As twilight ends this winter, look to the east. You may see a very bright “star” that does not twinkle. This is Jupiter, the largest planet of the Solar System, eleven times the diameter of the Earth. It was named after the King of the Gods in Roman mythology. But Jupiter is king in another sense as well; it is host to 67 moons, compared to Earth’s one. Most of these moons are quite tiny, less than 10 kilometers wide. But four are large enough to be seen easily from Earth.
These are the four Galilean moons — Io, Europa, Ganymede, and Callisto – discovered by Galileo Galilei in January, 1610. Galileo, who devised the telescope to see them, questioned the prevailing belief that all celestial bodies revolved around the Earth. This eventually gave way to what we know today — that the Earth orbits the Sun, the Sun orbits the center of the Milky Way, and planets can have moons that orbit them.
The Galilean moons couldn’t possibly be any less alike one another. Fiery Io orbits so close to Jupiter that it is tidally stretched and compressed like pizza dough, causing massive volcanic eruptions to continually occur. Europa, the next furthest moon, is a frozen world on its surface with a global subsurface ocean many miles deep. Scientists speculate that this ocean may harbor extraterrestrial life. Ganymede, the next furthest, is larger than the planet Mercury, and has its own oxygen atmosphere and magnetic field. Last, Callisto is heavily cratered from countless objects pulled into Jupiter’s immense gravity. It is one of the most impacted bodies in the Solar System.
The four moons orbit Jupiter fast enough that, observed through a small telescope or binoculars, their positions visibly change nightly. This is the dance that Galileo watched four hundred years ago - and you can, too. In the evening, watch for that bright yellowish "star" in the east, see four dimmer "stars" in a line with it. It will be the King of the Planets, and the biggest members of his court.
Western Slope Skies is produced by members of the Black Canyon Astronomical Society. This episode was written & recorded by Michael T. Williams.
