On the evening of May 8, 2026, we turned Dyer Observatory’s Seyfert Telescope 24” mirror toward Jupiter to get a look at the gas giant and its moons. The timing wasn’t coincidental; predictions showed that two of the moons would simultaneously cast shadows on the planet for about 45 minutes. As seen in the featured animation above, the moons were right on schedule.
The Galilean Moons
As of late April 2026, Jupiter is home to 115 known natural moons, but only four are visible to backyard telescopes thanks to their large size. Galileo Galilei is credited with their discovery in January 1610, so they are often referred to as the Galilean moons. They are a favorite target of both amateur and professional astronomers as they change positions on an hourly to nightly basis. The remaining 111 moons are much smaller.
In order of distance from Jupiter, the Galilean moons are Io, Europa, Ganymede and Callisto.
Ganymede is the largest moon in the Solar System, measuring about 50 percent larger than Earth’s Moon and even slightly bigger than the planet Mercury.
Io is the most volcanically active body in the solar system because of the intense gravitational pull forces from Jupiter. Its close proximity to the planet creates powerful internal friction, generating enough heat to fuel constant volcanic activity. In contrast, Callisto is considered a geologically inactive world. It has very little internal heat to drive volcanism or plate tectonics, and its heavily cratered surface has remainedlargely unchanged for billions of years.
Finally, Europa, which is about 16 percent smaller than Earth’s Moon, is covered by a thick, icy crust that overlays a vast ocean. Although Earth is more than four times wider than Europa, scientists believe that this smallest of the Galilean moons may contain nearly twice the amount of water than all Earth’s oceans, lakes, rivers, groundwater and ice deposits combined!
Fun fact: the fifth largest moon, Amalthea, was discovered in 1892 by Nashvillian and director of Vanderbilt’s first observatory, Edward Emerson Barnard, while using the 36″ refractor at California’s Lick Observatory. It is only 250 km at its widest point (5 percent of Ganymede’s diameter), making it a miracle that Barnard was able to detect it among the glare of brilliant Jupiter.
Gravitational interactions have created an orbital resonance between Io, Europa and Ganymede. This means their orbital times are linked in a regular pattern by gravity. For every orbit of Ganymede, Europa completes two orbits and Io completes four. More distant Callisto has almost achieved an orbital resonance with the group. It nearly completes one orbit for every two orbits of Ganymede. Scientists estimate that in about 1.5 billion years Callisto will achieve resonance with the other moons, creating a 8:4:2:1 orbital resonance.
Are Jovian Shadows Rare?
Seeing the shadow of one of Jupiter’s moons pass across the planet’s cloud tops, called a transit, is fairly common. Io creates a shadow transit about every 1.8 days because that is how long it takes to orbit Jupiter. Europa does the same every 3.6 days, while Ganymede produces one every 7.2 days.
It is less common to see two moon shadows crossing Jupiter at the same time, and seeing three shadows together is much rarer.
The reason this is rare comes down to two factors. First, the orbital resonance of Io, Europa and Ganymede guarantees that when two of these moons line up to cast shadows on Jupiter at the same time, the third is positioned elsewhere in its orbit and cannot also cast its shadow on Jupiter at that moment. Therefore, if a third shadow is seen at the same time, it must come from more distant Callisto.
Second, Callisto’s greater distance from Jupiter than the other moons mean its placement between the Sun and Jupiter must be more precise for its shadow to cross the planet. Callisto’s orbit is above Jupiter’s equator, which means that the Sun also must be positioned above Jupiter’s equator for Callisto’s shadow to fall on Jupiter.
Just as with Earth, the Sun is above Jupiter’s equator during the planet’s equinox. The key difference is timing. Jupiter’s equinoxes come around about every six years, while Earth has an equinox about every six months, making triple-shadow transits possible for a length of time about every six years.
The last time this occurred was January 24, 2015. The next three occurrences will be:
- March 20, 2032 (visible just before sunrise in Nashville)
- December 20, 2032 (visible mostly in Eastern Asia)
- August 5, 2038 (too close to the Sun to observe)
Another Double-Shadow Transit is Coming
If you’d like to catch a double-shadow transit with your own telescope, Europa and Callisto will give observers in Nashville and surrounding areas a show on the evening of June 9, 2026.
By 7:45 p.m. CDT, it will be dark enough to spot Jupiter, which will be positioned right next to brighter Venus low in the western sky. Europa will already be casting a shadow near the center of the planet. Just before 8 p.m. CDT, Callisto’s fuzzier shadow will begin creeping in from the planet’s edge.
Observers will have about an hour for a decent view before Jupiter gets ready to set, so make sure you have a clear west-northwest horizon.
Don’t Miss Jupiter’s Encounters with our Own Moon
Looking ahead, Jupiter has other notable lunar encounters, but this time it will be with our own Moon. If you have ever wanted to try to see a planet through a telescope in the daytime, September 8, 2026, provides a great opportunity.
The waning crescent Moon will pass in front of Jupiter starting around 2:03 p.m. CDT, taking about a minute to completely cover it. Use the Moon as a guidepost to see if you can spot faint Jupiter against the blue sky. The gas giant will re-emerge from behind the dark edge of the Moon at 3 p.m. CDT for areas around Nashville.
Just before sunrise on October 8, 2026, the waning crescent Moon will pass in front of Jupiter and its moons once again. The Moon begins to cover Europa around 3:11 a.m. CDT. Two minutes later it will have covered Io and Callisto, and beginning at 3:16 a.m. CDT, the Moon will take about a full minute to pass in front of Jupiter. Finally, by 3:19 a.m. CDT, it will have covered Ganymede.
Seeing the Jovian system next to the cratered lunar surface makes for a great photo opportunity and a very pleasing view in a telescopeeyepiece. Some, however, might argue that the Jupiter re-emerging from behind the Moon will be even more spectacular.
Since the Moon is in a waning crescent phase, its sunlit side is very thin. Jupiter will first slip behind the bright edge of the Moon, and it will later reappear from behind the dark edge of the Moon. Europa will reappear around 4:11 a.m. CDT, and the globe of Jupiter will slowly be unveiled over the course of a minute beginning at 4:16 a.m. CDT.
If conditions are right, it will appear that Jupiter is spontaneously emerging from the desolate void of space. So set your alarm and make sure you reserve a spot with a good view of the eastern horizon.
If you are observing these occultations from a location significantly distant from the Nashville area (e.g., another state), the listed times may vary significantly, and/or the Moon may completely miss Jupiter. You can find timings for your location at In-The-Sky.org.