Feature

Giovanni Domenico Cassini, 1625-1712

Here Cassini was able to indulge his passion for the skies using the Marquis’s instruments and he completed his education being taught by Jesuit scientists. Cassini’s work was exceptional in its quality and precision and this was the grounding for later prestigious academic positions.

As a young man in his thirties, Cassini then worked for the Bolognese government and also held the chair at the University of Bologna. His early work included observations of the Sun but, as he obtained more powerful telescopes, he became more interested in the planets.

Cassini needed bigger buildings to house his new telescopes, and one was the San Petronio Cathedral. Here he placed his instrument, called the 'Meridiana', which could compute the exact date for Easter. In 1666, after observations of Mars, he found the value for Mars’s rotational period to be 24 hours 40 minutes. We now know it to be 24 hours 37 minutes 22.6 seconds.

In 1668 Cassini compiled tables showing the positions of Jupiter’s satellites and this was used by the Danish astronomer Ole Rømer to establish that the speed of light is finite.

In 1669 on hearing of Cassini’s work and discoveries, King Louis XIV of France invited him to Paris to join the recently formed Acadèmie Royale des Sciences. In 1671, Cassini held the directorship to the Observatoire de Paris and two years later became a French citizen, changing his name to Jean-Dominique.  
 

	Cassini's Sun calendar in the Basilica of San Petronio

Cassini discovered the Saturnian satellites Lapetus in 1671, Rhea in 1672, and both Tethys and Dione in 1684. In 1675 he discovered what is known today as the 'Cassini Division', the narrow gap separating Saturn's rings into two parts, and that the rings were in fact swarms of tiny moonlets which were too small to be seen individually.

After a study of zodiacal light, in 1683 he judged that this was of cosmic origin and not a meteorological phenomenon as some had thought. Also, in the same year, he began the measurement of the arc of the meridian (longitude line) through Paris. He concluded that Earth was elongated, like a rugby ball, whereas it is actually wider around the equator.
 
 

Technicians join Huygens to Cassini

He died in Paris in 1712, after starting the long and distinguished dynasty of French astronomers working at the Observatoire de Paris.

Cassini was a traditionalist by nature and, although he accepted some theories but also dismissed others which have since proved to be true, he stands among some of the most important scientists of the 17th and 18th centuries.

The joint NASA/ESA/ASI spacecraft due to arrive at Saturn in 2004 now bears his name - the NASA/ESA/ASI Cassini-Huygens mission.
 
 
Last update: 18 June 2009