He wanted to use the precious measures of the famous Danish naked eye observer. The young Danish astronomer, Olaus Roemer, helped him. During Winter 1671 - 72, observations of the eclipses of Io were made simultaneously by Roemer and Picard in Uraniborg and by Cassini in Paris. Picard, who had appreciated the work of Roemer came back to Paris with him during Summer 1672. The precise determination of the positions of the satellites is very difficult .
T is the period of revolution of Io.
Both explanations given by Cassini and Roemer are similar but Cassini will then change his mind and will even contradict Roemer's arguments, and for many years on, Roemer, for his part, will confirm his theory despite a lack of precise measurements. There is no article written by Roemer summarizing the measurements of the speed of light. Roemer took few notes and the fire of Copenhagen in 1728 damaged most of his registers where he put his observations down and his manuscripts. A letter from Roemer to Huygens reveals that his theory about the delay of light is exclusively based on observations realized by Picard and himself. One of Roemer's manuscripts found in 1913 at the library of the university of Copenhagen, and studied by K Meyer, contains, along with a few calculations, a series of observations carried out between 1668 and 1677 on the satellite of Jupiter.