Three lights that would not stay

Galileo Galilei, forty-five, teaching at Padua, has been grinding and combining lenses. The tube he points at Jupiter this night is his own work, about twenty power. He sees the planet, and three small points of light nearby, arranged in a line.

He takes them for distant stars, accidentally near. The unaided sky had not shown them. A new census can still be a census of stars.

He looks again. On the eighth the arrangement has changed. One of the points is gone from where it was, or the set has shifted relative to Jupiter. By the eleventh he is sure they are not fixed. By the thirteenth a fourth has appeared. They keep Jupiter’s company as the nights pass. Distant stars do not do that.

What he can say in January is limited: these lights belong with Jupiter. They move. They are not the same kind of thing he first thought. What to call a body that attends a planet, and whether the Earth may move, are fights that follow the book, not the first night.

Sidereus Nuncius is dated 13 March 1610. The Moon has mountains. The Milky Way resolves into innumerable stars. Orion’s belt and the Pleiades hold lights the unaided eye had missed. The Jupiter sequence is in the observing record, night after night, positions drawn.

The philosophical fight that followed is another story. The instrument had already changed what could be counted.

The four are Io, Europa, Ganymede, and Callisto. They orbit Jupiter. A cosmos can have more than one center of motion. That is what the nights showed, once they were allowed to be nights and not a proof already won. What people made of it is later.