Io Transit

by Paul Sterngold


We were lucky here in Alameda. The early clouds dissipated, so I set up my 4" Tak refractor on the sidewalk and watched as the moon approached the planet's limb. I shared with my daughter, Claire (10), and she really enjoyed watching it approach the planet, then merge into the limb. After a short while, we were just able to see the shadow begin to appear.

I noticed an interesting effect: when Io and its shadow first appeared, it was easy to see both (they were nearly touching). But when Io was more than 1/4 into the planet's disk, it became quite challenging to see the moon. The shadow was distinct and clear as could be (the seeing improved *dramatically* around 9:30pm). Later, when Io was about 3/4 across the planet, it began to appear a little more distinctly again. Did anyone else notice this? I noticed the same effect last Wednesday (22-Dec) in my 6" refractor, as did my fellow observers: we could see the shadow but not the moon until both were within 1/4 of the edge of Jupe's disk.

Last night, I used a Tak FC100 with magnifications up to 286x (Tak 2.8mm Or) on both Jupiter and Saturn. Jupiter was best with a 5mm Radian at 160x. The GRS showed internal detail, with a darker ruddy-brown swirl in its south, a lighter internal section rolling over this swirl, and a sharp ruddy-brown line at the delimitation with the SEB. I was also able to see an elongated white "barge" (correct term?) just south of the NEB, with a darkening in the NEB at the barge's leading edge. The SEB showed a distinctive split behind the GRS. It's really enjoyable to observe the detail Jupiter has to offer!