Janus. Phoebe. Hyperion. Europa. Most of the planetary satellites in our solar system have cool names like these — names pulled mainly from ancient mythology. But our own natural satellite? In English anyway, the single moon that orbits our planet goes by the unimaginative name, “the moon.” Why?
It turns out the name “Moon” wasn’t lame until we started discovering satellites orbiting other planets, and we made the fatal mistake of calling them “moons.” It’s somewhat confusing that the proper noun "Moon," derived from the old English word “Mona,” was co-opted to become a generic name for any planetary satellite.
Happily, languages other than English are not so afflicted. The Moon is known in Spanish as Luna, Selene in Greek, Mweze in Swahili, and Kuu in Estonian.
Regarding the other satellites in our solar system whose names are higher on the cool scale: they come in all shapes and sizes, from irregular chunks of rock and ice to giant, complex spheres. Of the over 100 moons that have been observed in our solar system (with many more waiting to be discovered), here are the five largest.
5. Earth's Moon
It happens that the fifth-largest satellite in our solar system is our own un-inspirationally named moon, at a diameter of 3,475 kilometers (about 2,160 miles).
Fun fact: It’s not by accident that the same side of the moon faces the Earth throughout its orbit: The moon rotates on its axis with the same period as its revolution around Earth. This phenomenon, called “tidal locking,” is caused by the influence of Earth’s gravity on the moon, and is not unique to the Earth-moon system; it is found in many other planetary satellites in our solar system.
At a diameter of 3,636 kilometers (2,260 miles), Io is only slightly larger than our moon and orbits Jupiter. It’s Jupiter’s fifth-closest known moon, and was discovered in 1610 by none other than telescope inventor Galileo Galilei.
Fun fact: Io has over 400 active volcanoes on its surface, making it the most geologically active object in our solar system.
Another of Galileo’s 1610 discoveries, Callisto also orbits Jupiter. At 4,800 kilometers (just under 3,000 miles) in diameter, it is almost the same size as the planet Mercury. Like our Moon, Callisto is tidally locked to its parent planet.
Fun fact: Although it is nearly the size of Mercury, Callisto has only one-third of Mercury’s density. This suggests that about half of Callisto is composed of water ice, and there may be a liquid-water ocean under the satellite’s crust.
The aptly named Titan is quite large indeed, at a diameter of 5,151 kilometers (3,200 miles). Unique among the many satellites in the solar system, Titan has an atmosphere, although you can’t breathe it: It’s 95 percent nitrogen and 5 percent methane. It was discovered by Dutch astronomer Christiaan Huygens in 1655.
Fun fact: Titan’s dense atmosphere causes a greenhouse effect that traps solar energy near the moon’s surface. Temperatures on Titan are a comparatively balmy −179° C (290° below-zero Fahrenheit).
Galileo was on a hot streak in 1610. In addition to finding Io and Callisto, he used his newly-invented telescope to spot Europa and Ganymede orbiting Jupiter.
To be fair, Ganymede was the low-hanging fruit in that bunch. At 5,268 kilometers (3,273 miles_ in diameter, Ganymede edges Titan for the “largest moon” honor; it is larger than not only Mercury but also Pluto, the one-time planet that was demoted to dwarf planet status in 2006.
Fun fact: Like Callisto, Ganymede is believed to have a water ocean beneath its crust that completely covers the moon’s interior.
Bonus Fun Fact
Although Ganymede is the largest moon in the solar system, its size relative to its planet, Jupiter, is super small — less than 4 percent of Jupiter’s diameter. By comparison, the diameter of our moon is about 27 percent of Earth’s diameter. The big winner by this measure is Charon, Pluto’s biggest moon: Charon’s diameter is half that of Pluto, and some astronomers think of Pluto and Charon as a double dwarf planet.