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The article presents an encyclopedia entry for astronomer Jean Picard. He first introduced in the practice of astronomy the technique of corresponding altitudes, and also the determination of right ascensions of stars by the time of their passage across the meridian. Picard also contributed to the contents of the first five volumes of La connaissance des temps, the first nautical almanac. However, the popular scientific result of Picard rests on his precise measurement of a meridional arc, of which he gave an account in his Mesure de la terre, a work that exercised considerable influence on the development of geodesy, celestial mechanics, and cartography.

The article presents an encyclopedia entry for Observatoire Paris, the official observatory in Paris, France. Building an official observatory was important to the organization of the Académie Royales des Sciences. It was learned that the structure was initially intended to serve not only for astronomical observation, but also as a place of assembly and experiment. The building was designed by Claude Perrault (1613-1688), a member of the Académie. Modifications were demanded by his colleagues Adrien Auzout (1621-1619), Jean Picard (1620-1682) and Jean-Dominique Cassini (1625-1712). It was said that the inauguration of the Observatoire de Paris marks the start of a new era in astronomical precision.

The article presents an encyclopedia entry for French engineer and astronomer Jean Richer. His scientific activities essentially date from 1666 to 1673 when he was an assistant at the Académie Royale des Sciences in Paris, France. Two missions where entrusted to Richer. The first mission was in Acadia in 1670 which gave him the opportunity to carry out the first astronomical observations on American soil, made with instruments equipped with telescopic sights. The second is at Cayenne, French Guiana from 1672-1673 which furnished the learned world with two fundamental elements concerning knowledge of the universe.

The article presents an encyclopedia entry for Pierre Petit, a military engineer who was an active figure in French science for half a century. A skillful experiment, he became popular in 1625 for designing an original proportional compass and also for some determinations of specific weights and comparisons of standards of length and weight, which were valued by his contemporaries. An active member of several Parisian scientific circles, which were a prelude to the Académie Royale des Sciences, Petit, to his great disappointment, was not among those selected as a member of the core group of the academy organized in 1666. However, on April 24, 1667, he was among the first foreigners to be elected to the Royal Society of London.