Jupiter Moon Shadows

In town, Sept 9? 1997

Speaking of Jupiter's moon shadows, Io made an excellent pass last night.

I'd been curious how my new-to-me 80mm refractor would do on the moon, and last night was my first opportunity to find out. From my driveway, the seeing was fairly unsteady, but who cares when you have an 80mm? (Akkana discovers the joys of small aperture ...) The sun was just rising over the Straight Wall, and Birt and Birt A were gleaming goggle-eyed out of the darkness. I made a sketch of that area, and of the Bradley and Hadley Rilles over in Mare Imbrium (there was a beautiful ray of light dividing the Apennines near the end of Hadley Rille, but my drawing didn't capture it well at all) then turned to Jupiter.

At a little after 10pm PDT, viewed at 182x, the GRS was nearly on the meridian, and also on the meridian was a very large, prominent moon shadow. A shadow so large must be a close moon -- Io? Just on the other side of the meridian, on the NEB, one dark spot (barge?) was visible -- and in the light-colored equatorial zone, right next to the barge, was an odd, very circular, dusky spot, only slightly darker than the tawny equatorial zone around it. I'd never seen much detail in the equatorial zone. Could it be -- the moon that was causing the shadow?

I'd never actually seen a moon against the disk of Jupiter before. I ran inside and fired up the computer to run my Java Jupiter applet -- and there it was, Io right next to its shadow, exactly as I'd seen them.

I watched for a while, then went inside for a while and got distracted by something, so by the time I went back outside, I'd missed seeing Io at the edge of the limb -- the shadow was still visible on the planet's face, but the moon itself hung tangent just off the limb. Lovely. I watched for a few minutes longer as the tiny disk of Io broke free of the disk of its primary.

Akkana Peck ; last updated: 1997 Sept 25