The spacecraft Juno made history Monday night. But before that, it captured footage of a phenomenon that has, until now, only existed in the human imagination: the serene trajectory of moons circling a planet.
Juno’s video, taken as the probe approached Jupiter, marks “the first time humanity’s been able to see one celestial object go around another,” Scott Bolton, Juno’s principal investigator, told me. The objects, in this case, are the Galilean moons of Jupiter: Io, Europa, Ganymede, and Callisto.
If it’s not immediately clear why this is an awe-inspiring sight, some context might help. On January 7, 1610, Galileo noticed what he thought might be a cluster of three bright stars near Jupiter. The odd formation of the stars piqued his curiosity—they were like three pinpricks of light on a line running through the planet. So he looked for them again the next night. And the night after that. Here’s how the Galileo Project, at Rice University, explains what happened:
Galileo’s expectation was that Jupiter, which was then in its retrograde loop, would have moved from east to west and had left the three little stars behind. Instead, he saw all three stars to the west of Jupiter. It appeared as though Jupiter had not moved to the west but rather to the east. This was an anomaly, and Galileo returned to this formation again and again. Over the next week he found out several things. First, the little stars never left Jupiter; they appeared to be carried along with the planet. Second, as they were carried along, they changed their position with respect to each other and Jupiter. Third, there were not three but four of these little stars. By the 15th of January he had figured it out: these were not fixed stars but rather planetary bodies that revolved around Jupiter. Jupiter had four moons.
Today, scientists know that Jupiter actually has hundreds of moons. But Galileo’s discovery was far more profound that the existence of the moons themselves. If enormous moons were dancing around a distant planet, Earth wasn’t the only celestial body with something revolving around it. And, culturally, Earth wasn’t really the center of the universe—at least not in the way it once seemed to be.
“And now, 500 years later almost, we’re able to see the motion ourselves,” Scott Bolton, Juno’s principal investigator, said in a live broadcast from the Jet Propulsion Laboratory in Pasadena, California, Monday night. It’s a rare view of “natural harmony,” that underpins countless mechanisms that make our universe what it is, he said. “The harmony is there in nature at every scale, even if I go right down to the atom.”