From: kaydin@jarthur.claremont.edu (puck)
Subject: Re: Full moon and tides
Organization: Forest of Satyrs, City of Vampires, the wilds.
Distribution: usa
Date: Mon, 14 Dec 1992 17:36:33 GMT

In article <1992Dec14.153154.22191@newsgate.sps.mot.com> rapw20@email.sps.mot.com writes:

>                   I find it hard to believe that a full moon has any  
>effect on the tides. The mass of the moon, and hence its gravitational  
>pull on the ocean doesn't change, nor does its distance from the earth (at  
>least not because the moon is full). The only thing that really changes is  
>that we on earth can see more of the moon. Is this an urban legend or  
>what?
>
>Mark

   No, not an urban legend, as in surfer or marine biologist could tell
you.  Just look at a tide chart, and you'll see that the height of the
high tides varies greatly from day to day.  Why?  Not just because of the
moon (which alone could not cause the change) but due to the combined
influence of the sun and the moon...to use my handy ASCII graphic
blackboard...

   Here is the Earth, with parentheses 
representing the water levels:                         (E)

   Here is the Sun:                                    (S)
   Here is the Moon:                                   (M)

Now then...

   The sun and the moon BOTH like to pull at the earth, causing our
friendly tides.  During a full or new moon, the earth, sun and moon
are in a line, like this:

                   (S)                   (  E  )         (M)

  or this:

                   (S)      (M)          (  E  )         [not to scale]

  As you can see, the combined influence of the sun and moon pull the
Earth's water a good deal away from her body.  This is called, for reasons
lost to antiquity, the Spring tide.  These tides have higher highs and
lower lows than the overall average.
  When the half moon comes, the picture changes:

                  (S)                    ( E )

                                          (M)

  Now, the gravitational forces of the sun and the moon pull at right
angles to each other, contradicting each other and causing lower highs
and higher lows.  These, of course, are called Neap tides, for reasons
which remain obvious to the most casual observer (though not to me).
  Note:  the moon pulls stronger that the sun, so the above picture is
misleading.  But the idea...that's the important thing.

  In reality, the picture is even more complicated.  After all, if you
imagine the moon and sun on a 3-D surface, they can be anywhere around
the earth, as they most usually are...

               (S)         

                                  ( E )

                                       (M)

  is a more likely situation.  Which is why, of course, we don't get
eclipses every day (Eclipses accompany unusually high tides).  
Indeed, the tides at a single location may be modelled by no fewer 
than four (4!) sinusoidal waves.

  Notice I said "a single location."  Because some coasts have one high
tide and one low tide a day, some have two---height and time can vary 
greatly depending on geography, from mile to mile along a coast---the
take home message, check your tide charts, or expect to dive into a
bayfull of mud.

-kerim,
surfer, (sometimes) marine biologist.

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