Paris Observatory Telescope (Cassini Era)

Timeline

1. 1671–1673Paris Observatory · ParisThe French Royal Academy of Sciences completed the Paris Observatory; Cassini took up the post of director in 1671 with ultra-long-focus aerial telescopes made in the Netherlands. These telescopes had limited achromatic correction but extremely long focal lengths, capable of resolving surface details on planets. Cassini immediately used them to precisely determine the orbital periods of Jupiter's four Galilean moons, compiling satellite ephemerides to support longitude determination.
2. 1675Paris Observatory · ParisIn 1675 Cassini observed Saturn with the Paris Observatory's long-focus aerial telescope and first discovered a distinct dark gap between Saturn's A and B rings, later named the "Cassini Division." This discovery proved that Saturn's rings were not a single continuous structure, marking a milestone in planetary astronomy and demonstrating the limit of resolution of long-focus telescopes at the time.
3. 1676Paris Observatory · ParisDanish astronomer Ole Rømer, at the Paris Observatory using Cassini's Jupiter satellite ephemerides and telescopes, observed systematic deviations in the timing of Io's emersions from eclipse and in 1676 gave the first proof of the finite speed of light and an estimate of its value (about 220,000 km/s), an important milestone in the history of physics. This result depended on the Paris Observatory's long-focus telescope facilities and Cassini's precise ephemeris data.