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On Dec 3, 2004, at 5:39 PM, Frank Reed wrote:

>
> Of DeltaT, Omar R wrote:
> "I wonder why can't this value be deterministic, like everything else
> in astronomy ?"
>
> Can you predict the dust storms on Mars? How about the shape of the
> next sunspot? You would probably counter that those things are
> "weather" not astronomy. And I think that's basically how we define
> traditional computational astronomy. Anything that is subject to
> computation in the foreseeable future, we count in, otherwise it's
> out. Changes in delta-T derive from the complexities of the Earth's
> weather and its interior, channeling angular momentum from one part of
> the total system to another. The net angular momentum of the Earth, if
> we could measure it, presumably shows a much more "deterministic"
> decline since it's under the influence of the luni-solar tides. But
> even there, the exact damping effect depends on the shapes of the
> ocean basins (which are changing on a geological time scale).
>
> Frank R
> [ ] Mystic, Connecticut
> [X] Chicago, Illinois
>

I'm certainly no geologist --often regret never having had a year of
geology in college--  but I believe the rate of sedimentation from a
big river such as the Mississippi, Amazon, Rhine, etc, can be fast
enough to change the tidal frequency in enclosed bays, such as Fundy,
over periods of thousands of years.  This would be a shorter time scale
than what I envision for the "geological time scale," which I see as
ranging between tens of thousands to tens of millions of years.  Now
whether these changes in tidal frequency in local bays would have a
significant effect on global tides is an additional question.  However,
the ice ages would have had a profound, global effect on tidal
modulation of the earth's angular momentum.  The end of the last ice
age was rather sudden, but the onset depended on accumulation of snow
over extended periods, and would have been longer, I expect: imagine
building a mile thick sheet of ice at the rate of 20 inches per year.

   

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