On the evening of 14 March 2016 it was a clear and cool 55 degrees. I went out to the observatory to look at Jupiter for a while before going to bed and immediately saw two moons on the following side pretty far away from the planet. These were later identified as Callisto (the outermost) and Ganymede. I then saw another moon very close to the following limb of Jupiter and, just within the limb, a moon shadow which I initially thought came from that moon. After a little research in the March 2016 Sky & Telescope, I found that the moon was Io but the shadow was from Europa which was transiting the planet somewhere ahead of the shadow but unseen due to a lack of contrast between the sunlit moon and the cloud-tops of Jupiter. A little more research showed that a double transit was in progress starring Io and Europa and their shadows.
After a few more minutes, Io crossed the following limb as Europa’s shadow progressed toward the preceding side. After about ten more minutes the shadow of Io began to show itself along the following limb. I could still see Io as a bright spot along Jupiter’s Northern Equatorial belt. The shadows of Io and Europa were also along this belt. I looked as carefully as I could but could never see Europa.
I watched a few more minutes of the double transit but could not watch all of it. I had to stop and go to bed so I could get up at 5:45 am and go to work the next morning.