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Roemer’s Determination of Speed of Light

07-DEC-2016 14:42

    Earlier, many scientists believed that the speed of light was infinite and instantaneously could not travel any distance. In the early 17th century an Italian physicist Galileo Galilee devised an experiment and became the first person to try to measure the speed of light. In his experiment, he made two persons stand on hilltops which were at a distance of a few kilometres. He took lanterns with these two persons covering it and stood at a known distance apart. Then in the working on an experiment one person uncovered his lantern and as soon as the other person saw the light, he also uncovered his own lantern.

    Actually, while doing so Galileo wanted to record the time between lantern signals, but was unsuccessful because light moves so fast to be measured and  the distance involved was too small. Also, it needed inter-planetary determinations.


    Olaus Roemer

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    In 1676, the first successful measurement of the speed of light or we can say the determination of the speed of light was made by Olaus Roemer and became the first person to prove that light travels at a finite speed. His method was based on observations of the eclipses of the moons of Jupiter. So, while studying one of Jupiter’s moons, he noticed that the time between eclipses would vary throughout the year based on whether the Earth was moving towards Jupiter or away from it.

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    Olaus Roemer experiment

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    A discrepancy was observed for the time between the eclipses, increasing when the Earth was moving away from Jupiter and decreasing when the Earth was approaching. In half a year, there are a total of 102 eclipses of Io (the moon, he was observing), giving a maximum delay of 16.5 minutes. Roemer interpreted this as the difference in the times needed for the light to travel between Jupiter and Earth. He obtained a value of 214,000 km/s compared to the current value 299,792 km/s. The diameter of the Earth's orbit was not accurately known and there was also an error in the measurement of the delay. Nevertheless, it was a first confirmation that the speed of light is finite and 7 December, 2016 is 340th anniversary of Roemer’s determination of speed of light.


    Olaus Roemer discovered the speed of light


    He had been working at the Royal Observatory in Paris, where Giovani Domenico Cassini was his director. But Roemer was not able to convince his boss. However, many other scientists have supported him like Sir Issac Newton and it gained recognition. But the accurate speed was determined much later in 1975.

    Facts related to speed of light

    The Speed of light is denoted as c and it is a physical constant known as a universal physical constant which means that its value can never be changed. Its exact value is 299,792,458 metres per second (approx 3.00×108 m/s). We all know that Albert Einstein theory of relativity also says that the speed of light in a vacuum does not change.

    Do you know that nothing can travel faster than light? In miles per hour speed of light is about 670,616,629 mph. That is if you could travel at the speed of light, you could go around the Earth 7.5 times in one second. Also, the speed of light in air is about 300 million meters per second.

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    DISCLAIMER: JPL and its affiliates shall have no liability for any views, thoughts and comments expressed on this article.

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