Triple transits on Jupiter
The three inner satellites of Jupiter – Io, Europa and Ganymede – are locked in an orbital resonance as a result of the tidal effects of giant Jupiter. The resonance between the three moons is described as 1:2:4, which means that for every one orbit that Ganymede completes, Europa completes two orbits and Io swings around Jupiter four times.
Watch this unique video that shows the 1:2:4 resonance of the moons and assigns a different sound to each moon once they complete an orbit from the top. The orbits are gradually sped up so you can hear the beat more easily.
Because of the way that these three moons are tidally locked into specific orbits, they can never all be in front of (or behind) Jupiter at the same time. So any transit event that occurs with three of the Galilean moons must necessarily include the most distant Callisto. And Callisto is not always at the correct angle for us to see transits or eclipses every time it circles Jupiter. Every six years, lasting for about three years, the farthest Galilean moon is in position to allow us a chance to see a triple transit event. A triple transit event involves three of the Galilean moons or shadows transiting the face of Jupiter at the same time as seen from Earth.
August 15, 2021, triple transit
Before 2021, the last [triple transit event] was in 2015. The next will be August 15, 2021! And the next such event won’t be until 2025. A triple transit of only shadows on Jupiter is rarer, with the last in 2015 and the next in 2032.
The August 15 event will begin around 15:17 [UTC]. At this time, Callisto will be in front of Jupiter while both Ganymede and its shadow are also in front of Jupiter. The triple event begins when Europa’s shadow edges onto the planet’s surface, thus Callisto, Ganymede and Europa are all transiting the planet whether by their actual satellite or the satellite’s shadow.
At the beginning of the transit you can still spot Io off to one side of Jupiter (the side that Callisto is on) while Europa is off to the other side of the planet in front of the blackness of space. But it won’t stay there. By 15:20 UTC, Europa itself will join its shadow and the other two planets in front of Jupiter. Then, around 15:42 UTC, Io disappears behind Jupiter, leaving no specks of light from Jupiter’s moons to be seen around the planet. The triple transit event ends soon after, as Callisto edges off the planet and back in front of dark space by 15:54 UTC.
Now for the bad news. Because Jupiter is not always above the horizon, or for some it’s above the horizon in daylight, not everyone gets to see this rare event. Only observers in parts of Australia and Asia will get a glimpse. For example, in Kuala Lumpur, Malaysia, the event begins on August 15 at 11:17 p.m., while in Sydney, Australia, the event begins on August 16 at 1:17 a.m.
As a bonus to the observers on this night, if you keep observing after the triple transit event is over, you’ll get to see the quicker Europa pass under Ganymede, so that it will appear that the two moons temporarily merge, along with their shadows!