moon than the far side.
Since the near side has a greater acceleration towards the moon than the
"region in between", it falls away from the "region in between" - producing
a bulge on the near side. In the same way, the bulge on the far side occurs
because the "region in between" falls towards the moon at a rate greater
than that of the far side. In doing so, it leaves the far side behind and,
because of this, we see a bulge on the far side as the water on the far
side trails behind the falling water of the "region in between".
In other words, when you place a sphere of liquid water in a non-uniform
gravitational field in which the force gets weaker with the distance from
the source of the gravitational field, it's shape changes to that of a
football (actually, more like the shape of a ball used in rugby) with the
axis of rotation pointing towards the source of the gravitational field.
These tidal bulges are an important element in the explanation of tides.
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If we replaced most of the center of this huge body of water with a dense
ball of matter (sort of like what we have right now), the thin shell of
water remaining would still have tidal bulges. Here the near side is the
half of the shell of water near the moon, the "region in between" is the
earth, and the far side is the remaining half shell of water. All three
regions continue to fall to the moon at different rates and thus what
applied to the sphere of water applies here as well.
Because the earth spins on it's axis, points on the earth move from one
bulge to the other and back again over the course of a day. This is what
brings about the apparent, twice-a-day rise and fall of the sea level (i.e.
the tides). It's this spinning of the earth in conjunction with the bulges
of the shell of water that is responsible for the tides. Any bulges the
earth might have would simply affect the tidal range and not their
occurrence.
Since the moon moves around the earth, the tidal bulges move around
accordingly advancing the time when high and low tide occurs as the days of
the month go by (about 50 minutes later each day).
A footnote: the drag caused by the earth spinning under the
thin film of water pulls the tidal bulges out of alignment
with the line passing through the earth and the moon.
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As it turns out, the earth has tidal bulges too.
We are on a very thin, plastic shell called the earth's crust and it, in
turn, sits on a thicker, viscous shell called the asthenosphere (the part
of the upper mantle that moves around and, as a result, carries the various
plates of the crust from one location to another).
Although these two regions are not as fluid as the ocean they will flow
from one place to another. So what is true for the "solid" earth and it's
oceans turns out to be true for the solid core of the earth and its viscous
outer layers.
So the earth also has tides - for exactly the same reason as the oceans.
As was pointed out by others, The earth's greater rigidity leads to tides
of smaller amplitude. Whereas the oceans rise and fall over an average
distance of about 2.5 meters, the earth does so over a distance of only
about 0.25 meters.
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I hope some of this proved informative. If you already knew all of this,
then please accept my apologies.
ron
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