On this day in 1610 Galileo saw the four largest moons of Jupiter for the first time. He was 45. It was the first time anyone had seen the moons of Jupiter, and it's hard to think how anyone Galileo spoke to made sense of what he was describing. The moon, our moon, was a large bright disk in the sky; Jupiter was a pin prick by comparison. Brighter, certainly, than the other stars, but it was hard to imagine moons revolving around something so insignificant.

Galileo was a revolutionary. His te...