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Frank wrote:

> ...The only
> case, in fact,  where it leads to a significant error is when you do
> limb-limb index error measurements with the Sun. Otherwise index "error" is a
> bit of a  misnomer.

Very interesting.  It has long been my belief that side error would indeed
matter in limb-to-limb IE measurement.  My reasoning?  If side error was
*way* off, I would not see tangency until the images were almost side by
side.  In extreme cases, no tangency at all.

If my mental image is correct, side error would shift the IE off the arc.
Is that correct?

> For sights, I have been happiest recently getting index  correction from a
> nearly full moon in early twilight. The edge definition is  excellent. There's
> no additional shade error involved and no eye strain from the glare of the
> Sun
> around the line of sight to the Sun. For the next week or so  there are good
> morning twilight opportunities to try this. It's somewhat  important to align
> the Moon pole-to-pole when doing this though if you're near  Full Moon, a
> little tilt won't do much harm.

Excellent idea, thank you.

Bill


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