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Jupiter double transit 3/20/04



 
 
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  #1  
Old March 22nd 04, 01:12 AM
Axel
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Default Jupiter double transit 3/20/04

Anyone here in the U.S. catch the simultaneous transits of Ganymede
and Io across Jupiter last night, around 10:30 PM to midnight CST or
so? We observed them using my friend's 12" LX-200. Ganymede and its
shadow were transiting over the NTrZ, just north of the NEB. I was
surprised at how easy it was to see Ganymede, visible as a grayish
indistinct dot over the white zone.

Io and its shadow transited over the southern edge of the NEB. I
figured that this would make Io easy to spot, but it was actually more
difficult than Ganymede, probably due to its smaller size. It was
visible as a tiny white "hole punch" in the NEB, immediately preceding
its shadow.

While observing, we remarked on how the distance between Ganymede and
its shadow really made the sun's angle of illumination (with respect
to our line of sight) evident. It seems strange to me that Jupiter's
gibbous phase never gets smaller than 97% or so (it was barely visible
last night), yet the effect of the indirect illumination is so much
more dramatic on the moons and their shadows.

Anyone have an explanation for this?

Cheers,
Ritesh
  #2  
Old March 22nd 04, 07:22 AM
Milton Aupperle
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Default Jupiter double transit 3/20/04

In article , Axel
wrote:

Anyone here in the U.S. catch the simultaneous transits of Ganymede
and Io across Jupiter last night, around 10:30 PM to midnight CST or
so? We observed them using my friend's 12" LX-200. Ganymede and its
shadow were transiting over the NTrZ, just north of the NEB. I was
surprised at how easy it was to see Ganymede, visible as a grayish
indistinct dot over the white zone.

Io and its shadow transited over the southern edge of the NEB. I
figured that this would make Io easy to spot, but it was actually more
difficult than Ganymede, probably due to its smaller size. It was
visible as a tiny white "hole punch" in the NEB, immediately preceding
its shadow.

While observing, we remarked on how the distance between Ganymede and
its shadow really made the sun's angle of illumination (with respect
to our line of sight) evident. It seems strange to me that Jupiter's
gibbous phase never gets smaller than 97% or so (it was barely visible
last night), yet the effect of the indirect illumination is so much
more dramatic on the moons and their shadows.

Anyone have an explanation for this?

Cheers,
Ritesh


I noticed that too. Also the emergence of Europa from Jupiter's shadow
also reinforced the off angle for the sun relative to us.

I also photographed 3 hours of it and am now processing the 40
gigabytes of collected video data. Ganymede is easily visible as a
grayish disk on the footage and like you observed, Io was just a "hole"
on the darkish brown belt.

Milton Aupperle

http://www.outcastsoft.com/AstroImages/AstroIndex.html
  #4  
Old March 23rd 04, 07:33 AM
Axel
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Default Jupiter double transit 3/20/04

This depiction is by no means an explanation, but it begins to show how the
larger distance of Ganymede enters the equation.

http://www.psiaz.com/polakis/jupiter_shadow_cones.jpg


That's neat! Thx.

Ritesh
 




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