Jupiter was the star on Friday night as astronomers and enthusiasts turned to the skies. A rare celestial event unfolded involving the gas giant's three out of four largest moons.
Callisto started off the event, with its shadow appearing at around 7:11 p.m. PST. At about 8:35 p.m. PST, Io's shadow joined in, followed by Europa's shadow at around 10:27 p.m. PST. Once Io's shadow moved away at 10:52 p.m. PST, the event concluded. It wasn't until midnight that Callisto's shadow disappeared from Jupiter's disk, while Europa's shadow lingered until 1:22 a.m. PST.
During the event, another phenomenon occurred, with Io being eclipsed by Callisto. Io started crossing the front of Jupiter's disk around 19 minutes after its shadow first appeared. It ran into Callisto's shadow, darkening Io and making it appear as a very visible black dot against the gas giant's bright backdrop. Io's shadow merged with Callisto's before Io became visible again. The eclipse ended with Io appearing as a bright speck in front of Callisto's shadow.
The last time three moon shadows appeared simultaneously on Jupiter was in 2013 but the next one won't be happening until 2032. The Griffith Observatory in Los Angeles held a viewing activity for the event, releasing as well a video beforehand of what the gas giant's moons' overlapping shadows would look like. The observatory also provided a live video stream of the shadows in transit and has said it will be uploading a time-lapse video of the shadows meeting in the days following the event.
The moons, as well as Ganymede, were discovered by Galileo Galilei in 1610. Thus, collectively, Jupiter's largest moons are called Galilean moons. Ganymede was also visible during the event but it did not cast a shadow on Jupiter.
Shadow transits, as the event is called, involves Jupiter's moons passing in front of the gas giant to project their own shadows on the planet. It's not unusual to observe one shadow transit a week but it's already considered a treat to witness two at the same time. That's why having three simultaneous shadow transits is a big deal. With the event concluded, astronomers and enthusiasts in the U.S. will have to wait until December 30, 2032 for the next one.
Below is the animation of the shadow transit:
Those who can spare around four hours can also replay the coverage of Griffith Observatory: