A simplified illustration of the parallax of an object against a distant background due to a perspective shift. When viewed from Viewpoint A, the object appears to be in front of the blue square. When the viewpoint is changed to Viewpoint B, the object appears to have moved in front of the red square.
From top to bottom: Io, Europa, Ganymede, Callisto. The Galilean moons are the four largest moons of JupiterâIo, Europa, Ganymede, and Callisto. They were discovered by Galileo Galilei around January 1610 and were the first group of objects found to orbit another planet. Their names derive from the lovers of Zeus.
By the 15th of January he had figured it out: these were not fixed stars but rather planetary bodies that revolved around Jupiter. Jupiter had four moons. His book, Sidereus Nuncius, in which his discovery was described, came off the press in Venice in the middle of March 1610 and made Galileo famous.