THE GALILEAN MOONS AND THE SPEED OF LIGHT

On 7 January 1610, Galileo Galilei (1564-1642), professor of mathematics at the University of Padua, aimed a new device of some glass lenses mounted in a piece of organ pipe-he called it a “perspicillum”-toward the southeastern sky. There, to the upper right of the gibbous Moon, was Jupiter, a bright dot of light.

With his telescope yielding about 20x magnification, Galileo noted three little stars near Jupiter, one to the west and two to the east of it. The next night all three lay to the west of Jupiter. Two nights later, he saw only two stars, both to the east. This went on for two weeks before Galileo realized he was seeing a total me four worlds orbiting Jupiter. No one had ever seen these bodies before. Today, we know them as the Galileo moons (ganymede, io, callisto, europa)

by the 1670s, astronomers had determined the periods of revolution for the Galilean moons to within seconds of their modern values. But the predictions for when a moon would enter or leave Jupiter’s shadow were often up to several minutes in error. These events seemed to occur earlier when Earth was nearer the Jovian system and later when it was farther away from it. In 1675, Danish astronomer Ole Römer realized these errors arose from the fact that the speed of light was not infinitely quick. As a result of this study involving the Galilean moons, the first scientific determination for the speed of light was made.