On the cold night of 7 January 1610, an Italian professor named Galileo Galilei climbed up to a rooftop in Padua with a long brass tube he had built himself. His telescope could magnify objects only about twenty times - weaker than most pairs of binoculars today - but it was the best on Earth. He pointed it at the bright planet Jupiter and saw three tiny stars in a perfectly straight line beside it. That was odd. Stars were not supposed to line up like that.
Night after night he sketched what he saw. The little points of light shifted positions. Sometimes one disappeared, then came back. After a week Galileo realised the truth - those were not stars at all. They were moons, four of them, circling Jupiter the way our Moon circles Earth. He named them Io, Europa, Ganymede, and Callisto, after characters from old Greek myths.
This was an earthquake of an idea. For 1,500 years almost everyone had believed that every object in the sky orbited Earth. But here were four worlds clearly orbiting something else. Galileo's notebooks became proof that Earth was not the centre of everything. He got into serious trouble with church leaders for saying so, but the moons kept circling. Today the Galilean moons are some of the most fascinating places in our solar system - Europa hides a salty ocean under a shell of ice, and NASA is sending spacecraft there to look for signs of alien life.
