# Galilean moons

Also called: Io, Europa, Ganymede, Callisto.

Jupiter's four large moons — Io, Europa, Ganymede and Callisto — discovered by Galileo in 1610.

Galileo's first telescope showed four “stars” lined up beside Jupiter that moved night by night: the first moons found orbiting another world. Io circles Jupiter in under two days, Callisto in about 17, so their dance changes visibly within an evening.

With steady air you can watch a moon's black shadow crawl across Jupiter's clouds, or see a moon vanish into Jupiter's shadow.

**Examples:** [io](https://astropilot.space/object/io.md), [europa](https://astropilot.space/object/europa.md), [ganymede](https://astropilot.space/object/ganymede.md), [callisto](https://astropilot.space/object/callisto.md)

**Related:** [Planetary moon](https://astropilot.space/glossary/planetary-moon.md), [Transit](https://astropilot.space/glossary/transit.md), [Occultation](https://astropilot.space/glossary/occultation.md), [Great Red Spot](https://astropilot.space/glossary/great-red-spot.md)

From the AstroPilot astronomy glossary: https://astropilot.space/glossary/galilean-moons