Galilean moons
Galileo’s discovery proved the importance of the telescope as a tool for astronomers. It showed there were objects in space to be found, which were not seen by the naked eye. More importantly, the discovery of celestial bodies orbiting something other than the Earth dealt a blow to the then-accepted Ptolemaic world system. This held that the Earth was at the center of the universe and all other celestial bodies revolved around it. That Jupiter has four moons while Earth has only one further undercut the near-universal belief that the Earth was the center of the universe both in position and in importance. Galileo's Sidereus Nuncius (Starry Messenger), which announced celestial observations through his telescope, does not mention Copernican heliocentrism, a theory that placed the Sun at the center of the universe. Nevertheless, Galileo believed in the Copernican theory.