Bulges

[aepstein@polar.Bowdoin.EDU (Ari W Epstein)]

I had hoped to stay out of this (too soon after the last time to
go at it again), but I can't help myself. Gary Karshner wrote:

> Edmund Halley, the editor/publisher of Newton's Principia is
> credited with generating the first accurate tide tables for
> England. A feat that requires both a knowledge of the position of the
> earth's tidal buldge and the rate of flow of water into and out of
> rivers, bays and harbors.


This is really not true--the prediction of real tides is not helped
(and may actually be hindered) by one's knowledge of "the position of
the earth's tidal bulge," since there is no single (or rather double),
simple bulge. There are surface displacements due to the tide, and
they are dynamically important, and they do travel around in the
ocean, but they *don't* follow the moon around the way the textbook
says they should. Look at a cotidal chart for the North Atlantic to
see what I mean--the tide travels around and around the ocean basin,
not acrosss it. To make up a good tide chart, you really want to have
spent a lot of time gathering data at your particular site. Predicting
actual tides from first principles is just barely possible with
intricate numerical models, and those require much better knowledge of
the bathymetry than Halley could possibly have had.



Back into the woodwork,

Ari Epstein
aepstein@bowdoin.edu