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The Galilean moons are Jupiter's largest four moons, discovered in 1610 by the Italian astronomer and physicist using one of the earliest telescopes. The discovery of the Galilean moons, and the implications about the universe they signified, are considered a landmark event in the Renaissance. By discovering the first moons orbiting around another planet, Galileo showed that other planets possess their own gravity, supporting the Copernican heliocentrist theory published 67 years earlier.
The Galilean moons were, in order of size: Ganymede, Callisto, Io, and Europa. Ganymede has a diameter of 5262 km, Callisto is 4820 km, Io is 3660 km, and Europa is 3121 km. In order of distance from Jupiter, they are Io, Europa, Ganymede, and Callisto. Ganymede is even larger than the planet Mercury, although its mass is only about half. Callisto approaches Mercury in size. For comparison, our own Moon is 3474 km in size, making it only larger than Europa.
Along with the Moon, Saturn's Titan, and Neptune's Triton, the Galilean satellites make up the seven largest satellites in the solar system, sometimes called "the Big Seven." They are larger than all other satellites in the solar system by a significant margin. Looking at the variation between these satellites is a good way of learning about different planetary geographies and histories.
To describe the Galilean moons in terms of appearance: Io, the closest to Jupiter, looks like a yellow pizza, because it is covered in erupting volcanoes and sulfur dioxide frosts. Ganymede: the largest, which looks like a giant shaded sandstone, including huge sectors of ice smashed by asteroids. Callisto: a very dark moon with bright splotches of white, barraged by so many craters that no new one can be created without erasing one of the more ancient. And Europa: the smallest, a mostly white sphere with brown linea criss-crossing its surface, and some large brown smudges.
The Galilean moons are some of the most impressive celestial sights that can be observed with a high-end telescope. Although huge, any one of these moons could be swallowed up in Jupiter's Great Red Spot.