His results obtained from 1656 to
1659, made him famous as a mathematician and as an astronomer.
He gave a new value of the inclination of the ecliptic
of 23°29' 15 ' ( the best at his time!), a table of the atmospheric
refractions that remained the most accurate for over a century, and
he confirmed that the orbital velocity of the Earth was not uniform, in
line with Kepler's laws of planetary motion. The latter went
against aristotelician physics that assumed uniform movements in
the sky.
He was sent to Pope Alexander
VII, by the Senate of Bologna to arrange the difficulties that had arisen
between Bologna and Ferrara about the navigation and of the courses of
the rivers Po and Reno(1657-59), later on he was named superintendant
of the fortifications of Fort Urban and of the fortress of Perugia(1663),
and then he was delegated for solving the problems about the course of
river Chiana, alternate affluent of the Tiber and Arno(1663-67), which
brought into conflict the Pope and the Grand Duke of Tuscany.
All this time, he continued his astronomical
work, and devised a method for the determination of longitudes through
observing the solar eclipses (1661), the publication of which was censored
by the Inquisitor of Modena. It will be published later in
France (1670).
Using one of the first simple
long focal lenses, manufactured by Giuseppe Campani, he turned to the
study of planets.
He discovered the shadows of Jupiter's satellites
on that planet, which one took for spots, and deduced their
rotational period; he observed an "exceptional" spot (the Red
Spot), that he called the "big permanent spot" and which enabled him
to evaluate Jupiter's rotational period (9h56')(1665). He noticed the
flatness of the disc and started to draw up tables of Jupiter's
satellites(1668), for the determination of longitudes, which will be
improved in France (1693). He also evaluated Mars's rotational
period (24h40) (1666) and tried to find the more tricky one of Venus (23h20')(1667).
With all this work Cassini became famous
beyond the limits of Italy.
Louis XIVth, who wished " to make France as well
flourishing and illustrious by the letters as it was it by the
weapons", entrusted his minister Colbert with inviting him
to join the recently formed Royal Academy
of Sciences,
and urging him to come to Paris, at the Observatory which was in construction.
The Pope (Clement IX) , his employer agreed to lend
him to France, temporarily.
Cassini reached Paris in April 1669, and two years
later, he went to live in the Observatory. In 1673, at his request
he obtained the French nationality. He never returned to Italy, except
for personal trips, or observations. The same year he married Genevieve
de Laistre, daughter of the Count de Clermont's lieutenant-general, who
was a King's adviser, and bought the castle of Thury, near Beauvais,
which became his family residence and whose area was crossed- amazingly!-
by the Paris meridian.
As soon as Cassini arrived at the Observatory, he began
a series of observations of the lunar surface which was to lead to the
realization of an Atlas (1678), a large Map(1692), and to a theory of
the libration and three laws of the Moon rotation, that bear his
name.
At the same time as Richer in Cayenne(1672), he observed
the planet Mars at Paris Observatory , and thanks to his excellent
tables of refractions, he measured the Mars parallax and deduced from
it the Sun parallax to 9' ' 5 (correct to within 8 °/° ).
This value increased considerably the scale of the solar system (1674),
and gave to Jupiter and Saturn huge sizes. One still find the echo of
that event seventy years later, in the Voltaire's Micromégas.
But, above all, his name remains associated with the
planet Saturn.
Huygens had discovered the first Saturn satellite, Titan
(1655). Hardly settled at the Observatory, Cassini discovered two others,
Iapetus(1671) and Rhea(1672), then he discovered that the breadth of
the ring was divided into two parts, now known as 'Cassini's Division'(1675),
he saw for the first time, a cloudy strip parallel to its equator (1677),
the flatness of the disc and two other satellites, Dione and Thetys (1684).
Finally, he suggested that the ring, considered as
solid, could be formed " of a swarm of very small satellites with
various motions that cannot be seen separately" (1705).
