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All of the Galilean moons are large and close enough to Jupiter that they can completely eclipse the sun and allow a solar eclipse to happen.

My question is can an observer from Earth, see the moon(s)' shadow(s) on the surface of Jupiter with a telescope or are the shadows too small?

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    $\begingroup$ The Moon is the name of Earth's moon. It's confusing if you write "Moon" when talking about other moons. (Of course, you do need to capitalize "moon" at the start of a sentence). $\endgroup$
    – PM 2Ring
    Commented May 5, 2020 at 1:14
  • $\begingroup$ different but related phenomenon that's fun to watch: When will the next series of mutual eclipses of Jupiter's moons begin? $\endgroup$
    – uhoh
    Commented May 6, 2020 at 3:25
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    $\begingroup$ Google "shadow of moon on jupiter" and you'll find a number of images. $\endgroup$
    – Barmar
    Commented May 6, 2020 at 18:55

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Yes, you can see it (I have seen Io's shadow on Jupiter and we were happy it was visitor night so that we could share the view with guests)

A 50cm mirror and 125x magnification allows you to see it when the air is not too disturbed and when you know where and when to look. Likely a somewhat smaller telescope will do, too, as light sensitivity is not too crucial - more the resolution and magnification so that you can still see the tiny black dot of the shadow. The shadow has about the same diameter as the width of some major clouds bands on Jupiter.

Stellarium is a great tool to find the right time to look at Jupiter to find these transits.

Find a local amateur observatory and they sure will be happy to share this view with you (possibly not before late northern summer due to the rise time of jupiter in 2020)

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Yes, the shadows of the Galilean moons are easily seen in small telescopes. The technical name is "shadow transit".
I saw one last summer (2019) in a 3" (76mm) refractor. My notes from the time say that it was much easier to see than the great red spot. I recall that I was not even looking out for this event, but it was unmissable.

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    $\begingroup$ And indeed, a quick image search for "shadow transit jupiter" brings up a lot of nice pics, including this triple-shadow sequence from the Griffith Observatory: newsledge.com/… $\endgroup$
    – CCTO
    Commented May 5, 2020 at 18:22
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    $\begingroup$ @CCTO - great image, and in a small telescope the black shadows stand out much more than the belts of Jupiter. $\endgroup$
    – Dr Chuck
    Commented May 5, 2020 at 21:19
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    $\begingroup$ Worth making, I think, the obvious point that, to an inexperienced observer, using a 3-inch telescope, it can be quite difficult to be sure that what you are seeing is the shadow of a moon falling on the cloud layer, as distinct from seeing the moon itself transiting across the disc (i.e. passing between the planet and the observer). Easy to see, in a 200-inch telescope, which is a moon and which is a shadow. But in my 3-inch refractor, quite tricky to tell a shadow from an actual transit. $\endgroup$
    – Ed999
    Commented May 6, 2020 at 13:33
  • $\begingroup$ @Ed999 - that may be true if you were to chance upon a dark circle on Jupiter, but if you plan what you are looking for, then you will know in advance which it is. $\endgroup$
    – Dr Chuck
    Commented May 6, 2020 at 14:05
  • $\begingroup$ One practical difficulty is that if you do have access to a slightly larger instrument, the greater the magnification the smaller the field of view: in magnifying the image of the planet, to observe the cloud banding, the only moons you are likely to observe would be those in transit across the disc, because any which are not transiting will be outside the field of view. Worth making the point that, without a very large reflector, only the 4 major satellites can be seen: if you count them up, and spot 5 or more, that is a practical indicator that one of the objects is a moon's shadow. $\endgroup$
    – Ed999
    Commented May 6, 2020 at 14:42

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