As light is tremendously fast. measuring its speed was almost an impossible task. It was easier to calculate when light had travelled a long way. Ole Romer, a Danish astronomer accidentally found this in 1676. He noticed that the time between eclipses of Jupiter's moons when they are hidden behind the planet varied throughout the year. Romer realised that it was be-cause of variation in two things: the distance from Earth to Jupiter throughout the year and the distance travelled by light from the moons. His initial calculations estimated the speed of light to be about 220,000 km per second.
Roemer, working at the Paris Observatory, was not looking for the speed of light when he found it. Instead, he was compiling extensive observations of the orbit of Io, the innermost of the four big satellites of Jupiter discovered by Galileo in 1610. By timing the eclipses of Io by Jupiter, Roemer hoped to determine a more accurate value for the satellite’s orbital period. Such observations had a practical importance in the seventeenth century. Galileo himself had suggested that tables of the orbital motion of Jupiter’s satellites would provide a kind of “clock” in the sky. Navigators and mapmakers anywhere in the world might use this clock to read the absolute time (the standard time at a place of known longitude, like the Paris Observatory). Then, by determining the local solar time, they could calculate their longitude from the time difference. This method of finding longitude eventually turned out to be impractical and was abandoned after the development of accurate seagoing timepieces. But the Io eclipse data unexpectedly solved another important scientific problem—the speed of light.The orbital period of Io is now known to be 1.769 Earth days. The satellite is eclipsed by Jupiter once every orbit, as seen from the Earth. By timing these eclipses over many years, Roemer noticed something peculiar. The time interval between successive eclipses became steadily shorter as the Earth in its orbit moved toward Jupiter and became steadily longer as the Earth moved away from Jupiter. These differences accumulated. From his data, Roemer estimated that when the Earth was nearest to Jupiter (at E1), eclipses of Io would occur about eleven minutes earlier than predicted based on the average orbital period over many years. And 6.5 months later, when the Earth was farthest from Jupiter (at E2), the eclipses would occur about eleven minutes later than predicted.
Though this was 25 per cent slower than the correct speed, it was a significant start in the right direction. Christiaan Huygens later deduced that the speed of light is approximately 212,000 km/s. In 1809, astronomer Jean Baptiste Joseph Delambre estimated the time taken by light to travel from the Sun to Earth as 8 minutes and 12 seconds This is quite close to the modern value, which is 8 minutes and 19 seconds, at a speed of 299,792.458 metres per second.
In 1638, Galileo is often credited with being the first scientist to try to determine the speed of light. His method was quite simple. ... By measuring the elapsed time until Galileo saw his assistant's light and knowing how far apart the lamps were, Galileo reasoned he should be able to determine the speed of the light.
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