The controversial origins of naming moons

Earlier this year, a dozen moons were added to Jupiter’s official tally, bringing the total number for the giant planet to 92. At least half the newly discovered satellites have diameters greater than a kilometer and thus will receive official names from the International Astronomical Union. As per IAU guidelines, moons of Jupiter must be named after either the descendants of the Roman god Jupiter/the Greeks’ Zeus or the god’s (often nonconsensual) sexual partners. The names can have no current commercial, political, military, or religious connotations.

Keeping the solar system safely “mythological"—a perceived neutrality that nonetheless privileges Western traditions—has been a challenge for astronomers for hundreds of years. Even as European nations were building empires, their astronomers were taking steps to keep personal and imperial ambitions out of the heavens. Those attempts created the conventions that continue to dictate solar system nomenclature today.

A history of numbers

When Galileo Galilei discovered the four largest moons of Jupiter in 1610, he had no thought of naming them individually. Instead, he gave them a collective name: the Medician stars, after Grand Duke Cosimo de’ Medici of Tuscany, a potential patron. Four years later, Simon Marius published his own account of the moons and offered multiple naming possibilities, including one suggested to him by Johannes Kepler: Io, Europa, Ganymede, and Callisto, after Jupiter’s partners. The names did not catch on, however, and the satellites of Jupiter were designated by number in order from their distance to the planet.

Nearly half a century later, when Christiaan Huygens spotted the largest satellite of Saturn, he gave it no name. The French–Italian astronomer Giovanni Domenico Cassini named neither the two Saturnian moons he discovered in 1671 nor another pair in 1684. Like Jupiter’s moons, the satellites of Saturn were numbered in order from the planet, 1 through 5. And like Galileo, Cassini gave his moons a collective name only, also after a patron: He called them the Lodoicean stars, after King Louis XIV of France.

The situation was complicated in 1789 when the British astronomer William Herschel used his massive 40-foot reflecting telescope to confirm two additional moons of Saturn. Those were the sixth and seventh moons in order of discovery, but they were closer to the planet than the previous five. Instead of renumbering the moons, Herschel compromised convention: Going outward, the moons of Saturn became 6, 7, 1, 2, 3, 4, and 5.

William’s son, John Herschel, finally did away with the confusing numbering system. In an 1847 study of Saturn’s moons, he gave them their current mythological names: Mimas, Enceladus, Tethys, Dione, Rhea, Titan, and Iapetus, all of the mythical race of giants that were siblings of Saturn and gave birth to the gods. He wrote that “as Saturn devoured his children, his family could not be assembled round him, so that the choice lay among his brothers and sisters, the Titans and Titanesses.”

With the new convention, numbers gave way to mythological beings for the purpose of, according to Herschel, clearing up the confusion regarding Saturn’s moons. Astronomers have followed his approach for Saturn’s system ever since, though they have expanded it to encompass giants from Inuit, Celtic, and Norse mythologies.

It turns out, however, that Herschel wasn’t being completely honest. There was more to giving moons mythological names than just clarity.

A history of controversy

The truth is that John Herschel had been happy to use the numerical system for Saturn’s moons in his observations and correspondence up to a very specific date—28 November 1846—at which point mythical names for the moons abruptly appear without explanation in his diaries and notes. That day, he received a letter from the French astronomer Urbain Le Verrier regarding not moons but planets—in particular the planet whose discovery Le Verrier had mathematically predicted and whose existence had been confirmed not long before, a planet that was currently in the midst of its own naming controversy.

Originally content to call the new world “Neptune,” French astronomers had begun a movement to name it “Leverrier.” British astronomers, who claimed they had naming rights as well because of the contested co-prediction of the planet by John Couch Adams, protested such a flagrantly nationalistic name. Yet they did not have much ground to stand on: When William Herschel discovered Uranus in 1781, he had named it for the British king—Georgium Sidus (George’s Star) or “the Georgian,” as it was still referred to at the time in British almanacs. Le Verrier’s letter to John Herschel that November proposed a compromise: Le Verrier’s planet would be called “Leverrier,” and the planet discovered by John Herschel’s father would be “Herschel.”

John Herschel was horrified. The heavens, he felt, were no place for nationalistic labels. He had long ceased referring to his father’s discovered planet as anything but Uranus, and in his suggestions for the names of Southern Hemisphere constellations he had emphasized they must be free of any modern names or personalities (while apparently failing to recognize that classical Greek and Roman mythology nonetheless biased the skies for European observers). The moons of Saturn—which up to this point Herschel had been content to refer to by number—offered the ideal chance to put this “neutral” classicalism into play. If the names of Titans stuck, it would make names like Le Verrier, Herschel, or the Georgian seem even more out of place in the solar system.

The plan worked. His 1847 study of Saturn’s moons promulgated his new convention. The following year, when British astronomer William Lassell discovered an additional moon of Saturn, he dubbed it Hyperion. Le Verrier dropped his insistence on personal names for the planets, and Neptune was accepted. Though the Galilean moons of Jupiter were still usually referred to by number, the proper names suggested two centuries earlier by Marius came more into common usage. When a new moon of Jupiter was discovered by E. E. Barnard in 1892, it was dubbed Amalthea, after the infant Jupiter’s nurse. Swapping classical names for numbers had saved the skies from controversy by keeping the solar system nomenclaturally neutral, an impartiality astronomers strive to maintain today.

An ironic epilogue

There were still a few troubling moons in the solar system, however. William Herschel had detected six around Uranus during his years of observing. Two turned out to be spurious, but in 1851 Lassell confirmed the orbits of the others. Since John Herschel had provided names for the previously discovered Saturnian moons, Lassell turned to him for names for the new ones as well.

Herschel suggested naming them after “sprites, or airy spirits” and offered Oberon, Titania, Ariel, and Umbriel, all but the last from the works of William Shakespeare. (Umbriel is from Alexander Pope’s poem “The Rape of the Lock.”) The idea made sense: spirits of air accompanying the Greek god of the sky. Lassell liked the names but had a concern: Were they classical enough, or was this simply a subtle way of surrounding the planet first named for a British king with names lifted from English literature?

Herschel was undeterred, and his influence at the time was enough that his suggestion held sway. Somewhere along the line, however, his original intention of naming the moons after magical spirits was forgotten. When the next Uranian moon was discovered a century later by Gerard Kuiper, it was named Miranda, for the heroine of Shakespeare’s The Tempest. The Voyager 2 science team followed suit when the NASA spacecraft’s 1986 Uranus flyby revealed a host of new moons, all of which were named for characters from Shakespeare’s plays.

In the end, John Herschel was able to have his cake and eat it too: King George’s legacy in the sky is forgotten, but the nomenclatural nod to English literature endures.

Stephen Case is a professor in the department of chemistry and geosciences at Olivet Nazarene University in Bourbonnais, Illinois. A historian of astronomy, he is the author of Making Stars Physical: The Astronomy of Sir John Herschel and a coeditor of the forthcoming Cambridge Companion to John Herschel.