Jean-Dominique Cassini and "the oracle of Apollo"

Jean-Dominique Cassini was born an Italian gentleman at Perinaldo in the county of Nice on June 8, 1625. In his twenty-fifth year, in 1650, he was given the chair of astronomy at the University of Bologna. He owed this appointment to the powerful marquess of Malvasia (1603-1664 ), his sponsor, who also allowed him use of his private observatory at the castle of Panzano of which Cassini would later remark that it was the "Italian Uraniborg", in reference to the observatory of the famous astronomer Tycho Brahé ( 1546-1601 ), built on the island of Hven in Denmark.

Birthplace of Jean-Dominique Cassini in Perinaldo (Courtesy of Marina Muzzi)

During the winter solstice of the year 1652, the young Cassini observed and followed a comet that provided him with the material for his first treatise, De Cometa, which he wrote the following year. In it he stated that the star has no discernible parallax (i.e. the angle under which the Earth's radius can be seen from the object in question), therefore it must be located beyond the orbit of Saturn. This observation of the passing of a comet, which was, after all, relatively common, would mark the starting point of Cassini's research. He was, at the time, a firm supporter of the view of the system of the World as espoused by Tycho Brahé, according to which the Earth always stays at the center - whilst the planets circulate round the sun - unlike the heliocentric Copernican system. Nevertheless, being a scientist used to proceeding by experimentation, he decided to question nature itself; for that, he had to call upon the « oracle of Apollo », which was the name he gave to the great meridian line that he planned to build.

The opportunity to do so practically fell into his lap just a few years later when the major demolition and expansion works of the church of San Petronio were agreed by the Senate of Bologna. The church already had a meridian line drawn in 1575 by the Dominican priest Ignazio Danti (1536-1586), but it declines by more than 9°from the meridian axis. Cassini offered to build another one, provided with a gnomon of almost 27 m, which extended on the ground of the Basilica to a length of over 67 m to the point marking the winter solstice, while somehow successfully managing to make its way through the middle of the dense colonnade of the nave. This would yield the height of the Sun at any time of year with unprecedented accuracy! The Senate was reluctant but his patron was powerful. On the day of the summer solstice of the year 1655, Cassini organized a ceremony with great pomp and all due solemnity to mark the first time that the site was used to track the image of the Sun as it passed the meridian. Anybody who was anybody in Bolognese society was there. Among them were various Bolognese scholars who were already recognized figures in the field of astronomy, such as the Jesuit, Giovanni Riccioli (1598-1671) and his disciple, Francesco Grimaldi (1618-1663), who would earn renown a few years later, in 1665, thanks to his discovery of the phenomenon of diffraction of light (as published in his Physicomathesis de lumine, coloribus, et iride, 1665). In this staging of how the early work of laying out the great meridian line was performed, there is more than a touch of Galileo about Cassini.

Solar image around the winter solstice on the meridian line of the Basilica of San Petronio in Bologna taken on 14 December 2004 (photo courtesy of Giovanni Paltrinieri)

His great meridian, for which he completely rethought the principles of construction in order to obtain the necessary accuracy, was unlike any other built before. Cassini conceived of it as an astronomical instrument rather than merely as a tool for determining the dates of movable feasts or equinoxes. He also named it the "heliometer", so as to highlight its function as an instrument. From his meridian observations of the height of the Sun, he would go on to discover that atmospheric refraction does not stop after 45°, as Tycho had thought, and he would be led into deducing that the solar parallax is smaller than 10" (hence the Earth is seen from the Sun at a very small angle, measuring no more than 10 seconds of a degree, less than three thousandths of a degree!), thus putting the Sun at a far greater distance from the Earth than anyone had ever dared imagine, and he would also demonstrate the "bisection of the eccentricity" thanks to his measurements of solar diameter. This was to be the culmination of his work on the "theory of the Sun": that Kepler and Copernicus were both right ... Ultimately, his Bologna meridian would provide him with answers to all his questions and would radically change his view of the system of the World.

Cassini eventually left Bologna on February 25, 1669, to join the Royal Observatory in Paris on April 4, 1669, at the invitation of Colbert and on the recommendation of the astronomer, Jean Picard (1620-1682). After a short stay in the royal apartments of the Louvre, he moved over to the Royal Paris Observatory on 14 September 1671. He obtained his naturalization papers on June 14, 1673. From that date on, as far as his French civil status is concerned, he was named Jean-Dominique, which is the French translation of his name in Italian, Giovanni Domenico. However, he continued to sign his writings as those of Gio. Domenico; this being the way in which he is always referred to in Italy. In November 1673, he married Genevieve de Laistre, the "woman of his heart's desire," who bore him a son, Jacques, in 1677. He returned to Bologna only in 1695, accompanied by his son, Jacques, where he went to restore the Meridian. Cassini died in Paris on September 14, 1712, at the age of 87. He is buried not far from the Observatory, in the Church of St. Jacques du Haut-Pas: the inscription on his tombstone reads "Jean-Dominique Cassini - Astronomer."


    Engravings of the great meridian line restored by Cassini in the Basilica of San Petronio in Bologna, in La Meridiana del Tempio di S. Petronio, Gio: Domenico Cassini, 1695. (Library of the Paris Observatory)

  • The great meridian line in the Basilica of San Petronio in Bologna. (courtesy of Giovanni Paltrinieri)

  • Drawing of the meridian line by the hand of G.D. Cassini. (Meridiana di S. Petronio, riguardante la meridiana e datato 1653, Manuscrits kept in the Archivio di Stato di Bologna, Assunteria di Istituto, diversorum, b. 22, n. 4)