Understanding the Speed of Light

• Scientists have defined the speed of light as 299,792,458 m/s, redefining the metre as a yardstick in terms of light speed.

• Prior calculations were based on the time it took a light pulse to cover a known distance.

• The first practical estimate was made in 1676 by Danish astronomer Ole Roemer.

• Roemer noticed that the intervals between the disappearance of some moons behind Jupiter varied with distances between Jupiter and the earth.

• He calculated the speed of light to be around 225,300 km/s, a significantly different estimate from modern figures of around 299,792 km/s, which are made using a laser beam and an atomic clock.