Light travels through empty space at a speed of 300,000 km/s

• In 1676, Danish astronomer Olaus Rømer discovered that the exact time of eclipses of Jupiter’s moons depended on the distance of Jupiter to Earth

• This happens because it takes varying times for light to travel the varying distance between Earth and Jupiter

• Using d=rt with a known distance and a measured time gave the speed (rate) of the light