Moon shadow?

[Paddy de Klerk]

Like Iceman said;Don`t be afraid.Being a big sky with lots to discover I recently followed a transit of a moon across Jupiter.How does one distinguish wether its a shadow or the real thing.What I viewed was distinctly dark.I do know that it was Io though.Cheers

[Blue Skies]

I think you're asking "How can I tell I'm looking at a moon of Jupiter, such as Io, or just a shadow of a moon?"

The shadows of the moons will definitely be black.

The moons themselves are generally bright and tend to blend in well with the background cloudscape of Jupiter! Europa, the smallest of the four Galilean satellites, can be hard to spot on Jupiter if you only have a smallish scope (say, an 8").

In my experience Io is best followed onto the face of the planet (if you can) and from there you know where exactly it is, although it is big enough to be picked up at anytime with careful observations (again, this is through an 8" scope). It will look like a small white-yellow dot.

I don't recall seeing either Europa or Callisto in transit - indeed I wouldn't have seen Callisto, now I think about it, as I've just read its only starting to do transits again after some time. Ah yes, the latest Australian Sky and Telescope has all the info! pages 56 to 59. There is a picture of Callisto in transit and it's quite dark. I recommend you do your best to get hold of this article, it will probably answer any question you have! :thumbsup:

[iceman]

Hi Paddy

Jacqui has explained most of it. The shadows will be black where as most of the moons are bright and are hard to distinguish across the face of Jupiter, except when they're near the limb.

However Ganymede and Callisto can be quite dark.

This is an image (http://www.iceinspace.com.au/mygallery/displayimage.php?pos=-716) I did recently showing Callisto transitting and it's very dark! Yes, that's Callisto and not its shadow!

This is an image (http://www.iceinspace.com.au/mygallery/displayimage.php?pos=-708) I did showing Ganymede transitting with Europa's shadow as well. While Ganymede is still very dark, you can see the difference compared to Europa's shadow.

The best way is to just know what you're looking at.

I use a program called Jupiter 2 (http://www.astrosurf.com/rondi/jupiter/) to know what transits are coming up that night, or to confirm what I saw last night, or to plan an imaging session next week for example.

Hope that helps!

[Paddy de Klerk]

Finally got to sit down .I thank you for the knowledge to go further in space.Jupiter 2 covers it nicely .What I saw up there was definitely a shadow!!!Looking forward to Lostock.Paddy