Occultation Practice and an Eclipsed Galilean Moon: First Session of the New Year

Thursday, January 9th had a lot going for it. First, I worked Sunday through Thursday this week, so I would have Friday off. Next, the forecast was for clear skies. There was an air stagnation advisory, so the atmosphere looked to be relatively steady. And there were some interesting lunar and planetary events:

  1. Io and its shadow transited Jupiter in the early evening.
  2. Two of the main stars in the Pleiades were hidden by the Moon, and would reappear off its bright limb.
  3. Ganymede would emerge from occultation by Jupiter, and later would be eclipsed by Jupiter’s shadow.
  4. The Great Red Spot would be visible on Jupiter.

Besides that, Venus was half-lit in the western sky, with Saturn above it. The Moon was in an 83% waxing gibbous phase. Mars was a week away from opposition, and 4 days away from being occulted by the Full Moon. I hadn’t been out with the scope since early November, so it was a good night for a shakedown.

To prepare, I packed my 10” Dob in the car the previous evening, so that it would be ready for me to scamper to my local site after work. I didn’t have a lot of energy during the day, but I started feeling better later. I arrived at my site just after 5:30pm, and the ground was wet from all of the recent rain, but I found some gravel to set up on. Seeing was a bit rough early on, as the air temperature was dropping quickly. Io’s shadow was visible, but the moon itself didn’t show until it was on the limb of Jupiter at around 6:00. A few minutes later, Atlas and then Pleione were to emerge from the Moon’s bright limb. The Moon was in a spectacular phase. I don’t spend much time looking at the Moon, but the bright rims at various angles on the craters drew my eye, as well as the terrain near the terminator. I knew where Atlas would pop out, and it did, an ember against the bright sky. As the Moon left it behind, the bluish color of this hot star was more apparent. Dimmer Pleione followed a few minutes later.

I took time to look at Venus and Saturn before returning to the Moon. Seeing was still a little wobbly. The four main craterlets on the floor of Plato were visible, but b and d only some of the time.

Sky & Telescope's tool indicated that Ganymede would exit occultation at 7:28pm. The moon was first visible as a pimple on the limb of Jupiter at 7:19, and I was caught off guard a bit. Around 7:28 was when the moon was fully visible.

Seeing was still average. I did a sketch of Jupiter. I wasn’t getting a lot of fine detail, but the Great Red Spot and the array of belts were prominent enough. The air temperature was hovering just below freezing, it was very moist, and I had printed my sketch template on regular copy paper rather than the heavier paper I sometimes use. I was having a bit of a rough time rendering detail. I probably should have just re-drawn the sketch afterward, but I didn’t. So, my colorized version retains the wavy blurriness of the original.



Ganymede was supposed to enter eclipse at 8:22pm according to my table. In this case, the table indicated the very start of the eclipse, and I could notice a decrease in brightness almost immediately. The moon gradually faded out. It took 12 minutes until it was gone completely. By this time, I was also fighting eyepiece fogging. For a couple of minutes, everything was still, and the detail on Jupiter increased tremendously. I considered making another sketch, but the moment of excellent seeing was fleeting.

As my Dob accumulated a coat of frost, I turned to Mars. Mars wasn’t as high as Jupiter, and the seeing was still moderate. I could see a sharply defined North Polar Cap. There appeared to be bright limb clouds along the preceding limb, and the following limb was also bright. Surface markings were a bit muted, but I could make out the main complexes of Mare Acidalium and Mare Erythraeum, as well as some detail within. Argyre in the far south was bright. Again, my paper was suffering from the humidity, and again I decided to just colorize and scan my original sketch when I sat down afterward. It’s been a while since I sketched planets, so I decided to go rough.



My fingers were getting cold and creaky, so I packed up after an eventful and rewarding 4-hour session.

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