Thread View: alt.folklore.science
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Started by Steve Crabtree
Fri, 18 Dec 1992 20:33
Re: Full moon and tides
Author: Steve Crabtree
Date: Fri, 18 Dec 1992 20:33
Date: Fri, 18 Dec 1992 20:33
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To: alt.folklore.science
From: steve.crabtree@synapse.org
mayor@ee.umanitoba.ca writes:
M(> In article 1gigbeINNk69@gap.caltech.edu, carl@SOL1.GPS.CALTECH.EDU
M(> (Carl J Lydick) writes:
M(> >The moon is not the sole source of tides. The tidal effects of
M(> >the sun are also significant. During new and full moons, the
M(> >tidal effects of the sun and the moon reinforce each other.
M(> >Hence higher tides during new and full moons, ceteris paribus.
M(> During the new moon, the moon is on the same side of the earth as
M(> the sun. During full moon, the moon is on the OPPOSITE side of
M(> earth as the sun. So shouldn't the gravitational tugs oppose each
M(> other during full moon and reinforce each other during the new
M(> moon only?
M(> I think so. Maybe I'm wrong. Think about it.
The reason I don't think this is so is that there are two raised
humps of water on the earth's surface, one facing the moon (offset by
effects of the earth's rotation) and one on the opposite side. This
gives the normal two high tides per day (roughly).
When I first thought about this, it gave me some trouble until I
realized that the earth-moon system does not revolve around the
earths centre, but around it's common centre of gravity.
This means that water on the far side of the centre of gravity
experiences a centrifugal pseudo-force which causes it to rise up on
the side facing away from the moon.
Similarly, the earth-sun system revolves around a common centre of
gravity which similarly creates a hump on the far side of the earth in
addition to a hump caused by the differential gravitational force on
the side facing the sun.
Clearly either the "gravitational attraction" humps or the
"centrifugal" humps can add together and therefore the sun and moon
should cause greater tides together when they line up on opposite
sides of the earth or on the same side.
One thing I've never tried to work out is whether the centrifugal hump
and the gravitational hump are the same size, and further, if they
are, whether they have to be or whether it's coincidence. I know from
personal experience (living by the banks of a tidal river) that they
are approximately equal.
Re: Full moon and tides
Author: puck
Date: Sun, 20 Dec 1992 04:30
Date: Sun, 20 Dec 1992 04:30
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In article <647.1561.uupcb@synapse.org> steve.crabtree@synapse.org (Steve Crabtree) writes:
>
>One thing I've never tried to work out is whether the centrifugal hump
>and the gravitational hump are the same size, and further, if they
>are, whether they have to be or whether it's coincidence. I know from
>personal experience (living by the banks of a tidal river) that they
>are approximately equal.
>
The relative strength of the centrifugal hump depends a great
deal on your delclination (latititude). Since the Earth is spinning
on it's pole, a point on the equator is farther from the center of
spin than a point near a pole. This causes tides in different areas
to have different effects.
For example, the tides on the U.S. west coast have two peaks daily.
The difference in height between the two peaks differs from day to
day---sometime the peaks are the same height, sometimes the peaks
have several feet of difference between them.
Local geography also affects tides greatly.
.k.
--
``Much of what is greatest in human |kaydin@jarthur.claremont.edu
achievement involves some element of |kaydin@hmcvax.bitnet
intoxication.'' |!uunet!muddcs!jarthur!kaydin
--Bertrand Russell |or send mail by W.A.S.T.E.
Re: Full moon and tides
Author: David Breneman
Date: Wed, 23 Dec 1992 00:36
Date: Wed, 23 Dec 1992 00:36
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In article <647.1561.uupcb@synapse.org> steve.crabtree@synapse.org (Steve Crabtree) writes: > >One thing I've never tried to work out is whether the centrifugal hump >and the gravitational hump are the same size, and further, if they >are, whether they have to be or whether it's coincidence. I know from >personal experience (living by the banks of a tidal river) that they >are approximately equal. > Well, living on Puget Sound, I observe that they vary. Either you have two relatively moderate high and low tides (ie 8' high, 6' low, 9' high, 5' low) in a 25-hour period, or one extreme and one fairly moderate tide (ie 12' high, -2' low, 9' high, 7' low) and you transition between these tide-pairs on a roughly 4-week cycle. Also, during the winter you tend to have the extreme high tides during the day; and during the summer you tend to have the extreme low tides during the day, which wrecks havoc with boat ramps. :-) -- David Breneman Sys Admin, Tacoma Screw Products, Inc. | ____ ____ ____ dcb@tacoma.uucp | SCREWIE the TSP CLOWN sez- | / /___ /___/ ..!uunet!tacoma!dcb | "Zinc & cadmium plating stops | / ____/ / CompuServe: <DEAD> | rust & acts as a lubricant!" |
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