Unveiling the size of the Universe: the first accurate measurement of the Earth-Sun distance by Giovanni Domenico Cassini

In the ambitious plan of King Louis XIV, France should have become culturally dominant in Europe. The First Minister of State Jean Baptiste Colbert suggested him to finance not only military campaigns but also scientific expeditions aimed to determine, with the highest possible accuracy, the extension of the colonial possessions, to show that France was the largest European power. The astronomers of the Académie des Sciences (founded in 1666), being also geographers and cartographers, were the “leading actors” of those dangerous expeditions, as measuring the terrestrial coordinates (latitude and longitude) strongly required their skills: the desire for glory of the Roi Soleil had unexpectedly turned out into an improvement of astronomy. All the expeditions were supervised by the Italian astronomer Giovanni Domenico Cassini (1625-1712) who had been warmly welcomed at the court of the Sun King, in 1669, and was living in the Observatoire Royal in Paris. Cassini instructed the scientists who were chosen for the expeditions, checked their instruments and compiled a list of instructions concerning the observations they should have carried out. Among the several expeditions organized by Cassini, the one to Cayenne (French Guiana) deserves particular attention as thanks to some observations carried out there (and simultaneously in Paris) Cassini obtained the first accurate measurement of the Earth-Sun distance. Through a careful check and inspection of all the available original documents kept in the Archives in Paris, the history of the observations which were carried out in Cayenne, has been reconstructed and is presented in this work. Moreover, some almost unknown details concerning Cassini’s life and work are also shown. The ambitious aim of this work is to make the reader go back in the past to perceive the atmosphere of an epoch in which, thanks also to the overseas expeditions, began to bloom what it was going to become the Age of Enlightenment.