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The Arago Medallions, Paris, France

missing image file48° 504.23N, 2° 20 11.41 E

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François Arago and the Paris Meridian

François Arago was a French mathematician, physicist, and astronomer (and briefly the Prime Minister of France in 1848) who studied optics, magnetism, electricity, and astronomy. He strongly supported the then-controversial theory that light was made up of waves; he demonstrated that a rotating metal disc affects the motion of a magnetic needle suspended above it (for more on magnetism, see Chapter 75); and he showed that light moves more slowly in dense media. He was also a popular orator, and gave public lectures on astronomy for over 30 years.

But Arago is best known today as one of the directors of l’Observatoire de Paris (the Paris Observatory), where he lived and worked. The Observatory was established in 1667 with the support of King Louis XIV of France. Initially, the Observatory’s mission was to improve the instruments and maps needed for marine navigation. On Midsummer’s Day in 1667, the outline of the Observatory was traced on the ground, with measurements being made to determine the location for the Paris meridian. The Paris meridian (and hence the French definition ...

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