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I was doing my own "space exploration" last night with a telescope. Being a space noob I can't visually determine stars or planets (I know the moon, though), but I focused on one particularly bright one when I saw 4 smaller "stars" on each side. After a quick Google I learned I had discovered the moons of Jupiter!

Galilean Moons

I now know the moons I could see were Io, Europa, Ganymede, and Callisto. But which is which?

Is there an easy way to determine which Galilean moon is which?

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  • $\begingroup$ I don't understand why no one has said to use the Google Sky phone app $\endgroup$ – Zee May 9 '18 at 4:29
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Use an orrery that will let you specify a specific epoch and vantage point. There are some quite fancy ones online, for example this Solar System Scope:

enter image description here

What you do in this particular tool is click on the calendar bar below and enter date and time of your observation (if you forgot that, there's a good chance your photograph has a time stamp, either of the file or in its EXIF data). You can adjust epoch later on, too. Then double-click on Jupiter, then its Orbit button, then click on link As seen from Earth below, then click Go back button and use mouse wheel to zoom out so all four Galilean moons display. Then compare with the photograph that you took. Note that it might take a bit of rotation for you image to align perfectly. I use the old neck twisting and turning technique, but photo editing tools or even rotatable screens will do just as well, if not better.

Note that this mentioned tool does offer settings to switch between orrery and realistic model and large and realistic sizes of displayed celestial bodies. But there are many such tools online or downloadable for both computers and smartphones. I think even this one is available as an app for smartphones for free, but last time I tried to install it, it didn't work for me. Maybe they fixed that. Do check because it's a lot easier if you can check orbital alignment of Galilean moons as seen from Earth directly on the field, as you're doing your observations.

Have clear skies and I wish you happy hunting for planets and moons of our Solar system and beyond!

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Well, if "last night" means around 2015-04-22 at 2230 GMT, the order from left to right is: Europa, Io, Ganymede, Callisto.

Here is a stellarium snapshot of (roughly) this configuration. The snapshot doesn't label the moons, but I clicked on each individually to confirm the order above.

enter image description here

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One option is to sketch (or photograph) their approximate positions at the start of the night and again 4-5 hours later, if this is possible. Then repeat the next night. It should be easy to at least identify Io and Europa that way (they're the fastest moving).

(That's a pretty good picture for a beginner by the way!)

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    $\begingroup$ Excellent answer, this should be more highly voted $\endgroup$ – Jonathan Landrum May 9 '18 at 15:11
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The easiest way is to look it up, e.g. here: http://www.skyandtelescope.com/observing/a-jupiter-almanac/
Because the moons' positions change, and they are too small to distinguish in a small telescope, there are no general rules.

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The one far away is Ganymede, the brightest is Europa but not much of a way for the others (from any angle) I did find a diagram that lines up pretty well with your picture though.

enter image description here

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In-the-Sky.org has a dedicated calendar for the Galilean moons. I can't seem to reconcile your picture with the plot for the last day or two, though!

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As mentioned previously, to be certain you need to record the day and time of the picture and compare to a program or table of moon positions.

Short of doing that, the far right & left ones appear to be the smaller moons (IO & Europa) while the middle two appear to be Callisto & Ganymede (I base this upon apparent size of the moons).

If you forced me to guess, I would say they were (from left to right) Io, Callisto, Ganymede, & Europa.

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You can use Sky and Telescope's interactive tool for observing Jupiter's moons. This website describes how to use it to determine the positions of the Galilean moons.

It works in the past or future.

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