This opinion was not adopted, even though it was reformulated
in the XVIIIth century by Thomas Wright and despite Laplace's mathematical
theory of a subdivision in many narrow rings. The Huygens' hypothesis,
of the solidity and flatness of the ring prevailed
until the mid-nineteenth century.
To pay homage to him, the consortium ESA-NASA,
called 'Mission Cassini,' the unit, including the spacecraft and
the Huygens probe, launched in 1997, to reach the Saturn system in July
2004.
We also owe an important geographical work
to Cassini . He took an active part in the measurement of the Earth,
by extending the meridian line established from Paris to Amiens by Jean
Picard, and he applied his tables of the Jupiter satellites to the determination
of longitudes.
Civil and Jesuit missionaries came to the Observatory
to be trained to his method of observation of the eclipses of the Jupiter
satellites, and sent to him the results of their observations made in
Africa, America or China. Cassini marked them on a large " geographical
drawing ", a planisphere of 7,80 meters in diameter, a pen-and-ink
sketch on the ground of a tower of the Observatory. Thus, the longitude
of the far-off countries could be corrected of more than 20° and that
of France of almost 3°.
According to his contemporaries, Cassini " the Great"
had a calm and gentle character, a warm nature and a happy and sharp
mind. He knew how to interest the King and his court in his work, and
to assert himself by his talent and qualities of organizer, among
renowned astronomers such as Jean Picard, Christian Huygens, Olaüs
Römer, Giacomo Felippo Maraldi, Philippe de La Hire, who were sometimes
called the "Paris school" with Cassini as leader.
Endowed with a sound constitution, he could carry on
his research until 1711, when he became blind completely, and died
in Paris Observatory , on september 14th of the
following year, being 87 years old.
He gave birth to a dynasty of astronomers and Cassini's
name remained attached to the history of Paris Observatory during more than
one century.
The CASSINI's dynasty
Jean Dominique(Giovanni Domenico), or Cassini I (1625-1712)
(1669-1712) **
Jacques, or Cassini II (1677-1756) (1712-1756)
César François Cassini III of Thury, (1714-1784)
(1756-1784)
Jean Dominique Cassini, or Cassini IV(1748-1845) (1784-1793)
* Jean-Dominique and Jacques were never appointed
as director of the Paris Observatory. The first to be designed was
César-François, on November 12, 1771. But Cassini I is worth
to be called a director, in regard with the part he plaid in the developments
of the French astronomy and the fame he brought to the Paris Observatory
** The underlined dates correspond to years spent
as leader or director of the Paris Observatory.
Raymonde BARTHALOT
______________________________________________________________________________
References :
J.D. CASSINI IV,
Mémoires pour Servir à
l'Histoire des Sciences et à celle de l'Observatoire de Paris,
suivis de la vie de J.D Cassini écrite par lui-même, et des
Eloges de plusieurs académiciens morts pendant la Révolution,
Paris 1810
J.B. DELAMBRE,
Histoire de l'Astronomie Moderne, 2 vol.,
Courcier lib. pour les Sciences, Paris, 1821.
A.F.0'D. ALEXANDER,
The Planet Saturn, A History of Observation,
Theory and Discovery, (Toronto, London 1962), Dover Publications,
New York, 1980
R.BARTHALOT,
The story of Paris Observatory, Sky and telescope,
59, Cambridge, Mass., 1980.
R. BARTHALOT,
L'Observatoire de Paris : Histoire, Science, politique
(1667-1795), Paris I-Panthéon-Sorbonne, 1982.
A.CASSINI,
Gio: Domenico Cassini, Uno scienziato del Seicento,
Testi e Documenti, Edizione a cura del Comune di Perinaldo,(Imperia),
1994.
E.BAIADA, F. BONOLIi, A.BRACCESSE,
L'astronomia a Bologna, in
Museo della Specola, Parte I, Universita degli Studi di Bologna,
Cisma, Dipartimento di astronomia, Bologna, 1995.
