Text 23750, 68 rader
Skriven 2008-11-13 22:53:00 av Dale Shipp (1:261/1466.0)
     Kommentar till en text av Michiel Van Der Vlist
Ärende: Re: Mass of the moon
============================
 -=> On 11-13-08  12:51,  Michiel Van Der Vlist <=-
 -=> spoke to Dale Shipp about Mass of the moon <=-

 MVDV> As a first approximation assume the sun is a point source 
 MVDV> of light at infinite distance. The earth shadow will then 
 MVDV> be a cylinder and when the moon enters the shadow, it will 
 MVDV> show as an arc. It one extends the arc to a full circle, 
 MVDV> one can see the size of the earth relative to the size of 
 MVDV> the moon.

With those assumptions, what you say would be true.  My problem was
having the knowledge that the earth shadow is not a cylinder.  Depending
on which shadow one means, it is a cone going to a point in the far
distance or is an expanding cone shape.  The inner cone, or umbra, is
obtained by matching the top edge of the sun with the top edge of the
earth and drawing a line to get the upper part of the cone.  Ditto for
the lower line.  (of course, I am talking as if we are looking at a
vertical plane through both and sun and earth are viewed as circles).
It is this complete shadow that is the relevant one for lunar eclipse.
The other partial shadow is the expanding one obtained by matching top
of the sun with lower part of the earth and vice versus.

 MVDV> Now the Sun is not a point source, so the earth shadow will be a cone,
 MVDV> rather  than a cylinder and the edge wil be fuzzy. It will give you 
 MVDV> an upper limit on the size of the moon though.
 
I still do not see that, given that the shadow is a cone and not a
cylinder.

 DS> My second wondering is that even if I assume that circle is the best
 DS> fit, what it shows is that the shadow of the earth at the moon is
 DS> about 2.5 times the size of the moon.   However, if the moon were at
 DS> twice the distance it is, the earth shadow would be much larger
 DS> relative to the visual size of the moon.

 MVDV> No, it would not be larger.

True, it would be smaller -- not larger.  I was thinking of the wrong
shadow when I said that.  If the moon were far enough away, it would not
be covered by the earth shadow (the umbra).
 
 DS> How do you see a proof of the moon size from the geometry of this
 DS> data? I would like to know.

I've not yet been convinced, but will keep an open mind on hearing of
how the proof will go.

 MVDV> I was just trying to explain that one does not always need 
 MVDV> wikipedia. There is a lot that one can deduct from personal 
 MVDV> observation if one just knows what to look for and how to 
 MVDV> look at it. Observing a lunar eclipse will give you an 
 MVDV> estimate of the size of the moon and from that one can see 
 MVDV> that an estimate for the lunar mass of 1/8 of that of the 
 MVDV> earth is not realistic.

Wikipedia is information.  The reader needs to apply judgement as to its
validity.  Much of it is accurate, but not all by any means.

                               Dale Shipp
                  fido_261_1466 (at) comcast (dot) net
                              (1:261/1466)


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