Io pulls clear of the Jovian Disk

[John]

I've just been watching the final part of Io's transit of Jupiter tonight with my ED120 refractor. The seeing is not the greatest here at the moment but I was able to catch the pale disk of Io against the cloudtops of Jupiters North Equatorial Belt as it neared the end of it's transit. It's interesting that seeing the actual disks of the Jovian moons as they transit their parent planet (as opposed to their shadows) seems, to me at least, to get much harder when they are against the central area of the planets disk but they can "pop" into view when they are close to it's E or W limbs. I reckon this is because of the "limb darkening" effect when viewing the planet so the bright moon disks show up there but blend into the paler / lighter tone of the central disk area. Does that sound plausable ?

Anyway, it's always nice to see one of the moons coming off the disk as a tiny bump which grows into a brighter droplet as it pulls away from the planets limb to join it's Gallilean partners against the backdrop of space

 

[DRT]

Sounds very nice, John. I watched Io appear from behind the planet a few nights ago, also through an ED120. I wasn't aware that the event was about to happen and recall thinking there was a strange pimple on the limb. I soon realised what it was and watched intently as the moon drifted off into the darkness of space. It is quite magical to watch these things happening from so far away.

[Pig]

Nice report and indeed a nice experience John  I was hoping to catch this last night but unfortunately the clouds didn't play ball and I missed it. I did manage to get a nice lunar session in as there were some decent sized gaps in the clouds early on.

[chiltonstar]

In case you haven't spotted it John, there's a nice transit of Io and Europa and their shadows on Mon 29th Feb at about 23:00 - a good chance to see both of the moons against Jupiter's surface, together with the much easier shadows.

Chris

[Stu]

John, I'm sure you are right about the limb darkening effect making them easier to see. I normally struggle (read fail!) to track them across the centre of the disk, but then they appear quite strongly towards the limb.

The one to look out for is Callisto. It is much darker so the actual transit is easy to spot, it looks much like a shadow transit, although not as jet black.

[Size9Hex]

Great report! I've seen Io's shadow (by fluke) previously, but never a moon itself against Jupiter's disc. This may be a stupid question, but do the moons appear as anything larger than a point of light against the disk? It's something I've started looking for when I see the moons, but have never convinced myself I'm seeing anything that can't be explained by wobbly seeing, diffraction, bad collimation etc. It was Io's shadow that originally got me curious. It seemed like it might have been more than a point, but I'm honestly not sure what it even means to have a point source absence of light!

[Stu]

27 minutes ago, Size9Hex said:

Great report! I've seen Io's shadow (by fluke) previously, but never a moon itself against Jupiter's disc. This may be a stupid question, but do the moons appear as anything larger than a point of light against the disk? It's something I've started looking for when I see the moons, but have never convinced myself I'm seeing anything that can't be explained by wobbly seeing, diffraction, bad collimation etc. It was Io's shadow that originally got me curious. It seemed like it might have been more than a point, but I'm honestly not sure what it even means to have a point source absence of light!

To me, the moons and shadows appear as small discs rather than points. Strangely I find the moons easier to see as discs against the surface than when they are against the dark sky

[cgarry]

You are completely correct, it is the limb darkening effect that makes the moon show up better at the edge of the planet.  Here are a couple of unprocessed frames from an old capture that show the effect:

Io on the limb

 

Io on the disc

Cheers,
Chris

[laudropb]

Hi John was watching this myself last night. I have to agree Io was much easier to spot as it reached the limb. It's the first time I've seen a Moon "pop" from the planet and it was a special sight.

[Grotemobile]

That is a vey good picture of Io on the limb.

First time I have seen any thing like that. Do u

think that a bigger scope, allows U see more detail

on the planet Chris . 8" compared to a 12" scope.

 

Steve.

 

 

[John]

Thanks for the interesting replies folks and for the lovely photos Chris

I do indeed see the Gallilean moons as tiny disks rather than points and occasionally play a little game where I try and identify them just from their apparent sizes in the eyepiece and then check to see if I've got it right !. Despite the smallest apparent disk size being just under 1 arc second (Europa) and the largest around 1.6 arc seconds (Ganymede) I'm surprised how often it's possible to get this correct.

 

2 hours ago, chiltonstar said:

In case you haven't spotted it John, there's a nice transit of Io and Europa and their shadows on Mon 29th Feb at about 23:00 - a good chance to see both of the moons against Jupiter's surface, together with the much easier shadows.

Chris

Thanks for the "heads up" Chris - fingers crossed for clear skies on the 29th then !

[Stu]

2 hours ago, chiltonstar said:

In case you haven't spotted it John, there's a nice transit of Io and Europa and their shadows on Mon 29th Feb at about 23:00 - a good chance to see both of the moons against Jupiter's surface, together with the much easier shadows.

Chris

Thanks Chris, looks like a good one!

 

[Piero]

Very interesting John! I also thought to have seen the moons as discs rather than points on my Dob 200 although it left me the doubt whether it was real or due to the Airy discs. What magnification where you using? 

[John]

1 hour ago, Piero said:

Very interesting John! I also thought to have seen the moons as discs rather than points on my Dob 200 although it left me the doubt whether it was real or due to the Airy discs. What magnification where you using? 

Hi Piero,

I was using 134x and later (when Jupiter was higher) 191x. The moons seem to have different tints to them as well. Last night, as Io left the disk of Jupiter, it was really very white looking and Europa, which was on the same side of Jupiter at that point, seemed slightly duskier, if that makes any sense.

 

[chiltonstar]

With either of my Maks, the moons appear as disks distinctly larger than the Airy disks: these appear to be sharper as shadows than when they are against a black background for some reason. Additionally, when the moons are against Jupiter, they look sharper to me than when against dark sky. As well as the edge effect of Jupiter, making the moons more visible near the rim of the planet, sometimes when the moons are crossing Jupiter's disk, I see a darker halo around the moon which I assume (??) is an edge illumination effect on the moon itself.

Chris

[Size9Hex]

Thanks for the replies about the moons/discs everyone! I'll need to have a closer look next time I'm out! Sorry that it derailed the thread to some extent from the original report.