PSN-L Email List Message

Subject: Re: Re solar/lunar gravitationa
From: Ed Thelen ethelen@........
Date: Mon, 18 Sep 2000 03:06:06 -0700

Mariotti Mauro wrote:
> Sean-Thomas,
> i kindly request an explanation, since i'm not expert on mathematics
> and seismic and astronomy...

This is not Sean-Thomas, and I certainly cannot answer
for him - but I've got some fun info on the subject.
Read on (please)

> But when you said
> At 13.27 13/09/2000 -0500, you wrote:
> >Re solar/lunar gravitational forces with respect to earthquakes:
> >
> >
> >The stress in the earths' crust results in strain and tilts of the
> >order of 10^-7, and are so predictable that routine calculations of
> >them are used to calibrate broadband instruments. Over the years
> >many efforts have been made to correlate earthquake occurrence or
> What is 10^-7 ?
> degrees, meters, what?

G's,  units of earth gravity - 

> Sorry for the question, maybe it's stupid but....

Heck no,  vital!   With out the units you have nothing.

Now the fun - or troubled fun -

In the January 2000 "Scientific American" 
Amateur Scientist section
is an interesting article
   "Detecting Extraterrestrial Gravity"
which describes a home buildable instrument hopefully
capable of detecting the effect of the sun's and moon's
gravity on the local earth gravity you and I feel.

It is a clever scheme involving magnetic repulsion
(hopefully constant) vs local gravitational effect.
The necessary current to correct for the changes
of local gravity can be measured.

I did some quick calculations and figured that the
moon's gravity at the earth's surface was about 
10^-4 the earth's gravity.  (The effect of the sun
is roughly the same - off by a factor of about 2 or so.)

I figured that I could duplicate the instrument,
and with care, observe a 1/10,000 effect - probably.

THE BAD NEWS - and lots of it.

a) I lost an argument about the size of the effect 
   I had to measure!

   Because the earth falls toward the moon due to the 
   moon's gravity,  the effect you have to measure is
   the DIFFERENCE between 
      - the moon's gravity at the center of the earth  
         (which is about the gravity causing the earth
          to fall towards the moon)
      - the moon's gravity at your point on the 
        surface of the earth.

   The above subtraction yields about 10^-7 g's.

   That is REAL SERIOUS instrument building in a field
   of 1 gravity, by an amateur, or anyone.
   (In)Stability of temperature, magnetic fields, electronics,
   tilt of the instrument, and almost everything 
   can eat you alive.  

   The machine is also an excellent vertical seismometer.  
   Because of its tilt sensitivity it is also an excellent
   sensor of the wind's torque on your house, and ... 
   All you have to do is separate all of the various
   effects from eachother  ;-)  oh yes.


b) The guy that made the machine and reported to the
   author of the article was inadvertently "seeing"
   errors due to temperature differences day and night,
   and not the effect he thought he was seeing.

   And that is not much fun either, especially when
   the magazine is printed and in the mail!

   So - to see the resulting turmoil, you might wish
   to go to

   And be kind!  Fate can do the same to you and me  ;-)

I related the above true story in part to help emphasize
the relatively tiny effect major bodies relative the the
earth exert on the earth.  The astrologers who figure
that the position of Mars relative to the earth is important
to  some fetus have has some tall explaining to do.  
(Assuming we are talking of gravity of course. 
 If we are not - then we are into Social "Science".  ;-)

Best Wishes
  Ed Thelen

> Thank you
> Mauro

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Larry Cochrane <cochrane@..............>