What the speed of light is, and why it's so important to our understanding of the universe. The speed of light is more than a million times faster than a passenger airplane, so you have to admire anyone who finds a way to measure it. We know that the early universe expanded faster than light, and yet it's impossible to travel at the speed of light.A history of the speed of light :for centuries. People assumed light was instantaneous, travelling across space with infinite speed.The Arab mathematician and physicist Alhazen in the 11th century and Englishman Roger Bacon in the 13th century were among the first to say that, although light might be swift, it still requires time to travel between A and B. But light in a vacuum travels about a million times faster than sound in air, so it takes tremendous ingenuity to pin down its speed.Ole Rømer, Jupiter and the speed of light:The first person with that ingenuity was the Danish astronomer Ole Christensen Rømer.In the 1670s he discovered that Jupiter’s moons took 11 minutes longer to orbit when Earth was at its greatest distance from Jupiter than when it was at its closest.Ole Christensen Rømer’s idea was to time light crossing a known distance. Since light spans terrestrial distances too quickly for clocks to measure, the 17th century Danish astronomer looked to the night sky. Imagine there is a clock out in space that strikes midday when the Earth is closest to the clock in its orbit around the Sun. Six months later, when the Earth is at its farthest, the clock’s strike is delayed, because the light has to travel across the diameter of the Earth’s orbit. Rømer’s genius was to find a celestial ‘clock’ in the form of Jupiter and its moons. Instead of the striking of midday he used the instant at which moon Io went behind Jupiter. In 1676, he found that such eclipses were delayed by 22 minutes .16 minutes 40 secondsCombining this with an estimate of the diameter of Earths orbit around the Sun, he calculated the speed of light as 225,000 kilometre per second.