You'll be surprised to hear that the largest moon in the solar system is not the Moon that orbits Earth. The largest moon in the solar system is Ganymede which orbits Jupiter, the fifth planet from the Sun.. Ganymede's radius is 2,631.2 km which is larger than our Moon which is 1737.5 km. In comparison, the Earth's radius is 6,371km so roughly it is a third the size of our planet.
Our Moon is the largest in comparison to the planet it orbits. Ganymede when compared to the Jupiter which is 69,911km, it is considerably much smaller comparably than our moon. Below are some moon facts of other planets.
Neither Mercury or Venus have moons, if they did, they're not there now. Mercury is seen as being too weak to keep a moon in orbit because of the gravitational pull from the Sun which would have sucked it away. Venus could have been strong enough for a moon. Some researchers believe its moon was destroyed.
Mars' moon Phobos and Deimos are believed to have been asteroids that got too close to Mars and were pulled into orbit. The larger of the two is Phobos.
Saturn's largest moon is Titan and is the second largest moon after Ganymede and also the only moon in the Solar System to have an atmosphere, not one that we can breathe on though. Titan's largest moon is 2,576 km which is still bigger than our Moon.
Uranus's largest moon is Titania which you might think is a nod to Saturn's largest but its not. Titania is 788.9km and is the eighth largest moon in the Solar System. Titania comes from William Shakespeare's play 'A Midsummers Nights Dream'. With the exception of Ariel, Umbriel and Belinda, all of Uranus's moons are named after characters from William Shakespeare, instead they are named from 'Rape of the Lock', a poem by Alexander Pope.
Neptune's largest moon is Triton and is the only moon that orbits in the opposite direction of the planet that it orbits. The moon is named after the son of Poseidon, the Greek equivalent of the Roman God Neptune. It was the sole moon of Neptune until another moon was discovered in orbit over a hundred years later.
Although Pluto has been downgraded to that of a dwarf planet, it has moons in orbit round it. The largest moon is Charon. It is 603km in radius, just over half the size of the planet it orbits. The radius of Charon is larger than Ceres which is the largest asteroid in the asteroid belt between Mars and Jupiter and one that has been upgraded to dwarf planet from asteroid.
Eris, the dwarf planet on the outskirts of the solar system that after being discovered caused Pluto to be downgraded largest moon radius is 342 km.
Top 20 Largest Moons in the Solar System, Largest First
As for what is the largest moon in the galaxy. We've not got many contenders outside the solar system so we can't answer that yet. As for the universe, we could never know what is the largest moon in the universe as we could never see and measure every moon around every planet around every star in every galaxy. You could say at the moment, the largest moon we know of in the galaxy and the universe is Ganymede.