On this day, 415 years ago, on January 7, 1610 (December 13, 1609 lunar calendar), Galileo Galilei discovered the moons of Jupiter. Galileans are four large moons of Jupiter, first discovered by Galileo on January 7, 1610, so they are called Galilean moons. They are named "Io" (Io), "Europa" (Europa), "Ganymede" (Ganymede) and "Callisto" (Callisto) according to their numbering order. These four moons can be observed with low-power telescopes, and if there is no light damage and the environment is excellent, even Callisto can be barely seen with the naked eye. The four moons of Jupiter, Io, Europa, Ganymede, and Callisto, were discovered by the Italian astronomer Galileo Galilei in 1610, giving them their names. Their orbits are circular, their orbital planes almost coincide with Jupiter's equatorial plane, and their rotation periods are the same as those around Jupiter. They are the four largest moons in the solar system. The discovery of these moons played a historic role in the establishment of the heliocentric system. In the 1770s, the Danish astronomer Romer first measured the speed of light propagation through the phenomenon of Galileo's moons being eclipsed by Jupiter. Since the 17th century, the study of Galileo's moons has achieved many results.