Giovanni Domenico Cassini was also the first of his family to begin work on the project of creating a topographic map of France.

In 1648 Cassini accepted a position at the observatory at Panzano [it], near Bologna, to work with Marquis Cornelio Malvasia, a rich amateur astronomer, initiating the first part of his career.

[7] During his time at the Panzano Observatory, Cassini was able to complete his education under the scientists Giovanni Battista Riccioli and Francesco Maria Grimaldi.

[8] In San Petronio, Bologna, Cassini convinced church officials to create an improved sundial meridian line at the San Petronio Basilica, moving the pinhole gnomon that projected the Sun's image up into the church's vaults 66.8 meters (219 ft) away from the meridian inscribed in the floor.

[7] Cassini's determinations of the rotational periods of Jupiter and Mars in 1665–1667 enhanced his fame, and in 1669, with the reluctant assent of the Pope, he moved to France and through a grant from Louis XIV of France helped to set up the Paris Observatory, which opened in 1671;[8] he would remain the director of the observatory for the rest of his career until his death in 1712.

In 1661 he developed a method, inspired by Kepler's work, of mapping successive phases of solar eclipses; and in 1662 he published new tables of the sun, based on his observations at San Petronio.

"[7] Cassini also rejected Newton's theory of gravity, after measurements he conducted which wrongly suggested that the Earth was elongated at its poles.

More than forty years of controversy about the subject were closed in favour of Newton's theory after the measurements of the French Geodesic Mission (1736 to 1744) and the Lapponian expedition in 1737 led by Pierre Louis Moreau de Maupertuis Cassini was also the first to make successful measurements of longitude by the method suggested by Galileo, using eclipses of the Galilean satellites as a clock.

[8] Zodiacal light is a faint glow that extends away from the Sun in the ecliptic plane of the sky, caused by dusty objects in interplanetary space.

He was intrigued enough by it to spend considerable time and effort deciphering its cryptic contents,[citation needed] also determining on the way that the document originated in India.

Most of their time was spent calculating newer, better, and more accurate ephemerides for astrological purposes using the rapidly advancing astronomical methods and tools of the day.

"Cassini composed several memoirs on the flooding of the Po River and on the means of avoiding it; moreover, he also carried out experiments in applied hydraulics.

In the 1670s, Cassini began work on a project to create a topographic map of France, using Gemma Frisius's technique of triangulation.