How do we know the speed of light?

Modern estimates of the speed of light are made with a laser beam and an atomic clock.
| Photo Credit: Thomas Kinto

A: After centuries of developing improved instruments to measure everything in the physical world with increasing precision, scientists have agreed to define the speed of light as precisely 299,792,458 m/s. Their agreement redefines their yardstick, in this case the metre, in terms of the speed of light rather than the other way around.

Before that, calculations of the speed of light were conventionally based on how long it took a pulse of light to cover a known distance. Because the speed of light is so great, it had to be a very great distance, and comparatively accurate measurements had to await the development of sensitive modern instruments.

The first practical estimate was made in 1676 by Ole Roemer, a Danish astronomer then working at the Royal Observatory in Paris. Roemer noticed when studying the eclipses of Jupiter’s moons that the intervals between the disappearance of some of the moons behind the planet varied with distances between Jupiter and the earth. He reasoned that the velocity of light was responsible for an apparent delay in the eclipse when Jupiter was more distant from the earth. He calculated the speed of light to be around 225,300 km/s.

Roemer did not know the precise distance to Jupiter, so his estimate was considerably different from modern figures of around 299,792 km/s, give or take a few billionths of a second. These estimates are made with a laser beam and an atomic clock.