How fast is the speed of light

1676 demonstration of light's finite speed by Danish astronomer Ole Rømer

Rømer's determination of the speed of light was the demonstration in 1676 that light has an apprehensible, measurable speed and so does not travel instantaneously. The discovery is usually attributed to Danish astronomer Ole Rømer,^{[note 1]} who was working at the Royal Observatory in Paris at the time.

By timing the eclipses of Jupiter's moon Io, Rømer estimated that light would take about 22 minutes to travel a distance equal to the diameter of Earth's orbit around the Sun.^[1] Using modern orbits, this would imply a speed of light of 226,663 kilometres per second,^[2] 24.4% lower than the true value of 299,792 km/s.^[3] In his calculations Rømer used the idea and observations that the apparent time between eclipses would be greater while the Earth is moving further from Jupiter and lesser while moving closer. Fizeau!