Waiting for Cassini

The Cassinian year will be an opportunity to celebrate and rediscover the importance of Cassini through various initiatives with a high scientific profile and wide dissemination events for students, scholars and the public. The celebrations will include events for the general public, including the observation of the passage of the solstice in the Meridian line of San Petronio in Bologna, public conferences dedicated to the astrophysical topics addressed by Cassini, and a historical-scientific conference on the figure of Cassini which will take place in Bologna on 18-20 June 2025.

International  Conference:

"Giovanni Domenico Cassini, European astronomer of the 17th century on the fourth centenary of his birth"

Location and date: Bologna (Italy), 18-20 June 2025

Topics for discussion will include:

- Cassini and his era: society and scientists,
- From the Republic of Letters to the European scientific network of Academies,
- Cassini's cosmos: visions of the Solar System from the 17th century to today,
- Observations of Mars, Jupiter, Saturn,
- Cassini, Halley and the comets,
- Cassini and the telescopes of the 17th century,
- Cassini between heaven and earth: the Pope's engineer and the King's cartographer,
- Cassini in Bologna: the ephemerides and the great meridian line in San Petronio,
- Cassini at the court of Louis XIV

To express interest to attend, we kindly ask to fill this pre-registration form

For further inquiries: cassini400@inaf.it

Portrait of Giovanni Domenico Cassini, Quadreria dell'Università degli Studi di Bologna.

Giovanni Domenico Cassini

Born in Perinaldo (IM) on 8 June 1625, he was the most illustrious astronomer of his time, so much so that the Sun King Louis XIV wanted him at his side from 1669 to found the Observatoire de Paris, the first modern astronomical observatory. Cassini remained director of the Royal Observatory of Paris until his death in 1712 and gave rise to a long dynasty of astronomers.

The year 2025 will mark the 400th anniversary of the birth of this most important Italian scientist. In view of this anniversary, the University of Bologna, together with the National Institute of Astrophysics in Rome and Bologna, the University of Genoa and the Bologna Academy of Sciences, intend to celebrate an astronomer who left a fundamental mark on the European astronomy of his time and on the city of Bologna.

Sundial of the Basilica of San Petronio, detail.

Cassini and Bologna

Cassini, in fact, was appointed to the chair of astronomy at the University of Bologna in 1650. It was thanks to the studies undertaken during his time in Bologna that Cassini built his worldwide reputation. Inside the Basilica of San Petronio in 1655, he built the largest and most precise sundial in the world with the declared intention of verifying the validity of the calendar reform introduced some 70 years earlier by Pope Gregory XIII.

Diagram of the sundial in the Basilica of San Petronio by G.D.Cassini

In fact, Cassini had designed the sundial as a true astronomical instrument, which he called a ‘heliometer’, and with it he conducted systematic measurements of the diameter of the sun's disc, which allowed him to provide one of the first experimental proofs of Kepler's second law, corroborating the hypothesis of the heliocentric system at the expense of the geocentric one more in vogue at the time proposed by the Danish astronomer Tycho Brahe.

Saturn

Cassini's astronomical research

Among Cassini's interests, the study of the solar system plays a privileged role. Cassini observed the surface of Jupiter, in particular the red spot. He discovered the separation of Saturn's rings that bears his name. He studied the motion of the satellites of the two giant planets of the solar system, in particular he discovered 4 new satellites of Saturn Japetus, Rhea, Dione, Tethys and produced tables of the motions of Jupiter's Medicean satellites that were used for decades for the fundamental determination of the Earth's longitude.

He observed the spots on the surface of Mars and calculated the planet's rotation period with great precision. He studied the motion of comets, paving the way for Halley's work on cometary orbits. He produced a detailed map of the Moon and studied the anomalies of lunar motion. He discovered the cause of zodiacal light, as an effect of reflection and scattering of sunlight on interplanetary dust in the plane of the ecclliptic. He improved the accuracy in measuring the value of the inclination of the eccliptic. He measured the parallax value of Mars and the Sun, giving the solar system its true dimensions.