Tuesday, June 6, 2017

Double Shadow Transit on Jupiter

Three Moons and the Red Spot!

Orbiting Jovian moons sometimes pass in front of Jupiter and sometimes behind. When they pass in front, it's possible for their shadows to fall on Jupiter. There have been several recent opportunities to see two of these shadows on Jupiter at once! Unfortunately, it was cloudy at my location every time. Finally, on June 3rd, conditions were good enough to try observing this interesting phenomenon. Scattered clouds filled the sky as I set up my telescope. As darkness fell, clouds diminished leaving a slightly hazy sky. With no wind, and temperature at 74 degrees, it was very comfortable working outside at the telescope. Jupiter was about 1.5 degrees below the 75 percent illuminated Moon, forming an attractive pair.

I captured the image below at 10:15pm EDT, just after the first shadow entered Jupiter's disc. Two moons visible to the right of Jupiter are Ganymede and Callisto, with Ganymede above Callisto. Another moon, icy Europa, was left of Jupiter, but too dim to show in any images I took. The final Galilean moon, volcanic Io, cast the black shadow seen below. Io itself was located directly in front of Jupiter's disc to the right of the shadow. Consequently, Io is hard to distinguish from bright background planetary features in this image.
At 10:21pm EDT Ganymede's shadow can be seen making a small, black entrance notch in the upper left of Jupiter's disc.
Ten minutes later, at 10:31pm EDT, Ganymede's shadow was still entering the disc. Notice the small, pale red spot nearly centered in the bottom white zone.
In another 10 minutes, at 10:41pm EDT, Ganymede's shadow had fully entered the Jovian disc. An observer sitting within one of these shadows on Jupiter would see the Sun eclipsed by a Jovian moon!
In the next image, taken 30 minutes later at 11:11pm EDT, you can finally see the tiny white disc of Io about to exit the dark belt on Jupiter's right edge.
At 11:21pm EDT, Io had completely exited the Jovian disc and was now clearly visible on the right against a black sky. Jupiter's great Red Spot was also rotating into view on the lower left edge.
My hopes of following shadows entirely across the disc were dashed by increasing haziness over the whole sky. Jupiter was also getting closer to the horizon. Soon, a bright 22-degree circle formed around the nearby Moon. I had to increase my exposure times as Jupiter began to fade behind the thickening haze. My last image, at 12:01am EDT, June 4th, shows the Red Spot, two shadows, and three moons! Wow!
I made a time lapse animated GIF using 14 images recorded between 9:41pm EDT and midnight. This little video compresses 2 hours and 20 minutes of Jovian motion into about 2 seconds. Jupiter rotates, moons orbit, and shadows transit the disc!
By the time I got to bed after 1:00am, I was exhausted. My 52-mile bike ride the next morning completely wiped me out. But it was worth the effort to record this beautiful celestial event! 

People say I'm crazy doing what I'm doing
Well they give me all kinds of warnings to save me from ruin
When I say that I'm o.k. well they look at me kind of strange
Surely you're not happy now you no longer play the game

People say I'm lazy dreaming my life away
Well they give me all kinds of advice designed to enlighten me
When I tell them that I'm doing fine watching shadows on the wall
Don't you miss the big time boy you're no longer on the ball

I'm just sitting here watching the wheels go round and round
I really love to watch them roll
No longer riding on the merry-go-round
I just had to let it go

John Lennon