Giovanni Domenico Cassini
Giovanni Domenico Cassini was born in 1625 in the Liguria region of northwestern coastal Italy. He was raised by his mother's brother, and showed a notable talent for mathematics and astronomy. His abilities in the sciences were strengthened by being educated by Jesuit scientists.
In 1648 a rich amateur astronomer built an observatory near Bologna, and hired Cassini to help him make accurate tables of the positions of celestial objects. The tables were to aid astrologers in their work. The young Cassini had been interested in astrology, but found the science of astronomy more absorbing, and astrology unconvincing.
Despite his comparative youth, Cassini's obvious competence led in 1650 to his appointment to the principal chair of astronomy at the University of Bologna.
The meridian sundial of San Petronio
Visitors to Bologna can still see the meridian sundial that Cassini engineered in the Basilica of San Petronio. Perhaps you've made a pinhole camera in school or to view a solar eclipse. Light enters a box through a tiny hole and projects the image on a screen. In the basilica, the image of the Sun comes through a pinhole gnomon in the church vaults 27 m (89 ft) above the floor. It's projected somewhere along the 66.8 m (219 ft) long meridian line on the basilica floor. The position depends on the angle of the noon Sun throughout the year.
The image of the Sun is large enough to allow quite precise measurements of the apparent change in the Sun's size during the year. Cassini's measurements showed the variation matched the prediction of Kepler's theory of elliptical orbits, not that of Ptolemy's Earth-centered model.
Hydraulics, engineering, Jupiter
In addition to his astronomical work, Cassini was well versed in hydraulics and engineering. He was consulted about river management, and appointed by Pope Alexander VII to supervise the waters of the papal states.
Cassini was the first to measure Jupiter's rotation period, and to see the planet's flattening at the poles. He was also first to observe many surface features on Jupiter, including the Great Red Spot.
He made extensive observations of Jupiter's moons and prepared tables of their movements. Although surprised to find some oddities in the timings of the moon positions, Cassini rejected the idea that they resulted from light having a finite speed. Danish astronomer Ole Rømer later use Cassini's tables to show that the speed of light is, estimating that it took eleven minutes for light from the Sun to reach the Earth, a few seconds more than the modern value.
In 1669 Cassini's accomplishments prompted the French king Louis XIV to invite him to help set up an observatory in Paris. Cassini was also invited to join the recently formed Royal Academy of Sciences, and then to become the first director of the observatory. He held the post for over forty years.
The Cassini Dynasty
In 1673 Cassini became a French citizen and changed his name to Jean-Dominique. A year later he married Geneviève de Laistre, the daughter of a prominent family.
Three generations of Cassinis succeeded Jean-Dominique as director of the Paris Observatory, creating the Cassini Dynasty and some historical confusion. To keep them straight, the former Giovanni is called Cassini I, his son Jacques is Cassini II, Jacques's son César François is Cassini III and César's son Jean-Dominique is Cassini IV.
Cassini discovered four of the moons of Saturn – Iapetus, Rhea, Tethys and Dione. He also found a gap separating Saturn's rings into two parts, now called the Cassini Division. Observing variations in brightness on Iapetus, Cassini concluded that one hemisphere was covered in dark material. This dark terrain has been named Cassini Regio.
Although Cassini's name is associated with Saturn, he also continued to study Jupiter. He found that different parts of of the planet rotated at different speeds, establishing that he wasn't seeing a solid surface.
The king of France didn't build an observatory to acquire abstract knowledge. He wanted his realm mapped, starting with an accurate meridian (line of longitude) going through Paris. The surveying would use celestial bodies as reference points.
Several decades earlier, Galileo had proposed a way of measuring longitude using the eclipses of the moons of Jupiter for timing. The theory was good, but it needed more accurate measurements of the Jovian eclipses than existed in Galileo's day.
However Cassini had made tables of the movements of the moons, so in the 1670s French astronomers could survey many locations in France by making observations of Jupiter's moons. In 1679 the completed work showed revised east-west boundaries for France. The country was somewhat smaller than had been thought previously. The king supposedly remarked that he'd lost more territory to his astronomers than to his enemies.
Cassini also began began the work of creating a topographical map for France which would be the most accurate map ever made of any country. It took four generations of Cassinis to complete it, finally published as the Carte de France, but also often known as the Carte de Cassini.
Jean-Dominique died in Paris in 1712 and his son Jacques became director of the Paris Observatory.
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