Jupiter presented a stunning gift last weekend, at least to those who had a clear view from Earth. On Aug. 15, Jupiter’s largest moons, known as the Galilean moons, aligned to give two stunning, rare astronomical events.

First, Earthlings could see three of Jupiter’s Galilean moons — Europa, Ganymede and Callisto — simultaneously crossing in front of the planet, known as a triple transit. Then, a dance among Europa, Ganymede and their shadows delighted sky watchers with displays known as “mutual events.”

Amateur astronomer Christopher Go captured the marvels from the Philippines around midnight in what might be the one of the best documentations to date. Each second of the above animation represents 30 minutes of the event.

“This is a very difficult data to capture, and I am convinced that this is the best movie ever made of Jupiter’s triple transit event,” Kunio Sayanagi, a planetary scientist at Hampton University and an affiliate of the Imaging Science Team of NASA’s Cassini mission to Saturn, wrote in an email. Sayanagi helped assemble the above animation from Go’s frames.

Triple moons

Jupiter has four Galilean moons, Europa, Io, Callisto and Ganymede, which were discovered by Galileo Galilei in the 1600s. A transit occurs when one of these moons or its shadow passes in front of the planet as seen from Earth. 2021 has more than 600 transit events.

Single and double transits are fairly common and have several occurrences just this month. Three moons passing at the same time, however, is rarer. Before 2021, the last triple transit occurred in 2015; the next one will occur in 2032.

Sayanagi explains that triple transits are rare because only two of the three innermost Galilean moons (Io, Europa and Ganymede) ever line up due to their orbital periods.

Therefore, a triple transit only occurs when two of these moons happen to line up with the fourth moon, Callisto. Callisto, the farthest Galilean moon from Jupiter, is not in “orbital resonance” with the rest of the moons though.

“In the triple transit on Aug. 15, Europa and Ganymede were having the usual regularly-occurring alignment, and Callisto just happened to pass by them during that time,” wrote Sayanagi.

Due to the differences in orbital resonance, all four moons will never cross in front of Jupiter simultaneously.

During the transit, the moons can be difficult to distinguish from the light colors on Jupiter. The moons’ shadows, though, clearly darken the planet.

In the image above, Europa appears yellow-white and casts a shadow on Jupiter. Europa, which is slightly smaller than Earth’s moon, is the smallest of the Galilean moons and thus casts a relatively small shadow.

Ganymede and its shadow are the largest transits in the image. Not only is Ganymede the biggest satellite around Jupiter, it is the largest satellite in our solar system. Ganymede is only slightly smaller than Mars.

Callisto, which appears brown, has one of the most cratered surfaces in our solar system.

The triple transit was not visible from North America, but Go could see it from Cebu Island in the Philippines. However, good weather conditions were not a given. August falls within monsoon season in the Philippines and can bring winds from the mountains that interfere with astrophotography. Go said the winds from the mountain make everything look like mush and the planet “will dance around like a jellyfish.”

The winds shifted the night of the event though, which helped produce good photos. Rain also fell every day that week but miraculously cleared up the night before the event.

“I was so lucky. The sky was clear all throughout the event,” said Go, who shot the event from 11 p.m. to 2 a.m. “I was just so shocked because the following day it was raining again.”

Mutual moons

Shortly after the triple transit ended, two of the moons put on another show. Europa passed under Ganymede, appearing to hide behind the large moon from our perspective on Earth (known as an occultation). As Europa reappeared, Ganymede cast its shadow on Europa (known as an eclipse). Astronomers refer to these occultations and eclipses as “mutual events.”

The image on the left shows Europa hiding behind Ganymede, while the image on the right shows the eclipse. In the right image, Europa is partially darkened by Ganymede’s shadow. On Jupiter, Europa and Ganymede’s shadows overlap, hence why the large dark outline on the planet appears as a deformed circle.

“This thing this is so surreal,” said Go. “Ganymede is projecting a shadow toward Jupiter, but part of the shadow hit Europa.”

Mutual events are scientifically important because they allow researchers to easily record the moons’ positions, said Sayanagi. Researchers have been able to record the precise positions of Jupiter’s moons as far back as 1881.

Today’s instruments do not rely on mutual events for those position measurements. Sayanagi said NASA and the European Space Agency’s upcoming Europa Clipper and JUICE missions will use more sophisticated techniques to measure the moons’ orbital evolution. However, the older measurements from the 19th and 20th centuries provide valuable reference points.

Amateur photographs and data like Go’s are extremely valuable to planetary scientists and shared with the science community. Although he is not a professional astronomer (he works in furniture manufacturing full-time), he has made discoveries through his images and has been listed as an author on numerous studies, along with professional astronomers. He has also helped NASA process images of Jupiter from the Hubble Space Telescope.

“They’re actually a lot of us doing very good images of the planets all over the world from Australia to Japan, Thailand, Indonesia, Malaysia, Mexico, Brazil, Chile, Africa,” said Go, who describes himself as a citizen scientist. We need to get people to understand science, and this is one of the ways.”