It was 403 years ago this month, in 1610, that Galileo Galilei trained his telescope at distant Jupiter, and discovered the first four and the largest of its many moons. The first discovery came on January 7, when the Italian scientist wrote of seeing in front of Jupiter “three fixed stars, totally invisible by their smallness.” Lo and behold, when he peered at the objects the next night, he found that they had changed positions, which led to the realization that these were not stars but objects orbiting Jupiter.