by Tommaso Dorigo

This evening (April 14) I am blogging from a residence in Sesto val Pusteria, a beautiful mountain village in the Italian Alps. I came here for a few days of rest after a crazy work schedule in the past few days -the reason why my blogging has been intermittent. Sesto is surrounded by glorious mountains, and hiking around here is marvelous. But right now, as I sip a non-alcoholic beer (pretty good), chilling off after a day out, my thoughts are focused 500,000,000 kilometers away.

As an amateur astronomer since the age of seven, I have always loved to watch the night sky and its treasures. Nowadays I do that very infrequently, however, as work is a tyrant. But really the real reason why my observing sessions are rare is that I have grown picky. I cannot be content with watching the sky from a urban or suburban location: I want the best. There are places around Sesto where the background luminosity of the night sky is very low, which allows amateur telescopes like mine (a 16 inch dobsonian) to see wonderful details in the arms of many galaxies. So that’s one of the reasons why I came here.

In April, the constellation of Libra is up at nigth for northern observers, and with it a cluster of galaxies which is a real wonder to behold with medium to large telescopes (I remember counting 13 different galaxies in the same field of view by aiming at the center of the cluster). This evening, however, the moon will rise shortly after midnight, preventing a real serious galaxy observing session. So what is up?

It is Jupiter, the biggest planet of our solar system. Jupiter is these days at its closest point to the Earth, so observing it is easier. The planet has been a favourite target of observers since Galileo first aimed his rudimentary telescope at it in 1610 (forgive me if I don’t know the exact date). When I was a kid, the pictures that even giant telescopes like Mt. Palomar could snatch of the gas giant were blurry and indefinite -there was no technology to acquire digital images and stack the best shots, nor adaptive optics to react to varying atmospherical conditions in the field of view. And then came the first images from the probes we sent to the planet, and it was like, wow! THAT’s what those features we observe from Earth really look like!

Nowadays the planet is the subject of deep studies with the probes we send there, like Juno. But even modest equipment allows skilled amateurs to obtain pictures that were impossible for 5-meter telescopes twenty years before. And if you are a skilled amateur and you have access to a 1 meter telescope – well,  what you can do is amazing. Judge for yourself e.g. from the video below, obtained by Damian Peach last February by collating many fantastic images.

(the image is Copyright D. Peach/Chilescope Team. 1 metre telescope with ASI174MM camera.)
I do not pretend to be able to see similar details on Jupiter’s disk with my 16 incher. But tonight, if the sky clears, I will be aiming it at the planet with doubled interest. The reason is that the largest of the four main satellites of Jupiter, Ganymede, will pass in front of the disk as seen from the Earth, leaving a crisp dark shadow that can be followed during the three hours it takes to travel from one side to the other. It is a unforgettable view if you happen to see it.

The forecast says there will be clouds tonight, but the sky should clear up at some point. So I am praying for this to happen before 2AM, when the transit of Ganymede will end… I will let you know the result soon.

UPDATE (Apr 15): I am happy to report that the clouds finally left a window of opportunity after midnight, and I was able to follow the full transit of Ganymede and its shadow. The nice thing is that the seeing was surprisingly very good, allowing me to use 300x magnification and observe many small details on the tormented face of the planet. I could discern, for instance, three small white oval storms on the south-south-tropical band, which I later found in recent online pictures. The nice thing was observing the satellite, a tiny but discernible disc, a little brigther than the background of the NNTB, trailed by its shadow. This gave three-dimensionality to the view. When I dismounted the telescope and went to bed I was extremely satisfied to have stuck there watching layer after layer of clouds passing by…

The picture on the right shows more or less the kind of detail I could see, and the placement of Ganymede as I could see it on the planet’s disk. At 300x the planet had an angular appearance of about 4.5 degrees, i.e. nine times the full moon (credit: Valentin Lyakhovic).

And below, I attach a picture which shows me (ok a few years ago 😉 with my telescope.



giove2Update – after downloading some 30” footage I took “just for the sake of it” with a cellphone by attaching it to the eyepiece, I found that most of the time the planet was badly overexposed. I could only extract one snapshot which shows much less than what I could see at the eyepiece (e.g. the disk of Ganymede is not visible), but still gives some idea… Here it is on the left.

You can see some banding on the planet and the black dot due to Ganymede’s shadow. On the bottom right is Callisto.