FreeLists / az-observing / [AZ-Observing] Re: Jupiter Events

["Frank Kraljic" <FJKraljic@xxxxxxx>]

Hi Bill,

You were in fact observing Callisto and Io's shadow.  Callisto is a dark
body and when superimposed over Jupiter bright clouds, the contrast between
the two is like night and day.  I remember the first time observing Callisto
in transit and being shocked at how shadow-like it appeared.

When away from the planet and at low power, all the moons appear bright.
However, when seeing permits, be sure to increase the magnification beyond
300x.  At moderate-high magnification, the moons are no longer points of
light, but disks of various sizes with Ganymede being the largest at ~2",
Callisto, Io, and Europa.  Apart from their diameters, color characteristics
are visible for each.  Io is orange/yellow, Europa is white, Ganymede is
tan/gray in my opinion, and Callisto is gray and noticeably darker than the
other three.

So the question stands, why is it the moons still appear bright.  My best
analogy is think of a black card with a highly reflective surface.  When
light strikes the surface, regardless of the card being black, the varnish
still reflects a fair amount of light.  I believe the same is true with
Callisto and Ganymede.  Their surfaces are a mixture of ice and rock, and
probably reflect a fair amount of that light as a result of the ice on the
surface acting like a varnish.
        Honestly, I may be completely off on this interpretation.  Brian Skiff
would know better.

Hope this helps.

-FRANK


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