The Danish astronomer Ole Rømer (1644-1710) measured the orbital period of Jupiter’s innermost moon, Io. He found the period was shorter when Earth was approaching Jupiter than when it was moving away from it. He concluded that light travels at a finite speed and estimated that it would take light 22 minutes to travel the diameter of Earth’s orbit. Christiaan Huygens combined this estimate with an estimate for the diameter of Earth’s orbit. He concluded that the speed of light is 220,000 kilometers per second.

In 1725 the English astronomer James Bradley noted that the location of a star changed with the seasons. He proposed that the shift was due to the addition of the speed of light and the speed of Earth in its orbit. Bradley observed the shift in several stars and determined that light travelled 10,210 times faster than Earth in its orbit. The modern result is 10,066 times faster.