12 Facts About Galilean moons

Galilean moons were observed in either 1609 or 1610 when Galileo made improvements to his telescope, which enabled him to observe celestial bodies more distinctly than ever.

Four Galilean moons were the only known moons of Jupiter until the discovery of Amalthea in 1892.

The numbered names were used until the mid-20th century when other inner Galilean moons were discovered, and Marius' names became widely used.

The times of the eclipses of the Galilean moons could be precisely calculated in advance and compared with local observations on land or on ship to determine the local time and hence longitude.

Each generation of Galilean moons to have formed would have spiraled into Jupiter and been destroyed, due to tidal interactions with Jupiter's proto-satellite disk, with new Galilean moons forming from the remaining debris.

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Europa, the second of the four Galilean moons, is the second closest to Jupiter and the smallest at 3121.

Ganymede, the third Galilean moons moon, is named after the mythological Ganymede, cupbearer of the Greek gods and Zeus's beloved.

Fluctuations in the orbits of the Galilean moons indicate that their mean density decreases with distance from Jupiter.

Each generation of Galilean moons would have spiraled into Jupiter, due to drag from the disk, with new Galilean moons then forming from the new debris captured from the Solar nebula.

The current Galilean moons were still affected, falling into and being partially protected by an orbital resonance which still exists for Io, Europa, and Ganymede.

All four Galilean moons are bright enough to be viewed from Earth without a telescope, if only they could appear farther away from Jupiter.

The maximum angular separations of the Galilean moons are between 2 and 10 arcminutes from Jupiter, which is close to the limit of human visual acuity.