The Galilean moons of Jupiter were discovered by Galileo in 1610, proving the Copernican heliocentrist theory. They are Ganymede, Callisto, Io, and Europa, and are larger than all other satellites in the solar system. They have unique appearances and are impressive to observe with a telescope.
The Galilean moons are the four largest moons of Jupiter, discovered in 1610 by the Italian astronomer and physicist using one of the first telescopes. The discovery of the Galilean moons, and the implications they implied on the universe, are considered a seminal event in the Renaissance. By discovering the first moons orbiting another planet, Galileo proved that other planets have their own gravity, supporting the Copernican heliocentrist theory published 67 years earlier.
The Galilean moons were, in order of magnitude: 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 the size of Mercury. For comparison, our 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 of the solar system by a significant margin. Observing the variation between these satellites is a good way to learn about different planetary geographies and histories.
To describe the Galilean moons in terms of appearance: Io, closest to Jupiter, looks like a yellow pizza, because it’s covered in erupting volcanoes and sulfur dioxide freezes. Ganymede: the largest, which looks like a giant shaded sandstone, which includes huge sectors of ice shattered by asteroids. Callisto: A very dark moon with bright white patches, barred with so many craters that you can’t create a new one without obliterating one of the older ones. And Europa: the smallest, a mostly white sphere with brown lines criss-crossing its surface, and some large brown spots.
The Galilean moons are some of the most impressive celestial sights that can be observed with a high-end telescope. While huge, any of these moons could be engulfed in Jupiter’s Great Red Spot.
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