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If I were on a Galilean moon of Jupiter, like Europa or Ganymede, what would the planet look like? (Let's say I'm on a space station that provides atmosphere to make it slightly less unrealistic.)

Would it look like our moon, only a bit bigger and colored like Jupiter, with the stripes and the "eye", or would the moons be close enough for Jupiter to look gigantic and fill a significant portion of the sky?

The two moons that are closest to each other are Io and Europa. They seem to be about 250,000 kilometers away from each other. Would it be possible to see one from the other with the naked eye if they were aligned? For example, if Europa were in a position in its axis where it is lit by the sun, could I see it, looking at it from Io? Or would I need a telescope? A 3000 km sphere from 250,000 km away does not seem possible. The other way around, of course, Io is a bit bigger.

But the part I'm the most interested in is: what would Jupiter look like from one of these moons.

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You can simply startup Stellarium and have a look yourself. Choose any locations you are interested in. The Galilean satellites of Jupiter are inside the default list of locations.

The attached image shows Jupiter as viewed from Io at the given time, Europa is the bright object to the right; with -9 mag it is considerably brighter than Venus when viewed from Earth, but slightly less so than Earth's Moon as seen by us.

Interesting to note is also the huge extend of Jupiter in the field of view, if you choose the right hemisphere (Io is in a tidally-locked state). With roughly 15° you need about your entire hand at arm's length to block it while the sun is a lot smaller (~30x larger than Sun or Moon when viewed from Earth). Also the daily eclipses of the Sun by Jupiter might be noteworthy.

Jupiter viewed from Io on 23 June 2020 with the observer placed roughly half-way on the Northern Hemisphere

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  • $\begingroup$ I had no idea that software existed, but in retrospect, of course someone would have made something like that. Thanks! (I would have imagined Jupiter to look much bigger, by the way, while expecting it not to be THAT big because, after all, things are far away.) $\endgroup$
    – eje211
    Commented May 23, 2020 at 12:22
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    $\begingroup$ It's hard to capture in an image, but 15 degrees of arc in the sky is huge. The full moon looks big in the night sky and that's just 1/2 a degree of arc. Imagine 30 full moon across, 900 full moons in area. Europa, from Io - without checking, would probably be larger than full-moon size too, but not nearly as bright because the sunshine is so much less that far from the sun. The Jupiter eclipses would, as the answer says, be the most impressive. The sun behind the gas giant, visible refraction around the edges, the rings made visible, even if just a line due to being in the same plane. $\endgroup$
    – userLTK
    Commented May 23, 2020 at 18:00
  • $\begingroup$ I always point to an additional beautiful free package, Celestia tinyurl.com/ycab883c $\endgroup$
    – Alchimista
    Commented May 27, 2020 at 13:31
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Some time back, I made a jupyter notebook (in French, but the names of the satellites are similar enough) that compares the angular diameter of Jupiter as seen from its moons to the angular diameter of our own Moon seen from Earth. This way, you can get an idea of how big Jupiter would look in comparison to a more familiar sight.

For instance, if the Moon (seen from the surface of the Earth) is the blue dot, then Jupiter seen from the surface of Callisto is the big red dot.

enter image description here

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    $\begingroup$ (🇬🇧 below) Je n'avais pas vu cette réponse avant aujourd'hui. Ça me donne une idée de ce à quoi ça ressemble. On pourrait prendre une photo de la lune et multiplier le diamètre par huit et demie pour avoir une idée de la taille. Ça n'aiderait pas pour la couleur, bien sûr. Je vais regarder votre code. /// I hadn't seen this answer until today. It gives an idea of what it would look like.Taking a picture with the moon and multiplying the diameter by 8 might simulate the size if not the colors. I'll look at your code. $\endgroup$
    – eje211
    Commented Feb 18, 2023 at 2:50
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Pretty dang spectacular. NASA generated simulation of Appearance of Jupiter from Europa. enter image description here

Source

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  • $\begingroup$ This is EXACTLY what I needed! Thanks! Also, it must be really cool to see in person. Apart from the cold, the lack of air, the lack of good restaurants nearby, and all of that stuff. But still pretty cool. Since NASA generated the simulation, some people there must wonder about it too. $\endgroup$
    – eje211
    Commented Feb 18, 2023 at 2:45
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Science fiction writers and scientists have imagined the possibility of exomoons orbiting giant exoplanets in other star systems being habitable.

And there have been scientific articles discussing the possible limits of exomoon habitabilty.

In my answer to this question What do I bear in mind, creating an earth-like world with TWO moons? the last part of my answer links to this scientific article discussing exomoon habitabilty:

https://iopscience.iop.org/article/10.1088/2041-8205/776/2/L338

The author of that article calculates the inner and outer limits, in terms of radii of their planets, for the orbits of habitable exomoons.

And from that range of orbital distances I roughly calculated that a habitable exomoon would orbit at distances such that the giant exoplanet that it orbits would appear to be between 5.7295 to 22.9183 degrees wide, about 11 to 45 times the angular diameter of the Moon.

I may also point out that among those moonsof a giant plaent which are large and massive enough to be gravitationally rounded, the smallest moon with a rounded form would appear on the borderline between a dot of light and a disc with a visible diameter when it was about 5,729,582.7 kilometers, or 3,560,197.6 miles from another moon. So whenever a smallest possible rounded moon was closer than about 5,729,582.7 kilometers, or 3,560,197.6 miles, it would definately appear as a disc. And if a moon was larger than the smallest possible rounded moon, it would appear as disc in the sky of another moon even if the distance between them was more than about 5,729,582.7 kilometers, or 3,560,197.6 miles.

You ask:

The two moons that are closest to each other are Io and Europa. They seem to be about 250,000 kilometers away from each other. Would it be possible to see one from the other with the naked eye if they were aligned?

because Io and Europa orbit Jupiter in different orbits, the distance between them is not constant. Instead it is always changing.

The orbit of Io has a semi-major axis or radius of 421,700 kilometers, Europa's orbit has a radius of 671,034 kilometers, Ganymede's 1,070,412 kilometers, and Callisto's 1,882,709 kilometers.

So the distance between Io and Europa varies from about 249,000 to 1,092,734 kilometers. When they are farthest apart they are on opposite sides of Jupiter and hidden by it as well.

Europa, with a diameter of 3,121.6 kilometers, is the smallest of the Galilean moons. At its farthest from Callisto it is about 2,553,743 kilometers from Callisto, and according to my rough calculations should appear to be about 4.2 arc minutes wide from Callisto, about an eighth of the diameter of the Moon as seen from Earth. Of course Europa would be on the far side of Jupiter and hidden from Callisto anyway.

Io has a diameter of about 3,650 kilometers, and can be as close as about 249,000 kilometers to Europa. According to my rough calculations Io can appear as wide as 50 minutes wide in the sky of Europa, about one and two thirds the apparent width of the Moon as seen from Earth. Ganymede should appear approximately that wide from Europa when they are closest.

So all of the Galilean moons should appear as discs in the skies of the other Galilean moons whenever they are visible from them.

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  • $\begingroup$ Sorry for the late reply. That's AMAZING! Thanks! (And, yes, as you guessed, it is for as story where people are seeing things from the point of view of the orbit of Io. I was thinking: would they ever see another moon? If I get it wrong, even if the rest of the story is impossible, it will look very silly.) Like I told @LazyReader , even though it's sort of silly to think that way, it creates a desire to see it in person. $\endgroup$
    – eje211
    Commented Feb 18, 2023 at 2:55

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