We all are of course looking forward to see the great conjunction of Jupiter and Saturn. So I looked up the timeline of close approach in Stellarium and I saw that there might be an occultation with Ganymede and HIP 99314 (JD = 2459204.532313, date = 2020-12-21, 00:46:31 UTC). The star has a magnitude of around 7.5 and Ganymede has a magnitude of around 5.47 (combined magnitude is 5.31 -> change in magnitude is 0.16), so it might be interesting to see the perceivable dim (maybe not with the naked eye, but maybe on camera). Unfortunately, I don't think that Stellarium is built with that precision, so I am asking your opinion about this situation.

There are 2 options:

  1. Stellarium is built with that precision and this event is even more rare than previously thought.
  2. Stellarium is wrong: there are other data that don't support its results.

Help is very appreciated, but let's hope for the best.

Thank you!

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    $\begingroup$ An occultation is usually visible from a narrow stripe of land on the Earth, so figuring out whether this specific event does or does not happen depends on your location. Where are you located? (City is enough; it’s not like I need your home address…) $\endgroup$ Commented Dec 9, 2020 at 0:23
  • 4
    $\begingroup$ This is a software package folks commonly used to verify occultations. lunar-occultations.com/iota/occult4.htm $\endgroup$
    – Connor Garcia
    Commented Dec 9, 2020 at 3:44

2 Answers 2


For what it's worth, I just got a shot of what looked like a 5th Galilean moon near Ganymede tonight (from South Florida). Here's a quick, unprocessed image. I thought I was going crazy seeing an extra moon (in the correct plane) that kept showing up. But looks like it's HIP 99314 (2nd dot to the upper left of Jupiter).

Jupiter & Saturn

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    $\begingroup$ Beautiful answer, welcome to Astronomy SE! $\endgroup$
    – uhoh
    Commented Dec 21, 2020 at 2:30

The answer is a strong maybe.

The International Occultation Timing Association (IOTA) produces predictions for such events. Their webpage Major Planet Occultation Predictions lists the occultation of HIP 99314 by Jupiter, but it does not list an occultation by its moon Ganymede.

     Date       U.T.    Durn  Star      Star       Planet
   y   m  d    h   m   sec/m   mag      No.       No. Name
2020 Dec 21    6 45.3  60.6m   7.4  HIP 99314   P5M00 Jupiter

However, a discussion on the IOTA list/forum indicates that someone in San Diego California will attempt to observe the occultation by Ganymede "in bright twilight". This makes it seem likely that the occultation by Ganymede will occur. I suspect it is not included on the IOTA webpage because the area of visibility occurs in daylight (or too soon after sunset) and was filtered out of the list.

  • $\begingroup$ I looked it up in Stellarium for the Northern Chile. At the moment of the occultation is astronomical twilight. Also, pair will be around 13° high. Do you think that this is enough? $\endgroup$
    – User123
    Commented Dec 18, 2020 at 16:10
  • $\begingroup$ If the sky is clear, 13° altitude and astronomical twilight should be enough to see it. The steadiness of the air will determine how sharp the image will be. But the magnitude drop will be too small to detect visually. At some point, the star and Ganymede will merge together. There will not be an instantaneous drop in brightness as in most occultations. $\endgroup$
    – JohnHoltz
    Commented Dec 18, 2020 at 22:36

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