Last night after dinner I looked out the window to see Jupiter high in the sky with the nearly full Moon not far away. There were no signs of clouds so I decided to check if anything interesting might be visible on the planet. For this I use an app called Jupiter Simulator that shows me GRS and moon transits on my Android tablet. There are similar apps available for iOS and an online javascript utility at Shallow Sky.
I was pleasantly surprised to learn that Ganymede was currently in the middle of a transit. Even better, the Great Red Spot was just appearing on the Eastern limb. I grabbed my smallest aperture telescope that takes interchangeable eyepieces, a 60mm refractor, and headed outside.
I set the magnification to 124x. This is about my upper limit for a 60mm telescope as the resulting .5mm exit pupil makes the view dim and floaters in the eye become apparent. The view through the eyepiece was not immediately sharp. Even a small refractor takes a few minutes to cool down when there is a 38º difference between inside and outside temperatures. Until the thermal currents in the scope settled down the image often appeared blurry but after about 15 minutes it was much more steady.
I first noticed the disc of the planet and the visible moons arrayed to the sides. The two main belts were quickly recognizable and then more details perceived. Ganymede itself was visible against the planet’s disc, appearing as a small grayish dot. At first the Great Red Spot was just on the limb and a great deal of effort was required to make it out. Within 15-20 minutes it became much easier as it slowly moved across the face of the planet toward the meridian.
I spent a bit over an hour outside. Watching the show on Jupiter intently for a few minutes, then turning to another subject; Aristarchus and Vallis Schröteri on the Moon, the Trapezium in M42, Castor, Rigel. But in between them always back to Jupiter to see how the transits were progressing. This was the main event.
Eventually Ganymede’s shadow entered the scene. Compared to the moon itself the shadow was dark and crisp. Hard to miss, it stood out starkly against the planet. The view of all three objects, Ganymede, its shadow, and the GRS, simultaneously was fantastic. Not bad for a 60mm telescope.
-Dave
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