15 févr. 1564 - Galileo Galilei's birthday

Description:

Galileo discovered four of Jupiter’s moons: Europa, Ganymede, Callisto, and Io. He also discovered that Venus has phases. This helped people understand that, if moons can orbit a planet, then a planet can orbit a sun. Plus, Venus wouldn't have phases if it were orbiting Earth, as sunlight wouldn't be able to reach Venus.
(He also discovered Saturn's rings, but that's not very significant.)