25 September

1644: On 25 September 1644, Ole Christensen Rømer was born.

Rømer was a Danish astronomer who demonstrated conclusively that light travels at a finite speed. He measured the speed by precisely measuring the length of time between eclipses of Jupiter by one of its moons. This observation produces different results depending on the position of Earth in its orbit around the Sun. He reasoned that this meant light took longer to travel the greater distance when Earth was travelling in its orbit away from Jupiter.