 On 7 January 1610 , Galileo observed with his telescope what he described at the time as " three fixed stars , totally invisible by their smallness " , all close to Jupiter , and lying on a straight line through it . Observations on subsequent nights showed that the positions of these " stars " relative to Jupiter were changing in a way that would have been inexplicable if they had really been fixed stars . On 10 January , Galileo noted that one of them had disappeared , an observation which he attributed to its being hidden behind Jupiter . Within a few days , he concluded that they were orbiting Jupiter : he had discovered three of Jupiter 's four largest satellites ( moons ) . He discovered the fourth on 13 January . Galileo named the group of four the Medicean stars , in honour of his future patron , Cosimo II de ' Medici , Grand Duke of Tuscany , and Cosimo 's three brothers . Later astronomers , however , renamed them Galilean satellites in honour of their discoverer . These satellites are now called Io , Europa , Ganymede , and Callisto . 
