Galillean moons - differing albedo

[Stu]

A couple of threads today, and some observing experiences over the last few months prompted me to check this.

I confess I had been a little cynical about reports of people tracking the moon transits all the way across Jupiter. Shadow transits are easy but I have always found that moon transits start as bright disks, then fade into the background glare as they get away from the limb darkening and become invisible in any scope I have used.

Despite observing these many times over the years, the first time I consciously realised this was wrong was during the triple transit a little while ago, when Callisto appeared as a black dot, very similar to the two other shadows which I managed to spot through the appalling seeing!

It occurred to me that I must, in large part, have only been observing Io or Europa, or had been mistaking the others for shadows as I did not always have access to transit timings.

Last night I watched Ganymede start its transit, as normal like a tiny bright disk. It stayed visible as it progressed away from the limb but faded as it moved from the limb darkening. I had to take a break at that point, but returned later expecting it to be invisible. Wrong! It showed up as a nice darker dot transiting across the equatorial band and would have been visible right the way across.

I checked the albedo figures today:

Io 0.63

Europa 0.67

Ganymede 0.43

Callisto 0.22

This matches my observations. Callisto is very dark, and easy to mistake for a shadow transit. Ganymede appears dark, but not black so is easier to identify as not a shadow. The other two are much brighter and their visibility across the disk will very much depend upon the background they are highlighted against.

Callisto and Ganymede transit relatively less frequently given their longer orbits, so that is my excuse for not noticing this before ;-). Always learning something in this game.

One other effect to look for is an elongating of shadows as they approach the limb which also is something which obviously doesn't happen with moons!

Hope that is of some interest.

Stu

[Moonshane]

Great post mate. I agree with your findings too.

I Should pay more attention to this I reckon.

[Moonshane]

In fact I have just realised that I was actually looking at Ganymede and not the shadow last night. The shadow did not appear until after Jupiter went behind my house! My excuse being that it was already well on the Disk when I started observing. In other words I made an ass of u and me

[Stu]

In fact I have just realised that I was actually looking at Ganymede and not the shadow last night. The shadow did not appear until after Jupiter went behind my house! My excuse being that it was already well on the Disk when I started observing. In other words I made an ass of u and me

You see! I'm glad it's not just me getting confused.

I won't tell anyone else if you don't ;-)

[Moonshane]

Lol. No shame in errors it's how I learn. Ahem, I have learned a lot.........:0)

[SnakeyJ]

I thought exactly the same last night viewing jupiter on the capture laptop - it was only after stacking the data and double checking ephemerids in Win Jupos, that I realised I had Ganymede in transit. I've also got some albedo variation visible.

[Stu]

Nice one Jake, any images posted up yet?

EDIT: Just seen your post in the imaging section, lovely pic!

[christhebrit]

Good to know!

I'm planning on more observations of Jupiter over time in the same evening to see the moons travel.

I had only heard of the moons hiding in front of a bright background.

I'll make sure to check my findings :-)

[AndyWB]

Yeah, I'd agree with that - I've spotted Ganymede in front of Jupiter, and I think Callisto, but not Io or Europa.

I'm also pleased that someone else sees Ganymede as a little disc. I find at about x200 it doesn't look like a point source. I was wondering if my head was making it up.

[Stu]

Yeah, I'd agree with that - I've spotted Ganymede in front of Jupiter, and I think Callisto, but not Io or Europa.

I'm also pleased that someone else sees Ganymede as a little disc. I find at about x200 it doesn't look like a point source. I was wondering if my head was making it up.

I find they look more disk-like when transiting. When just on their own, I'm never sure if they look like disks or just an airy disk...

[John]

Yeah, I'd agree with that - I've spotted Ganymede in front of Jupiter, and I think Callisto, but not Io or Europa.

I'm also pleased that someone else sees Ganymede as a little disc. I find at about x200 it doesn't look like a point source. I was wondering if my head was making it up.

I see them as disks. A fun game to play is to try and identify which is which just from looking at their apparent diameters at high power. I tried it a few weeks back and was pleased to get it right when I later checked which was which. Ganymede's disk has an apparent diameter of around 1.8 arc seconds when Jupiter is at opposition.