Hi Bernd,

> Bernd Gaehrken states that Guide is using a sharp edge for
> Jupiter to produce its shadow...

Not so. Guide does provide a 'fuzzy' shadow, so that
both the umbra and penumbra are visible. The penumbra is
just _barely_ visible. I used lunar eclipses to "calibrate"
this, in the sense that I looked at lunar eclipses in Guide
and tuned penumbral eclipses to show a small amount of light
drop.

It's pretty darn small, which mostly reflects that (at
least to my eye) the light drop really _is_ very small. In
fact, a more realistic penumbra would probably be even less
noticeable than the one shown by Guide.

It occurs to me that Bernd Gaehrken may be noticing that
the edge between umbra and penumbra looks rather sharp in
Guide. This is true. I think it reflects reality -- the
similar edge during lunar eclipses is about as sharp -- but
could perhaps be persuaded that Callisto's eclipses would be
a different case. But some persuasion would be required;
Guide is already accounting correctly both for the fact that,
compared to a lunar eclipse, the sun appears five times smaller
(because we're five times further from it, resulting in a
sharper boundary); and that the Jupiter-Callisto distance is
almost five times that of the Earth-Moon distance (resulting
in a blurrier boundary; the effects ought to just about
cancel out, making the boundary only slightly less blurry
than that for a lunar eclipse.)

It's an interesting point. It would probably help if the
Galileo probe caught some instances of Galilean moons going into
or out of eclipse; you'd think that it would have taken several
such images over its long stay orbiting Jupiter.

-- Bill