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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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  • $\begingroup$ Related: Eclipses of Jupiter's Moon during Retrograde motion. $\endgroup$ – uhoh Nov 11 '17 at 3:40
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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 '17 at 5:18
  • $\begingroup$ @Mick only two more years to go now! ;-) $\endgroup$ – uhoh Feb 20 at 0:09
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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 at 0:50
  • $\begingroup$ @Mick if you are anything like I am, two years might be enough time to find something. I saw one mutual occultation decades ago, sometime in the 1980's. It's there, it's not there, then it's there again. As long as it's not too close to Jupiter at the time, I don't think the larger aperture helps particularly much. But don't let that be an excuse not to start looking! ;-) $\endgroup$ – uhoh Feb 20 at 1:04
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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 6 years.

The last series of events was in 2015, so the next will start in 2021. There will be a series of events between January and August.

The site http://lnfm1.sai.msu.ru/neb/nss/nsszph517he.htm can generate ephemerides for these events.

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    $\begingroup$ Thanks! That will make a nice 70th birthday treat for me. :-) $\endgroup$ – Mick Nov 11 '17 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 '17 at 8:32
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    $\begingroup$ Oh, and happy birthday in advance to @Mick! $\endgroup$ – uhoh Nov 18 '17 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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