To measure the distance to the Sun, Cassini had first to determine the distance to Mars, which passes to its nearest point of the Earth during the opposition of September 1672. He dispatched Jean Richer (1630-1696) to Cayenne in French Guiana to accurately record the position of Mars at the same time as Jean Picard (1620-1682) and himself in Paris. When the planet is observed at the same time from two different places, its position with respect to the stars is slightly different because of parallax effects. Its distance can then be determined, after which that of the Sun can be calculated using Kepler’s laws. In this way, Cassini determined a distance of 138 million kilometres (the modern value is 150 million kilometres). While not very accurate, this was the first reliable determination, which showed that the solar system was very much larger than originally thought.

caption : Brief on observations made at Cayenne - copyright : Observatoire de Paris