by Matt Tarlach
I got home in time to enjoy a nice view of the double shadow transit across Jupiter last night. I set up my 6" f5 Orion OTA on the Super Polaris mount I bought a couple of weeks ago, and popped in a 5mm eyepiece for 150x as soon as the planet rose above the neighbor's fence at about 17:30 PST (0130 UT). I was surprised at how good the view was considering the planet's altitude: When I added a #23a red filter to reduce atmospheric refraction, I could see 4 belts across the planet, with some lumpy detail in the NEB and two festoons in the equatorial zone.
When I first looked, the shadow of Io was well onto the planet, and Ganymede's shadow appeared as a deep bite near the souther polar region. I watched as Ganymede's shadow moved fully onto the disk, and was unable to detect either of the moons themselves at this time.
After taking a short break, I went back out at about 18:20 (0220 UT) and saw Ganymede had cleared the planet. As Jupiter had risen somewhat and the air gotten even steadier, I recalled the geometry of the Galilean moons' orbits and tried to predict where Io should be, based on the location of Ganymede and the two shadows that still crawled across Jupiter's cloudtops. Perhaps three times at powers up to 250x, I thought I could glimpse a dusky roundish lump on the southern part of the SEB, where I expected Io to be, but wasn't quite sure. At about 18:45 (0245 UT), as Io moved onto the planet's darkened limb, it became an easy detection. Shortly before third contact, with Io still fully inside the limb of Jupiter, I could see at 180x the entire disk of the moon standing out in front of the planet. With Ganymede standing off nearby, and both shadows still marking the Jovian cloud tops, it was really a wonderful view! I continued to watch as Io cleared the planet, then took another break to wait for Jupiter to approach the meridian. Considering the excellent views I was getting with the planet low in the sky, I was anticipating fine conditions for the transit of the GRS later in the evening.
When I stepped back outside at 2100 (0500 UT), some broken cloud cover had moved in. I caught peeks between the puffs until about 2145, when the narrow band of clouds moved east and Jupiter was again in the clear. Seeing was actually no better than earlier in the evening, when the planet had been below 30 degrees altitude; perhaps the clouds signalled the arrival of less stable air. I was still able to get some nice views of the planet, with Ganymede and Io now paired off the limb. There were two monstrous festoons looping down from the NEB; one was bold and dark at a longitude similar to the GRS, which rolled into view as I was gazing through the clouds. The other preceded it, and was less bold and more diffuse, but possibly even more broad; this festoon transited at about 2145 (0545 UT). Both appeared bolder through a #25 red filter, so I believe they have a strong blue component though I did not see the color directly in the unfiltered view.
Carmichael, CA