What is the largest moon in our solar system? It's fitting that the largest moon in our solar system orbits the largest planet. Ganymede measures 3,270 miles in diameter and is so big that if it orbited the sun instead of Jupiter, astronomers would classify it as a planet.
The seventh moon from Jupiter, Ganymede is larger than Mercury and one-third the size of Earth. It's named after a boy from Greek mythology who became a servant to the gods. The astronomer Galileo Galilei discovered Ganymede in 1610, along with three other moons of Jupiter. He saw the four moons for the first time through a telescope he made himself.