Up until the end of the 17th century, people believed that light arrived “instantly”, but in 1675 the Danish astronomer Ole Rømer discovered the “waiting of light”, as he called the phenomenon.
Rømer found that the greater the distance between Earth and Jupiter, the longer it took the light from Io, Jupiter’s moon, to reach here.
He calculated that it took light about 22 minutes to travel a distance that corresponded to the diameter of the Earth’s orbit around the Sun, and based on these results, the speed of light could be estimated at about 220,000 km per second.
Now we know that light travels this distance in less than 17 minutes, and Rømer’s conclusion regarding the speed of light was also on the lower side. Light travels through a vacuum at exactly 299,792,458 km per second.
