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Re: Tides and tidal bulge



On Tue, 2 Jul 1996, Daniel L. MacIsaac wrote:

Recently, Jim Green posted his annual claims about tidal bulges:


And neither is the spring tide in coincidence with New/Full Moon.

....on 30 July (this month) the Moon is at perigee, and this coincides with a
full moon. According to my astronomical almanacs that means the highest tides
of the year will occur on that date (with phase corrections for local
geometry and drag). Pretty coincident for me. Anyone living on the coasts
inclined to check local tide predictions and comment?
...

In plain english, the Earth and Moon exert gravitational forces on one
another as solid, semi-rigid bodies and fall (accelerate) towards one
another. The waters on the side of the Earth nearest the moon accelerates
towards the moon at an above average rate, because they experience more
gravitational force, 'cause they're closer by one Earth radius. This causes
a bulge to be pulled up by the moon. On the back side of the earth (away
from the moon) the waters are two earth radii further away from the moon,
so the waters on the side away from the moon fall towards the moon with an
acceleration less than that of the earth and are left behind, forming a
second bulge.
...

Tidal bulges are real and deserve to be taught as such. Didn't we have this
debate about a year ago? I seem to have the refs on a nearby shelf with
printouts from that last round. Do you, Jim?


Bravo for an excellent treatment once again! Yes, we do seem to have this
discussion annually, and it's hard to see why. To teach tidal theory without
the simple Newtonian moon-sun differential force cause and the resulting
tidal bulge would pedagogically be equivalent to omitting Kepler's elliptical
orbits in teaching planetary motion on the grounds that the actual motion is
much more complicated because of all the planetary perturbations. Some
people seem to insist on losing sight of the forest because of all the
trees.

A. R. Marlow E-MAIL: marlow@beta.loyno.edu
Department of Physics PHONE: (504) 865 3647 (Office)
Loyola University 865 2245 (Home)
New Orleans, LA 70118 FAX: (504) 865 2453