enter image description here

Sorry for the huge picture), anyway, as you can see, there are 2 specks of light on either side of the upper planet which is likely Jupiter as the lower one is clearly Saturn-shaped.

I was going through some old photos when I came across the picture I took of the great conjunction that happened last year(I can't post the picture here as it's too large, so I cropped out just Jupiter)

So I was wondering, could these be two of the Galilean moons(If so, I think these 2 should be Io and Ganymede) or is it just light being reflected?

I am asking this because I thought all of the Galilean moons were of equal brightness, so it seemed unlikely to me that only 2 would be visible.

Someone asked for specific information, and here they are:

Date and time: 25th December, UTC 14:51

Here's the link for the full image: https://i.stack.imgur.com/TAPIk.jpg

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    $\begingroup$ It would help to know the angular scale of the picture, and whether the resolution is what it appears to be. Also the orientation (e.g. relative to the plane of the ecliptic.) I think it would be unusual, but not impossible, to find two moons nearly on the line of sight to Jupiter at the same time. $\endgroup$
    – Andrew Steane
    Jun 29, 2021 at 8:37
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    $\begingroup$ There's a significant amount of missing information here which you do have at your disposal (specifically, the precise time and date the photo was taken, and the full-frame full-resolution image (which is essential to understand the orientation of the frame)) but which you have not included in your post. The missing information makes it highly unlikely that this post is answerable in its current form. $\endgroup$ Jun 29, 2021 at 8:41
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    $\begingroup$ Welcome to astronomy SE! $\endgroup$
    – B--rian
    Jun 29, 2021 at 9:50
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    $\begingroup$ I think since this question is about this image, it's perfectly fine to have a larger than average post for it. By the way there is a wonderful question in meta: Post your pictures of the Great Conjunction here! and most contain specific time and date information. Have a look and you can consider adding your image there as well as a new answer! By the way, can you mention the time zone that your date and time correspond to, or convert to UTC? Thanks and Welcome to Stack Exchange! $\endgroup$
    – uhoh
    Jun 29, 2021 at 14:25
  • $\begingroup$ @EmilioPisanty There's an answer; it would certainly be nice to know if the times match though. $\endgroup$
    – uhoh
    Jun 30, 2021 at 12:41

2 Answers 2


It's likely.

The Galilean moons have a visual brightness around 6mag while Jupiter has around -2mag at that time. Given the link to the full image which probably shows also Saturn it looks reasonable that those faint dots are the Galilean moons in your image.

Your time misses time zone information. The Galilean moons have revolution times in the order of hours, which means you can actually watch them passing Jupiter in transit or vanishing behind it. Thus without accurate time information it's hard to say, but throught that date there are configurations where these dots can be interpreted as the Galilean moons in relative alignment to Jupiter and Saturn. E.g. like at 26th December 1:20h UTC might be a fitting time - and with a bit of good will one can even then identify all four of the Galilean moons.

OP's cropped original image from the question, rotated

26th December 1:20h UTC

My suggestion is: Grab Stellarium, configure it to your observation place and time zone, set the exact time and date and have a look whether the Galilean moons are aligned as are your fuzzy faint dots adjacent to Jupiter to confirm.

  • $\begingroup$ some additional images in meta. It looks like you were able to deduce the OPs time zone before they could add it, cool! $\endgroup$
    – uhoh
    Jun 29, 2021 at 14:27
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    $\begingroup$ Thanks for adding the rotated original image for reference :) $\endgroup$ Jun 29, 2021 at 15:17
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    $\begingroup$ Thanks for the answer!, and yeah I added the time zone now. I also opened Stellarium and the configuration does look similar with Callisto being the upper one and Ganymede being the lower. $\endgroup$
    – Avid_Cloud
    Jun 30, 2021 at 1:01

Whether the specks of light are actually the Galilean moons or not, you probably can see Ganymede and Callisto more easily than Io and Europa. The latter are closer to Jupiter and more likely to be lost in Jupiter's glare even if they are about equally bright. So the fact that you see two favors them being Ganymede and Callisto, but you should still check against star charts.


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