January 7: The Galilean moons were first observed (1610)

Io, Europa, Ganymede,
and Callisto
(from top to bottom)

Galileo Galilei discovered four moons of Jupiter with his telescope and they are the largest of many moons of Jupiter and are called the Galilean moons. The moons were named after the lovers of Zeus (also known as Jupiter): Ganymede, Callisto, Io and Europa, by Simon Marius who claimed to have discovered them at the same time as Galileo. This discovery showed how important a telescope could be for astronomers. Galileo improved the magnifying capability of the telescope by 20 times and it enabled him to discover these moons. As a note, a Chinese historian claimed that the Ganymede was first observed in China  in 362 BC, which is two millennia earlier than Galileo's. In fact, all four moons are visible without a telescope if they are far away from Jupiter. Anyway, it was a good proof of the usefulness of the telescope in astronomy and important scientific discovery.