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When I was young (I won't tell you when) I saw one of the four Galilean moons of Jupiter suddenly disappear while watching through a small refractor. It wasn't a coincidence, I'd seen the prediction in some announcements in Sky and Telescope and it happened like clockwork. The orbital plane of the Galilean moons had rotated so that it included the Sun, and the moons were Eclipsing each other, or so I remember.

Will this happen again? If so, when will they start, and where can I find predictions of individual eclipses?

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    $\begingroup$ This is a really interesting question. Perhaps, not of much interest to professional astronomers, but an answer might encourage me to get my telescope out again. $\endgroup$
    – Mick
    Nov 11, 2017 at 5:18
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    $\begingroup$ Thanks for the reminder. I'd better get busy and finish my 8" scope, otherwise I'll have to use my 4" (although it shows Jupiter nicely enough). I'l be 70 when it happens. Gah! $\endgroup$
    – Mick
    Feb 20, 2019 at 0:50
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    $\begingroup$ @Mick how's the 8" scope? $\endgroup$
    – uhoh
    Oct 5, 2020 at 11:15
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    $\begingroup$ Oh, dear! The 4" scope is up the attic, gathering dust. I haven't used it for years. The 8" mirror is still half-finished. I doubt if I will ever complete it. Maybe I'll get the 4" down and take a look at Mars while it's nicely placed. I did have a decent squint at NEOWISE with my binocs, but the weather closed in, so I didn't bother with the scope. Thanks for asking. $\endgroup$
    – Mick
    Oct 5, 2020 at 11:25
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    $\begingroup$ @Mick lucky for you on NEOWISE, combination of cloudy evenings and schedule prevented me from getting somewhere where I'd have a reasonably dark sky. We'll there's always going to be another comet. $\endgroup$
    – uhoh
    Oct 5, 2020 at 12:26

2 Answers 2

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Mutual eclipses and occulations of Jupiter's moons occur during Jovian Equinox, twice in each orbit of Jupiter, and so roughly every 5.9 years.

The last two series of events were in 2015 and 2021 (there was a series of events between January and August http://lnfm1.sai.msu.ru/neb/nss/nsszph517he.htm), so the next will start sometime around late 2026 or 2027.

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    $\begingroup$ Thanks! That will make a nice 70th birthday treat for me. :-) $\endgroup$
    – Mick
    Nov 11, 2017 at 8:06
  • $\begingroup$ Excellent! Using the geocentric default (500) it looks like after August 30, there's one more straggler on November 16. Also thanks for mentioning that there will also be mutual occultations - the two moons would be unresolved in a small telescope, but the light contribution from the one behind would suddenly be removed from the line-of-sight. $\endgroup$
    – uhoh
    Nov 11, 2017 at 8:32
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    $\begingroup$ Oh, and happy birthday in advance to @Mick! $\endgroup$
    – uhoh
    Nov 18, 2017 at 12:44
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@JamesK nailed it, I'll just add a little more.

Starting from the link in that answer, I've found the figures below on this page. It looks like this cycle will be difficult to watch in the first months of 2021, until Jupiter moves far enough away from the Sun as seen from Earth. Also, the southern declination will make it more difficult to watch from the highest northern latitudes.

Parameters of the Earth-Sun-Jupiter configuration during the mutual occultations and eclipses of the Galilean satellites in 2021.

The number of events per week (Monday to Sunday). Note: Only 30 % of the events can be seen on one observatory. Note: Events before 3 March 2021 are not observable (Twilight).

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Planetocentric planetoequatorial latitudes (deg) of the Earth and the Sun.

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Geocentric angular distance (deg) between Jupiter and the Sun.

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Declination of Jupiter (deg).

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The Sun-Earth phase angle on Jupiter (deg).

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