How was the finiteness of the speed of light propagation first discovered? Detailed answer

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How was the finiteness of the speed of light propagation first discovered? Detailed answer

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How was the finiteness of the speed of light propagation first discovered?

In 1672, the director of the Paris Observatory, Jean Dominique Cassini (1625-1712), while exploring the satellites of Jupiter, noticed certain delays in the moments of entry of one of them - Io - into the cone of the planet's shadow and exit from it, as if the satellite's orbital time around Jupiter was longer when it is further from the earth.

No one could explain this phenomenon until the Danish astronomer Olaf Römer (1644-1710) took up his study, who came to the conclusion that the observed anomaly of Io's motion should be attributed to the finiteness of the speed of light propagation.

In September 1676, Römer predicted the lag that would be observed in Io's upcoming eclipse in November. Convinced of the correctness of the forecast, he presented his theory to the Paris Academy of Sciences, where it met with strong resistance. Even Cassini, who himself took part in the observations, disclaimed responsibility for Roemer's conclusions. Roemer's theory was finally confirmed by the English astronomer James Bradley (1693-1762), when he, trying to determine the parallax of some stars, discovered in 1725 that at their culmination they seem to be deviated to the south.

Observations that continued until 1728 showed that during the year these stars seem to describe an ellipse. Bradley interpreted this phenomenon as the result of adding the speed of light coming from the star to the speed of the Earth's orbital motion.

Although terrestrial measurements of the speed of light were made only in the next century, after Bradley the finiteness of the speed of light was unanimously accepted as an experimental fact.

Author: Kondrashov A.P.

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