In December 1609, German astronomer Simon Mayr, observing Jupiter through a telescope, noticed four dots of light in shifting alignments on either side of the planet. He concluded they were orbiting moons. Galileo Galilei spied the moons at the same time, but he published the discovery first and so got the credit. But Guy Ottewell's Astronomical Calendar notes it was Mayr's names for the moons - Io, Callisto, Ganymede and Europa - that stuck.