<p>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).</p>

<p>(That's a pretty good picture for a beginner by the way!)</p>

CC BY-SA 4.0
2018-05-09T15:11:40.543Z
Excellent answer, this should be more highly voted
Jonathan E. Landrum
CC BY-SA 3.0
2015-04-28T07:38:25.770Z
2015-04-28T07:38:25.770Z
Andy
<p>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.</p>

<p><a href="https://i.sstatic.net/8qbbq.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/8qbbq.jpg" alt="enter image description here"></a></p>

CC BY-SA 3.0
2018-03-06T21:35:39.143Z
2018-03-06T21:35:39.143Z
Homework Researcher who helps
<p>The easiest way is to look it up, e.g. here: <a href="http://www.skyandtelescope.com/observing/a-jupiter-almanac/" rel="nofollow">http://www.skyandtelescope.com/observing/a-jupiter-almanac/</a><br>
Because the moons' positions change, and they are too small to distinguish in a small telescope, there are no general rules. </p>

CC BY-SA 3.0
2015-04-28T06:57:56.780Z
2015-04-28T06:57:56.780Z
Hobbes
<p>In-the-Sky.org has a <a href="https://in-the-sky.org/jupiter.php" rel="nofollow">dedicated calendar for the Galilean moons</a>. I can't seem to reconcile your picture with the plot for the last day or two, though!</p>

CC BY-SA 3.0
2015-04-28T16:47:50.640Z
2015-04-28T16:47:50.640Z
Russell Borogove
<p>You can use <em>Sky and Telescope's interactive tool for observing Jupiter's moons</em>. <a href="https://www.skyandtelescope.com/observing/celestial-objects-to-watch/jupiters-moons-javascript-utility/#" rel="nofollow noreferrer">This website</a> describes how to use it to determine the positions of the Galilean moons.</p>

<p>It works in the past or future.</p>

CC BY-SA 4.0
2019-06-13T08:48:17.497Z
2019-06-13T09:33:29.907Z
2019-06-13T09:33:29.907Z
Nagarjuna Borra
<p>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.</p>

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

<p>If you forced me to <strong>guess</strong>, I would say they were (from left to right) Io, Callisto, Ganymede, &amp; Europa.</p>

CC BY-SA 3.0
2015-04-30T03:57:49.153Z
2015-04-30T03:57:49.153Z
Jim2B
<p>Use an orrery that will let you specify a specific epoch and vantage point. There are some quite fancy ones online, for example this <a href="http://www.solarsystemscope.com/" rel="noreferrer">Solar System Scope</a>:</p>

<p><img src="https://i.sstatic.net/r2B45.png" alt="enter image description here"></p>

<p>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 <a href="https://en.wikipedia.org/wiki/Exchangeable_image_file_format" rel="noreferrer">EXIF</a> data). You can adjust epoch later on, too. Then double-click on Jupiter, then its <em>Orbit</em> button, then click on link <em>As seen from Earth</em> below, then click <em>Go back</em> 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 <em>neck twisting and turning technique</em>, but photo editing tools or even rotatable screens will do just as well, if not better.</p>

<p>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.</p>

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

CC BY-SA 3.0
2015-04-28T08:19:06.337Z
2015-04-28T08:19:06.337Z
TildalWave
<p>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!</p>

<p><img src="https://i.sstatic.net/PbMtw.jpg" alt="Galilean Moons"></p>

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

<p>Is there an easy way to determine which Galilean moon is which?</p>

CC BY-SA 4.0
2018-05-09T04:29:04.920Z
I don't understand why no one has said to use the Google Sky phone app
Zee
<jupiter><telescope><europa>
Determine the moons of Jupiter through a telescope
Coomie
# Determine the moons of Jupiter through a telescope

(Asked by: username_0 on 2015-04-28)

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](https://i.sstatic.net/PbMtw.jpg)

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?

**Tags:** jupiter, telescope, europa

> **Comments:**
> **username_1**: I don't understand why no one has said to use the Google Sky phone app  

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## 7 answers

### Answer by username_9 on 2015-04-28. Score: 8 | Accepted answer

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](http://www.solarsystemscope.com/):

![enter image description here](https://i.sstatic.net/r2B45.png)

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](https://en.wikipedia.org/wiki/Exchangeable_image_file_format) 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!


---

### Answer by username_2 on 2015-04-28. Score: 9

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!)

> **Comments:**
> **username_3**: Excellent answer, this should be more highly voted  

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### Answer by username_4 on 2018-03-06. Score: 2

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](https://i.sstatic.net/8qbbq.jpg)](https://i.sstatic.net/8qbbq.jpg)


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### Answer by username_5 on 2015-04-28. Score: 2

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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### Answer by username_6 on 2015-04-28. Score: 1

In-the-Sky.org has a [dedicated calendar for the Galilean moons](https://in-the-sky.org/jupiter.php). I can't seem to reconcile your picture with the plot for the last day or two, though!


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### Answer by username_7 on 2019-06-13. Score: 1

You can use *Sky and Telescope's interactive tool for observing Jupiter's moons*. [This website](https://www.skyandtelescope.com/observing/celestial-objects-to-watch/jupiters-moons-javascript-utility/#) describes how to use it to determine the positions of the Galilean moons.

It works in the past or future.


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### Answer by username_8 on 2015-04-30. Score: 1

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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