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Jupiter's Moons: Visual differences in apparent diameter


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8 hours ago, John said:

The GRS was nicely on show last night. It is smaller than I recall from previous years though.

Interesting to think that the GRS has an apparent diameter at it's widest point of around 7-8 arc seconds.



Interesting thought John, the GRS itself would make a decent sized planet! It's a shame it's shrinking though. :(

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On 13/09/2021 at 22:26, mikeDnight said:

I believe its because the moon's are not true point sources, and so being extended objects they play a different game. In my experience telescopes that are supposedly incapable of resolving two stellar point sources are quite capable of resolving an extended object. When any of the Galilean's begin to cross the face of Jupiter and while they are still in the shaded limb area, they each display a definite disc, just as they do on leaving the disc. And as the moon's make 1st or 4th contact they display a definite disc in a 4" scope. 

Yes, this is a really interesting effect - I've seen Io appear as a beautiful 'ball' just as it transited across Jupiter's limb - breathtaking.  Off Jupiter's disc, they appear as having different sizes and colours to me, but only on limb transits do I see them as really sharp discs (15" dob) - too much glare from the moons against the black of space.

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