Re: Tides and tidal bulge

[Jim Green <Jim.Green@Snow.edu>]

Margaret, I don't know of an undergraduate book that gives a reasonably good
description of the ocean tides.  All talk about Newton's explanation for two
"tidal bulges"  None that I know of approach the tides as complicated damped
driven harmonic waves.  Further there are only Ari and me who know the
truth. (:-)

Ari, do you have a reference???

Newton's explanation is OK for the *forces* which if there were no rotation
of Earth/Sun/Moon/(or Jupiter for that matter) AND the Earth were aqueous,
THEN there would be two bulges in the water,

BUT for the real system one might look at the oceans as violin strings and
the Moon/Sun as the violin bow, can you see that there might be a distance
from the Americas to Europe/Africa which approaches resonance length and
some so out of resonance that there is no wave.  There would be nodes and
antinodes.  The ocean might slosh back and forth like an aquarium being
tilted back and forth. The amplitude in the aquarium would depend on the
frequency of the tilting.
 
BUT there are MANY driving frequencies: daily revolution of the Sun, of the
Moon, precession of the Moon's orbit, etc etc etc -- NOAA's equations have
*63* terms. Well then some of the frequencies must be resonant one says,
Yes, BUT the speed of the ocean waves depends on depth and there are things
called peninsulas, islands, continental shelves, etc etc etc.

The bottom line the tides are impossible to predict theoretically, so NOAA
gives up and does something different. (Too much for this post) 

If there *were* "tidal bulges" co tidal lines (where max tide -- high water
-- is synchronous) would be North/South -- along longitudes.  Instead they
are squiggly, circular, convoluted, and in general weird.

For example, if there *were* "tidal bulges" one would expect the bulge to
hit the Atlantic Coast at about the same time and be proceeding East to
West.  But the tides there go from *South* to *North* (Florida is several
hours ahead of Main -- but Main is East of Florida) and on the European
Coast from North to South.  Indeed the tides in the Atlantic proceed in a
*circular* pattern!  The tide height varies from *zero* to *big* and can be
twice a day or once a day or some mixture thereof.

The Pacific is not the same, but because it is big it has its own weirdness,
and then there is the Indian Ocean, and all the little seas, etc.

If there *were* "tidal bulges" the tides at moderate latitudes would be
about *one* foot on all coasts, instead we have from zero to many feet --
the Bay of Funday doesn't count for other reasons.

Happily the Mediterranean and the Red Sea *do* act something like violin
strings - like sloshing aquariums.   

This is a long story


Jim Green


At 09:41 AM 7/1/96 EST, you wrote:
> As a sidelight of the Rossby waves thread, there have been comments about
>  the non-existence of tidal bulges. As I am about to try to teach tidal
>  effects to an intro. astronomy course, I would appreciate it if someone
>  could set me straight on this. Tides as a forced wave oscillation sounds
>  good, but I'd like more details. Is there a good reference at somewhere
> below expert level which will give me a lead?
>  Margaret Mazzolini
> Swinburne University of Technology
> Melbourne, Australia
>  
>