Galilean Moon Mythology

Galilean moons; in order right to left, Io, Europa, Ganymede, and Callisto

The Galilean Moons’ True Identity

On January 7, 1610, Galileo had just recently invented the telescope and was panning around the night sky until consequently, after fixing the lens towards the planet Jupiter; he noticed three small orbs of light situated around the planet. These were to become the most famous and beloved moons outside of our own, and Galileo watched these ‘objects’ orbit the planet for weeks afterward (they ‘moved’ around it)—thus becoming the first extraterrestrial moons discovered.

Because of the funds given by the Medici family to support his advances in astronomy, Galileo planned to name the moons he discovered after the four Medici brothers. (Galileo discovered three moons at first, but soon found a fourth, supposed to be Callisto today, orbiting rather farther away from the planet than the rest—therefore making it logical that it would be Callisto). The four moons would be called the Medici planets, because Galileo had not yet actually noticed they were moons—he thought it was perhaps another ‘solar system,’ or something of the other.

So, when Galileo reported to the world of his discovery, he called the moons: Principharus, Victipharus, Cosmipharus and Ferdinandipharus. It is suggested that his ‘disciple’ Hodierna (creator of the first ephemerides) suggested these names, so we might be giving Galileo much more credit than he deserves.Galileo called these moons in general the Medici Sidera (Latin for ‘Medici stars’) and the moons with their newly given names were published in Galileo’s short, Latin treatise called the Sidereus Nuncius (usually translated ‘the Sidereal Messenger,’ but more literally ‘a report relating to the stars’) published in March of 1610. Now this brought up controversy—larger than Galileo had expected.

As you know today, the Galilean moons had names that are different than what they were above. We know these moons with rather uncommon names: Io, Callisto, Europa, and Ganymede. But how did these names stick? German astronomer Simon Marius (January 10, 1573 – December 26, 1624) in 1614 published his work Mundus Iovialis (‘The World of Jupiter,’ in Latin, respectively) describing the planet Jupiter and its moons. He claimed he discovered the moons days before Galileo did, leading both himself and Galileo into an argument of who discovered what first. It is considered possible that Marius discovered the moons independently, but at least some days later than Galileo, as resolved today.

Regardless of who first discovered these moons, the mythological names finally became what we know them of today. This quick verse from Marius’ Mundus Iovialis shows us how the moons became named after mythological characters and not the Medici brothers. Three of the four mythological characters come from the Roman poet Ovid in his Metamorphoses, or ‘changes,’ speaking of the fate of three mythological beings. 

Io, Europa, Ganimedes puer, atque Calisto
lascivo nimium perplacuere Iovi.

When translating this passage, we get: “Io, Europa, the boy Ganymede and (even) Callsito / pleased lustful Jupiter very much.” When examining this passage even more, we get a better literal translation: “Io, Europa, the boy Ganymede and (even) Callisto / gave pleasure beyond measure (excessive) to lustful Jupiter.” (perplacuere is a dative-relating verb, as lascivo...Iovi can be put in dative, as well as simplified when perplacuere is simplified to just ‘pleased,’ rather than its literal translation of ‘gave pleasure (to).’)

Although Galileo never accepted this (Marius was already an arch-enemy), he couldn’t use his original names because of how prevalent Marius’ names pertained to the picture. If you examine each of the character’s stories, you will notice how they did ‘please lustful Jupiter,’ as the Latin describes. Galileo started labeling the moons in Roman numerals (I is Io, II is Europa, III is Ganymede, and IIII (IV) is Callisto, written in distance order. Io is closest, and Callisto is farthest, etc.) So, even though Galileo never published his Roman numeral names (for the moons), we still use the numerals and their names—which their stories are told following.



(pronounced EE-oh) Io is Jupiter's innermost Galilean moon and orbits the planet every 1.77 days. The picture below of this 'orange' lunar body was taken by NASA's spacecraft Galileo (named correctly!).