Giovanni Cassini had been attracted to the heavens in his youth, his first interest being astrology.

Having read widely on the subject of astrology while young, had had soon become versed in its intricacies; this extensive knowledge of astrology had led to his first appointment as an astronomer at the Panzano Observatory in Bologna, from 1648 to 1669.

Later in life, as he becomes increasingly involved in the Scientific Revolution, he will focus almost exclusively on astronomy and all but denounce astrology.

Having in 1671 become director of the Paris Observatory that he has helped to set up, Cassini had in 1672 sent his colleague Jean Richer to Cayenne, French Guiana, while he himself stayed in Paris.

The two made simultaneous observations of Mars and, by computing the parallax, determined its distance, thus measuring for the first time the true dimensions of the solar system.

Cassini is the first to observe four of Saturn's moons, which he calls Sidera Lodoicea, including Iapetus, whose anomalous variations in brightness he correctly ascribes as being due to the presence of dark matter on one hemisphere (now called Cassini regio in his honor).

He discovers, in 1675, the Cassini Division, a forty-eight hundred kilometer- (two thousand nine hundred and eighty mile-) wide region between Saturn’s A Ring and B Ring: from Earth, the Division appears as a thin black gap in the rings.

There are few sources on Århus native Ole Christensen Rømer until his entry in 1662 at the University of Copenhagen, at which his mentor had been Rasmus Bartholin, who had published his discovery of the double refraction of a light ray by Iceland spar (calcite) in 1668 while Rømer was living in his home.

Rømer had been afforded every opportunity to learn mathematics and astronomy using Tycho Brahe's astronomical observations, as Bartholin had been given the task of preparing them for publication.

The determination of longitude is a significant practical problem in cartography and navigation.

Rømer had joined the observatory of Uraniborg on the island of Hven, near Copenhagen, in 1671.

Over a period of several months, Jean Picard and Rømer had observed about one hundred and forty eclipses of Jupiter's moon Io, while in Paris Giovanni Domenico Cassini had observed the same eclipses.

By comparing the times of the eclipses, the difference in longitude of Paris to Uraniborg could be calculated.

Cassini had observed the moons of Jupiter between 1666 and 1668, and discovered discrepancies in his measurements that, at first, he attributed to light having a finite speed.

Rømer had gone to Paris in 1672 and continued observing the satellites of Jupiter as Cassini's assistant.

Adding his own observations to Cassini's, Rømer had observed that times between eclipses (particularly those of Io) got shorter as Earth approached Jupiter, and longer as Earth moved farther away.

Cassini on August 22, 1676, had made an announcement to the Academy of Sciences: “This second inequality appears to be due to light taking some time to reach us from the satellite; light seems to take about ten to eleven minutes [to cross] a distance equal to the half-diameter of the terrestrial orbit.” Although Cassini seems to have abandoned this reasoning, Rømer had adopted and set about buttressing it in an irrefutable manner, using a selected number of observations performed since 1671 by Picard and himself.

Rømer had presented his results to the French Academy of Sciences, and it is summarized soon after by an anonymous reporter in a short paper, Démonstration touchant le mouvement de la lumiére trouvé par M. Roemer de l'Académie des sciences, published December 7, 1676, in the Journal des sçavans.

The reporter, failing to understand Rømer’s presentation, obfuscated his's reasoning in the process, and Rømer himself is never to publish is results, but he has made the first quantitative measurements of the speed of light.

Nicolas Fatio de Duillier, the seventh of fourteen children of Jean-Baptiste and Cathérine Fatio in Basel, Switzerland, had moved with his family in 1672 to Duillier.

Fatio, at the age of eighteen, had traveled to Paris in 1682 to perform astronomical studies under the astronomer Giovanni Domenico Cassini at the Parisian observatory.

His greatest scientific success is the explanation of the nature of the zodiacal light in 1684, which he attributed to particles reflecting the light of the sun.

Fatio had in 1686 by chance become a witness to a conspiracy aimed at William of Orange, which he helped to foil.

In the same year he had made the acquaintance of Jacob Bernoulli and Christiaan Huygens, with whom he developed a particularly close cooperation.

The main contents of their work were the calculus.

He had traveled to London in 1687 and made the acquaintance of John Wallis and Edward Bernard (1638-1697) and worked out a solution of the inverse tangent problem.

He is also connected by friendship to Gilbert Burnet, John Locke, Richard Hampden and his son John Hampden.

Becoming a fellow of the Royal Society in 1688 on the recommendation of founding member Sir John Hoskyns, he gives an account on the mechanical explanation of gravitation of Huygens before the Society, whereby he tries to connect Huygens' theory with that of Newton.

Gian Domenico Cassini formulates Cassini’s Laws, three empirical rules that accurately describe the rotation of the Moon, in 1693.

They are: (1) the Moon rotates uniformly about its own axis once in the same time that it takes to revolve around the Earth; (2) the Moon's equator is tilted at a constant angle (about 1°32¢ of arc) to the ecliptic, the plane of Earth's orbit around the Sun; and (3) the ascending node of the lunar orbit (i.e., the point where the lunar orbit passes from south to north on the ecliptic) always coincides with the descending node of the lunar equator (i.e., the point where the lunar equator passes from north to south on the ecliptic).