Noticing an internet forum post mentioning a partial transit of Io in front of Ganymede, I decided to attempt to view it even though it would be at 2am PST. I love observing transient events: it sounds corny but I have a feeling of connection with the universe, seeing change in the heavens in my finite, human timescale.
I started observing at 2:05am (10:05 UT as in my notes). Seeing was 6/7 stable, and transparency was not too bad, average. A little dewy, temperature in the low 40s. After trying a range of magnifications I settled on 277x. Jupiter looked mushy but I could still see the moons clearly enough to use it to observe the transit.
Attached are my log notes with sketches. I added an exaggerated view to the right hand side to better show the progress of the transit. Most interesting was to see an apparent size difference between Io and Ganymede -- probably not noticeable unless they are very close to each other.
Scope: 12.5” f/7 newt; 8mm eyepiece @ 277x 0.4* TFOV. 95% full moon; Seeing 6/7 Transparency 3/5. Times in UT:
10:13: Transparency not too bad; Trapezium in Orion showing A-F stars at 170x. Ganymede and Io look like a nice double star of near equal brightness. Callisto is far out to the West. Europa not visible as it is in Jupiter’s shadow
10:16: Split narrowing. Two distinct disks in contact, just separating when seeing stills. Ganymede is bigger than Io. Jupiter mushy at this magnification
10:18: Notched but not separating, figure “8”
10:21: Maximum overlap. Elongated egg shape. No split, no notching
10:24: Just touching, notched but not separating
10:26: Easy close split. Separation grows as I watch.