Light speed was discovered in 1676 by a Danish astronomer called Ole Roemer. But Roemer wasn't the first person to try and measure light's speed. Galileo set about looking for it in 1638.
Finding the speed of light required a distance far greater than a few miles. Roemer began his research decades later in 1673 when he noticed the time elapse between the eclipses of a Jupiter moon called Io, which Galilee had discovered in 1610.
By monitoring the time difference, Roemer estimated "light seems to take about 10 to 11 minutes [to cross] a distance equal to the half-diameter of the terrestrial orbit", or Earth's orbit around the Sun. This means that light travels at about 200,000,000 meters per second, which is around 26 per cent below the established speed.
The speed of light is exactly 299,792,458 meters per second. James Bradley came closer to this figure in 1728 with the discovery of what is called the "aberration" of starlight and a speed of 295,000,000 meters per second for light.
But the exact number wasn't decided upon until 1975, after decades of becoming increasingly specific, when the General Conference on Weights and Measures recommended an official figure.
The symbol for the speed of light is c and is known for being a "universal physical constant", which means that it is an exact quantifiable amount that doesn't change.
Why is it important?
Albert Einstein's theory of relativity, which underpins modern concepts of space and time, is built on the premise that the speed of light in a vacuum is always the same.