Re: astronomy

[Jim Green <JMGreen@sisna.com>]

Leigh, neither I nor my colleagues can tell if you are agreeing with my post
or not -- You quote the book's author who says (1) "stronger tides" and (2)
"the tidal bulges would not appear to move"  

If the so called "tidal bulges" don't move and I stand on the shore of a
Moon ocean, I see NO tide. It is true (to first order) that there _would_ be
two fixed (Newtonian) tidal bulges on an aqueous Moon or even on a watery
Moon with continents, because (again to first order) the water does not move
with respect to the land -- the "bulges" always point exactly toward and
away from the Earth -- and there is little opportunity for damped driven
harmonic motion to develop in the Moon oceans as occurs on Earth. 

I am not going to do the calculations to determine if the "bulges" are
"stronger" as this calculation depends on the local g and the shape of the
ellipsoidal land mass, which _also_ points toward (and away from) the Earth.
Yes, the force gradient is greater there, but the Moon ain't spherical.  Any
driving frequency is irrelevant if there can be no oscillations. 

It seems worth asking your friend, the author, for a comment, but, hey, this
is an astronomy book, why is he asking about Moon tides?

I think that the answer which follows _is_ correct (again to first order) --
unless I have missed something major, which in my advanced age is
increasingly probable. (:-) 

Jim Green
JMGreen@sisna.com

At 12:24 PM 3/19/98 -0800, Leigh wrote and quotes a bit of Jim -

> > The answer the book expects, however,  is that there 
> > woun't be no tides on a watery Moon, but we all know this, no?

Sorry for the miserable attempt at humor.  It's that senility again. (:-) J.



> It hadn't occurred to me that there was an answer book for this text.
> However I just found an unopened "Instructor's Manual" by Leo P. Connolly
> (with whom I've published, it turns out) which has answers to the review
> questions. Leo writes:
>
>  "The Moon would experience stronger tides than on Earth because the
>  Earth, which produces the tides, is much more massive than the Moon,
>  which produces tides on the Earth. A compensating factor is the Moon's
>  smaller size, which reduces the effect somewhat. Since the Moon rotates
>  once during each orbit, the tidal bulges would not appear to move
>  relative to the surface of the Moon."
>
> Somehow I can't find great fault with this answer. Leo has left out the
> explicit mention of hypothetical oceans, but I have to agree with his
> answer. Any tidal height variation would be miniscule given a 0.4 uHz
> (uHz = microhertz) driving force.
>
> Leigh