Hi Bernd,

Ah, I see what you mean. No, the atmospheric effects aren't treated;
the assumption is of an airless object.

Treating this properly would be an interesting problem. It's not enough
to handle atmospheric absorption, which (I think) would be a minimal
effect, especially considering that the scale height for Jupiter is
much less than that for the Earth. The big issue, I'd think, would
be refracted light. (Much as the totally eclipsed moon is quite visible
due to light refracted around the earth. Guide handles this by showing
the totally eclipsed area in orange.)

It does cause me wonder what one could learn from studies of the
magnitudes of eclipsed Galilean moons. The magnitude of our moon varies
quite a bit (very faint after the eruption of the volcano Pinatubo in
1990, for example). In this case, we have _four_ moons that can be
observed, going in and out of eclipses at different distances from
Jupiter and at different latitudes. Seems as if there ought to be
at least some useful data there, though puzzling it out wouldn't be easy.

Exactly how much this would explain Bernd Gaehrken's observations,
and the magnitudes shown by Guide, I dunno. I'd think that the magnitude
of a totally eclipsed Galilean could vary as much as the magnitude of
a totally eclipse moon. For a penumbral event such as this, I'd expect
a pretty small magnitude drop, just as with the earth's moon; I'd not
be too surprised if the effect shown in Guide is at least slightly
exaggerated.

-- Bill