Universe Today
Catching the dance of Jupiter’s major moons though late 2026.
It’s that time of the decade again. Late 2026 going into early 2027 is a special time for the major Jovian moons, as they trade places passing one in front of another as seen from our Earthly point of view. We call this mutual transit-eclipse season, a favorable time to catch the complex interplay of Jupiter’s major moons.
Mutual transits and eclipses of the Jovian moons occur in bidecadal seasons. This is centered on once every five years, about half the time it takes Jupiter to orbit the Sun. Like eclipses on Earth, this would be a continual state of affairs were the orbits of the major moons not tipped slightly versus Jupiter’s path around the Sun. The tilts of the moons from innermost Io, through Europa, Ganymede and outermost Callisto are all less than a degree versus the Jupiter’s equatorial plane, which is in turn tilted just over three degrees versus the planet’s orbit around the Sun. Jupiter’s orbit overall is tipped just over a degree versus the ecliptic plane, the path traced out by the Earth's course around the Sun.
We wrote about the 2015-2016 season here and the 2020-2021 season here.
Fun fact: Callisto is the only major Galilean moon that can ‘miss’ Jupiter on occasion. This moon resumed casting a shadow on Jove on May 20th, 2025 heralding the approach of mutual transit-eclipse season, and will do so until August 5th, 2028. The middle of this formal eclipse season occurs next month around mid-October 2026.
Though the current season is already well underway, it also commenced while Jupiter passed solar conjunction on July 29th. Jupiter is now prominent in the pre-dawn sky, and events involving the four moons are becoming more frequent.
Io casts a shadow on Ganymede in 2009. Credit: Christopher Go.
Note that Jupiter casts its shadow off to one side as it approaches quadrature 90 degrees west of the Sun on November 18th, and narrows back to nearly straight behind the planet nearing opposition on February 11th, 2027. The shadows of the moons also follow the same ‘wide to the side,’ then swinging back straight behind vanishing act.
The geometry of eclipses and occultations for Jupiter's moons. Credit: Dave Dickinson.
The types of events you’ll witness can get wonderfully complex. One moon can pass in front of another in an occultation, while the shadow of a moon can cross the disk of another in an eclipse. Like Earthly eclipses, these each come in three flavors: total (completely obscured), annular (ring-like) and partial (only a portion is obscured).
Types of mutual Jovian moon events. Credit: Dave Dickinson
A seasoned observer can actually tell which moon is currently casting a shadow based on its appearance alone, from the large, ragged diffuse shadow of Callisto, to the inky black period dot of Io.
Io casts a shadow on Jove. Credit: Ralph Smyth.
What’s more, the innermost three large moons Ganymede, Europa and Io are in a 1-2-4 resonance with one another. This means, for example, that Io makes two orbits around Jupiter, for Europa’s one.
This also means that on rare occasions, a triple shadow transit can occur. The last such instance was on January 24th, 2015, and the next one occurs on March 20th, 2032.
The 2015 triple shadow transit. Credit: Efrain Morales.
Resources for predicting phenomena for Jupiter’s moons include ICMME’s site, and SETI PDS rings node. Planetarium programs like Stellarium also do a serviceable job at predicting and simulating upcoming phenomena for Jupiter’s moons.
Observing and imaging these events can be challenging, owing to the tiny size of the moons themselves. The largest moon Ganymede presents a tiny 1.3” disk. Still, even a small telescope will reveal a moon in total eclipse behind the shadow of another, as it slowly fades from view and returns. Occultations will look more like mergers as the two moons seem to merge and then separate again, going on their separate ways.
Of course, what you’re seeing when the shadow of one moon crosses another is a solar eclipse. Standing on the surface of the moon in the shadow of another (be sure to pack a spacesuit and take proper radiation protection precautions) you’d see a total solar eclipse offering a similar close fit to what we see here on Earth:
It’s strange to think: I’ve been covering these mutual eclipse events for over one Jovian orbit now. Be sure to follow the Jovian action this mutual eclipse season, as a testament to the complex dance of Jupiter and its retinue of moons.
