Jupiter’s Satellites, from Cassini to Today’s Icy Worlds: 350 Years of Observation and Study of the Galilean Satellites of Jupiter at Paris Observatory

Abstract

The satellites of Jupiter discovered by Galileo in 1610 are among the most interesting and difficult objects to study. These are complex worlds whose size is close to that of Mars. The four satellites are similar to a small solar system, their motions bringing together all the difficulties of celestial mechanics. These satellites, known as the Galilean satellites also have the peculiarity of being regularly eclipsed by the planet Jupiter, a phenomenon easily observed from Earth. As soon as the Paris Observatory was created, it soon became clear that these satellites were not only objects of study for astronomers, but also valuable auxiliaries for geographers and explorers, helping to define a universal time. Thus, they were used to perfect the cartography of France as desired by Louis XIV during the creation of the observatory. They also helped demonstrate the finite speed of light. The greatest astronomers were interested by these satellites, especially Laplace, who first, understood and described the dynamics of the Jovian system. Delambre undertook to observe and collect many observations of the eclipses of these satellites which are still useful today. More recently, space exploration revived these studies; the Voyager probe sent us unprecedented images revealing their icy surface. The continuous gain in accuracy of astrometric observations allowed to refine their dynamics, modeling and constraining their internal structure: the tidal effects of Jupiter on its satellites are now observable from Earth. These satellites remain a chosen field of study at Paris Observatory; where 350 years later their story continues to be told.