Jupiter and IO transit

[Richard Navarrete]

Thanks to Mark Wagner for the heads up on the IO transit last night. I bought a used planetary camera a few months ago and maybe used it once. Last night, despite the iffy seeing, I managed an ok image of the transit. At least you can see a black dot! :-)

Richard

[Mark Wagner]

Actually, that's to Muriel who first mentioned it to me.  It was a fun short night in the backyard.

[Mark Wagner]

So after witnessing Io's shadow last night I wondered if Galileo, in his small rudimentary telescope saw one too?

From the eminently reputable ChatGPT:

"The first recorded person to see a shadow transit on Jupiter was Galileo Galilei.
After discovering Jupiter’s four large moons in January 1610, Galileo kept observing the system closely. On March 9, 1612, he noted a small dark spot moving across Jupiter’s disk — which we now know was the shadow of Io (one of the Galilean moons) cast onto the planet’s cloud tops.
At the time, Galileo didn’t immediately understand what he was seeing. He initially thought the dark spot might be a surface feature on Jupiter itself. Only later did astronomers realize these moving spots were satellite shadows, a huge conceptual leap because it meant:"

Wow.

[David Kirjassoff]

I bet he had bortle 1 skies!

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[Mark Wagner]

For sure!  Galileo always replied "Bortle 1" when asked the darkness of his observing location ;-)

Mark

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[Ted Hauter]

Galileo transit shadow: Unlikely. Looking though every aberration known to science. 

Clearer sky may have helped. But in town where he was smoke would be prevalent. 

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[Steve Gottlieb]

I believe this is the actual lens (mounted in center) Galileo used to discover Jupiter’s moons.

— Steve

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[Alan Agrawal]

Hi,

Yep, that’s the one:

The whole lens is 58mm, it’s stopped down to 38mm clear aperture, and was tested with an interferometer and is about an 1/8th of a wave in transmission - a very good lens -  which was why he achieved what he did before others, and which was why it was months after before anyone could replicate his results.

Alan

On Feb 5, 2026, at 2:43 PM, Steve Gottlieb <astrog...@gmail.com> wrote:

I believe this is the actual lens (mounted in center) Galileo used to discover Jupiter’s moons.

— Steve

<IMG_0036.jpg>

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[Ted Hauter]

If only had this lens...

(Galileoscope lens) 😉

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[Joel Lee]

Thanks Mark and Muriel for the heads up! I would've set up a different rig otherwise. I finally managed to get a video that I liked of Jupiter. Seeing was alright at best 2/5 (imaging rigs were showing a little worse than usual seeing). Jupiter was jumping around but would occasionally snap into detail.

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[Richard Navarrete]

Cool. I love that you can see Io as well as its shadow.

Richard

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[Ted Hauter]

The transit and moon combo added an immediate depthness to it and removed any illusion of a flat surface. Jupiter, our largest failed star.

When sunspots arch around the limb of our closest star the sun, the same effect happens and the surface suddenly appears rounded to my eye.

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[Joe Acosta]

I imaged Jupiter IO transit and got the GRS as well.  I was able to get a 60-90 second video and even got the 3 other Galilean moons.  I was quite happy

Hope the attachment goes through 

Joe