Having already observed Io’s transit of Ganymede earlier this week, I decided to make Io’s eclipse by Ganymede last night a little more challenging. Using a Micro Guide astrometric eyepiece, I attempted to measure their separation and position angles at regular intervals during the event to add some “numbers” behind my visual descriptions. I started observing at 2:30am (10:30 UT in my notes).
Seeing was 6/7, Transparency 2/5, 95% waning gibbous moon. There was heavy dew which fortunately it did not interfere with my observation (however both the primary & secondary mirrors were dewed in the morning).
My equipment was a 12.5” f/7 reflector on an equatorial platform. The Micro Guide eyepiece is a 12.5mm Abbe, which I used in a 2x barlow in order to more widely spread the linear scale over the targets to aid measurement. The magnification was 354x @ 0.1* TFOV, which badly washed out Jupiter’s detail. I have an ordinary dobsonian mount, so I had to manually center the moons in the linear scale to take visual readings; this was very difficult to do at this magnification. I attempted to measure position angle, which involves centering and lining up the objects on the linear scale, turning off the RA drive, letting the object drift to the 360* protractor scale circling the edge of the FOV, and turning the drive back on when the brighter object reaches protractor to take the reading. Since I do not have an on/off switch on the EQ platform’s hand pad, I managed the power by plugging the power cord in and out of the battery, which I held in my hands while standing on the ladder and looking through the eyepiece—an arrangement which did not lend itself to accuracy.
Halfway through the session I jammed the auto lighter plug too hard into the battery socket and broke one of its metal clips, thus losing my EQ drive. I could not properly center the moons in the linear scale to begin with, so I have to throw out all my PA measurements. I am more confident in the relative accuracy of my separation measures, but they should be taken with a large grain of salt given my equipment challenges. All of this adds to my self-justification to build a massive permanent mount for this scope in my back yard.
I was so absorbed in taking measurements I failed to notice Ganymede’s shadow on Jupiter until almost halfway through the session.
Jupiter’s bands were aligned nearly parallel to west in the FOV. Callisto was to the far west and beyond the FOV. Europa was to the west about midway between the planet and Callisto and just in the FOV. Io started out closest to the east of Jupiter with Ganymede close beyond, both about a quarter of the distance between Jupiter and Europa. Io and Ganymede were both moving “toward” each other; Io was coming out from behind Jupiter travelling east, Ganymede travelling west & in front of Jupiter, which is why only its shadow transited the planet’s surface. Moving toward each other means the eclipse itself happened very quickly, just a couple of minutes. If I had the math skills I could perhaps determine the velocity (eg. If a train leaves New York and another Chicago…), but alas…
10:40: Moons forming a nice widely separated pair. Separation 8.36 arcseconds (”)
10:46: Moons slightly closer. Separation 7.1”
10:51: still separated, a tight pair. Separation 4.6”
10:55: Noticed Ganymede’s shadow; it’s 12.5” from the limb on the South Equatorial band, just beneath the red spot which is poorly visible at this magnification. Io/Ganymede just touching, separation 3.35”
10:57: I/G very narrowly split, only when seeing stills. Separation 2.0” and getting difficult to measure. Can tell G is larger than Io.
11:00: I/G now fully overlapped, with a slight notching. I “think” I see the arc of Ganymede’s limb spanning Io, but the whole image is becoming a fuzz. Can only take a measurement of their end to end length, 3.35”. G’s shadow is very slightly further in from the limb, ~13.5”
11:03: I/G are more significantly notched, beginning to separate; their combined length 4.2”
11:04: I/G just split. Broke my power cord!
11:07: I/G clear split, separation 4.0”. G’s shadow 14.6” from limb
11:12: I/G separation 5.0”
At this point it was too difficult to keep taking separation measurements, so I switched to regular eyepieces to enjoy the view. At 277x I observed the red spot better but only when seeing stilled, so I went down to 170x, which was much smaller and more challenging to the eye to pick out detail, but crisper. The red spot was remarkable: It appeared to be rolling on top of the south temporal belt, which compressed and flowed underneath the spot. So if the spot is a storm, and the band is a different or heavier type of gas, it is not mixing or getting sucked into the swirling forces of the spot? I could discern a darker “eyelid” along the opposite curve near the apex of the oval.