Galileo Discovers Callisto – 1610
News flash, this day in 1610: Galileo Galilei discovered a fourth moon of Jupiter. But he did not choose the name “Callisto”!
To some extent people who discover new lands, comets, animals, elements, moons (and so on!) get the privilege of naming them—but at times those names are changed later. Galileo called this moon IV, the Roman numeral “4,” to distinguish it from Jupiter's other moons, I, II, and III. This turned out to be confusing, as many other planets turned out to have their own moons—quite a lot of moons, in some cases!—and we now know that Jupiter alone has 63 moons (at least! —stay tuned!).
A German astronomer who lived at the same time as Galileo, Simon Marius, claimed to have discovered Jupiter's moons before Galileo. Looking at the evidence, it seems possible that Marius discovered the Jovian moons independently of Galileo—but a few days later. Marius suggested naming Jupiter's moons for female mythological characters associated with Jupiter: Io, Europa, Ganymede, and Callisto. But because of the dispute over priority, Galileo never adopted those names—he stuck with his Roman numerals.
It seems that Marius's suggested names were officially adopted in the mid-1800s and became commonly used in the 1900s. At that point, Roman numerals were used for the smaller, newly-discovered V through XII, but in the 1970s the smaller Jovian moons were named after other lovers, favorites, and daughters of Jupiter. A few of the smaller moons are still waiting for a name, as far as I can tell.
Take a Peek at Jupiter's Moons
With a small telescope or a good pair of binoculars, you can spot Callisto and Jupiter's other largest moons. Check the diagram on the Kids' Astronomy website about the locations of the moons relative to Jupiter—it's updated constantly.
Here's a great video about why Callisto might be home to a human colony some day!