Hello Bill,

thank you for your fast answer. But I think that's not just the point. The
question is more: How is the atmosphere of Jupiter treated? Is it a sharp edge
that produces the boundary of the umbra and the penumbra or are several layers of
different density (hence absorbtion) modelled?

I know this is a very subtile effect. But I think what Bernd Gaehrken expresses
is the fact that his observations show a smaller magnitude drop than would be
expected if Jupiter's atmospere had a sharp edge (or the polar diameter of
Jupiter is slightly different).

But don't invest too much time in this question. Guide as it is, is really a
great tool for simulations of the movement of planetary system bodies.

Clear skies
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

Bernd Brinkmann

Sternwarte Herne, MPC code A18
Herne, Germany

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