This was important evidence supporting a sun-centered, rather than Earth-centered, solar system. Ganymede is the largest of all solar system satellites and is even bigger than Mercury and Pluto. His predictions were accurate enough that they could be used to set a watch, the key to determining longitude. To view the satellites, Galileo designed a special helmet which he called a celatone. Despite these obstacles, this method of determining longitude was used for years, albeit primarily on land.

Source: Daily Sun October 02, 2021 15:45 UTC