In most books on the history of science one finds Galileo Galilei credited with being the founder of telescopic astronomy. This is only true in a very limited sense; Galileo was along with Thomas Harriot, Simon Marius and others one of the comparatively small group of astronomers who turned the telescope towards the heavens in 1609 and 1610 and he was the first to publish the discoveries that he and the others made there. However to describe these achievements, impressive as they were, as telescopic astronomy is somewhat stretching the point; as I wrote in my recent deflation of the Galileo myth what these people did is best described as wow-look-sensationalism rather than scientific astronomy. It worked on the basis of if you have a good enough instrument you can see all of these extraordinary things: moons of Jupiter, phases of Venus, lots of new stars, mountains on the moon and spots on the face of the sun. Very little attempt was made to make any real astronomical work with the new instrument and this remained the situation well into the second half of the 17th century.
The man who changed the situation and should really be credited as the founder of scientific telescopic astronomy is Giovanni Domenico Cassini who was born on the 8th June 1625 in Perinaldo in the Republic of Genova.
Motivated to study the heavens through his interest in astrology (a discipline he later rejected), a student of Riccioli and Grimaldi, Cassini established himself as one of the leading astronomers in Italy, becoming professor at the University of Bologna in 1650. Cassini became the leading observational astronomer of the age discovering the red spot and the flattened poles of Jupiter as well as determining its period of rotation and that of Mars. Because of his international reputation Louis XIV invited him to become director of the newly conceived Paris Observatory in 1668. Cassini accepted the post and oversaw the final construction of the observatory, which was opened in 1671.
In his new position Cassini established new standards for systematic telescopic observations, amongst other things determining the exact orbits of the moons of Jupiter and Saturn, four of which he discovered himself, the former allowing him to put into practice a scheme, original suggested by Galileo, using the moons of Jupiter as a clock to determine longitude, at least on land. This led to the famous comment by Louis that the cartographers had lost him more territory than all his wars because it turned out that the maps of France were seriously inaccurate in their longitude determinations and France was no where near as wide as had been thought. Ole Rømer also used orbital tables of the moons of Jupiter to determine the speed of light, which had been previously thought by most experts to be infinite. Cassini’s calculations also showed that the moons of Jupiter and Saturn obeyed Kepler’s third law of planetary motions, a fact used by Newton in his determination of the law of gravity. He explicitly names Cassini as the source of this information.
As well as effectively establishing the discipline of scientific telescopic astronomy Cassini and his pupils such as Picard and his own son modernised the disciplines of cartography and geodesy establishing new standards for accuracy. Cassini became such a part of the French establishment that he even changed his name to Jean-Dominique. The Cassinis became a French scientific dynasty with Jean-Dominique’s son, Jacques, succeeding him as observatory director at his death in 1712. Jacques was, in turn, succeeded by his son, César-Francois Cassini de Thury, in 1756. He in turn was succeeded by Jean-Dominique’s great grandson, Dominique Comte de Cassini who died in 1793 ending a more than 120-year reign of the Cassinis at the Paris Observatory. The four generations constructed a topological map of France the first such map of a complete country.