by David Kingsley
I was out looking at the moon last night in the backyard with my 7 inch Starmaster Oak Classic. A little after 10 pm I noticed Jupiter clearing the neighbors trees, and swung over to take a quick look. The planet was only 10 to 15 degrees above the horizon. But even through the murk, I could see that the Great red spot was nearly smack in the middle of the planet, and was being chased by the small black shadow of one of Jupiter's moons. I tried playing with some colored filters to see if they would help the view. Long wave length red light is less susceptible to refraction by the atmosphere, and I have heard that viewing through red filters may therefore help steady the view of objects sitting low in the sky. Any effect last night was small, and I found I preferred the natural color view.
Fumbling around with the filters did have the good effect of taking some time however. Just as I was about to move to something else, I noticed a second black dot had also appeared on Jupiter. Both the first shadow and this second one were superimposed on the southern equatorial belt . Two small moons were still visible off the trailing side of Jupiter, clearly the source of the shadows. I often have fun trying to guess which moon is which in an eyepiece view of Jupiter. When the moons are spread out, it relatively easy. Ganymede is always the biggest, and Caliisto is the duskiest and can travel much further from Jupiter than any of the other moons. Both Ganymede and Callisto were obvious last night as two distant dots far away from the trailing side of the planet. The remaining moons Io and Europa can be harder to distinguish because they only differ in diameter by about 15%, and often cross each other in their near Jupiter orbits. I couldn't really make out the size difference last nigh in the low altitude view. However, the bright dot closest to the following edge of Jupiter was slightly yellowish and the nearby moon was whiter. Io is a volcanic moon with lots of yellow sulfur deposits, and Europa is the remarkable frozen world with a a surface of bright ice. The big difference in surface character of the two moons is apparent in their colors at the eyepiece.
Although the size of the moon themselves was hard to distinguish, the size of the shadows were very obviously different. The first black shadow (Io's) I was larger and had sharper edges than the second (Europas). The line up of GRS and the double shadow transit made a great race of red and black features across the face of Jupiter. The GRS was followed by an obvious white oval break in the Southern belt. Io's shadow was obviously moving faster than the rotation of Jupiter's clouds, because it started out trailing this white feature within the belt, and was superimposed upon it by midnight. A little after midnight, Io itself reached the edge of Jupiter. I sketched a bit as Io went from a small dot, to a pimple on the edge of the planet, to a visible white dot superimposed on the same ruddy race track that led to the shadows further ahead.
I always enjoy watching transits, but Jupiter has been out of the evening skies for so long that I have not had a chance to see them for a long time. Double shadow transits are rarer. And double transits with a well positioned GRS are rarer still. For anyone else who enjoys watching the play of shadows and orbits on the moon, there is a very well positioned double shadow transit coming up for next Friday night/Saturday morning as well.
Night of November 9/10th (from Sky and Tel, pacific time zones shown)
Io shadow ingress: 11:51 pm
Io transit begin: 12:06 am
Europa shadow ingress: 12:06 am
Io shadow egress: 2:05 am
Europa transit begin: 3:09 am
Io Transit egress: 3:10 am
Europa shadow egress: 3:42 am
Note that two shadow transits will appear within 15 minutes of each other. The second shadow begins almost at the same time as the Io moon transit itself. It will be interesting to see how close Io and Europas shadow will be on the face of Jupiter. The transit of the first moon and the shadow of the second begin almost simultaneously around 12:06
Nice to have the King of Planets back.