Remaining in the field of Cassini's planetary studies, his "Astronomical Almanac" of  Jupiter's satellites was of particular importance.
Finding the positions of the moons of Jupiter allowed Cassini to prepare tables showing the moments when the satellites disappeared behind the planet. The observation of the eclipse of a satellite allowed him to read from the tables the exact time at which this occurred, which gave an extremely accurate time measurement.  This meant that it was possible to determine precisely the longitude of the place where the observation occurred, thus helping to solve one of the biggest problems of the time.

In addition, by establishing the periods of revolution of the satellites around Jupiter, Cassini was then able to discover that the satellites were eclipsed by the planet with a short delay. This was due to the time needed for the light to reach us from the planet.  Thanks to this effect discovered by Cassini and thanks to his accurate observations and his precise calculations, his collaborator Ole Roemer succeeded in 1675 in measuring the speed of light.

Fig. 1: While the satellite Io is disappearing behind Jupiter (on the left), the shadow of the other satellite Ganymede is coming into view in the distance on the planet's atmosphere. On the right of the planet, the great "red spot" is visible. (Credit: Foto Charles F. Capen, Jr.al McDonald Observatory)

	Fig. 2: The great map of the Moon drawn by Gian Domenico Cassini in 1679. (Credit: Biblioteca del Dipartimento di Astronomia di Bologna)

Cassini's great map of the Moon, with its delicate micrometric observations, achieved vast fame. These observations allowed him to study the variations in the orbit of our satellite, which led him to elaborate what is considered to be the first modern theory of lunar movements.
Three laws, expressed by Cassini in 1693, arose from his research into tidal attractions between planets and their satellites - similar to those found between the Moon and the Earth. Verification of this has been published recently in Icarus, the most important international journal of planetary studies.
Few scientists have had the honour which has been bestowed on Gian Domenico Cassini – that of still being quoted in scientific literature after more than 300 years, thanks to the current relevance of his research and not just because of his importance in history.