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> No.  There is no "obvious" way to do this that is unambiguously correct.
> Also, if  phase is large enough to worry about, you can *see* it (once you
> know
> what  you're looking for at least). As Venus gets closer to us, take a look at
> it with  your sextant's telescope. If it's approaching one arc-minute in
> diameter, then  through a 7x telescope, it will have an apparent diameter of
> more
> han five  arc-minutes. That's plenty big enough to resolve with the human eye.
> You'll  actually see a tiny crescent through the sextant. So you should still
> be able to  estimate the location of the planet's center and park that spot
> right on the  lunar limb when you're shooting lunars. And by the way, when in
> doubt about the  size and phase, pull out a little telescope and take a  look!

Frank

Thanks.  Even with Alex's excellent binoculars I cannot see what he can, so
would need a telescope.

It strikes me that for practical purposes a combination of phase and
apparent size would dictate use.  At maximum elongation Venus would be half
illuminated, but only 23" diameter, so offset from true center would be
<0.6". Even if apparent elevations were equal, 0.1' would not be too much to
worry about for most of us.

Bill

   

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