Who first determined the speed of light and how did they do it ?

The velocity of sound in air is slow enough that we can easily detect time delays as it travels. For instance, the slap of distant thunder is delayed five seconds for each mile traveled. But light must have a much larger velocity because it appears to arrive at all observers instantaneously. To produce an easily measurable time delay, the light must travel a very large distance.

The first person who observed and measured such a delay was the Danish astronomer Olaf Romer in 1676. He made careful observations of the "moons" of Jupiter. The innermost moon goes around Jupiter in less than two days and is eclipsed by Jupiter once in each orbit. Romer found that this moon did not reappear at exactly the time he expected. Instead, the time of the eclipses varied periodically during the year. When the Earth was farthest from Jupiter, the eclipses were about 11 minute late; when the Earth was nearest Jupiter (about 6 months later), they were about 11 minuets early. Romer deduced that this variation must be due to the annual variation in the distance the light had to travel to get to the Earth from near Jupiter. The 11-minute delay must be about he time that it takes light to travel a distance equal to the radius of the Earth's orbit. More refined measurements of the eclipses gave a value of about 186,000 miles per second for the velocity of light. That is a million times faster than sound!