is the birthday of Dominique François Jean Arago, a French astronomer and physicist born in 1786. Arago is best known for his discovery of the chromosphere--the sun's lower atmosphere--which is composed primarily of hydrogen gas, and for his accurate estimates of the diameters of the planets. In physics, Arago found that a rotating copper disk deflects a magnetic needle held above it, a phenomenon later explained in terms of magnetic induction. He also showed that light waves move more slowly through a dense medium than through air. Arago entered politics in 1848 as Minister of War and Marine and was responsible for abolishing slavery in the French colonies.
[Source: Trevor I. Williams, Ed., A Biographical Dictionary of Scientists (John Wiley & Sons, ed. 3, New York, 1982).]