Cassini started his studies in Genoa at the Jesuit College. He was a brilliant pupil, well up in literary [he loved Latin,], and in scientific subjects. 

At the Abbey of San Fruttuoso [near Camogli], under the guidance of Abbot Doria, he thoroughly studied the Rudolphine Tables, the Alfonsine Tables and the Elements of Euclid. 

Soon after, Senator Giambattista Baliani, a man with broad views and very much open minded toward science, in close relationship by letter with Galileo, became an important teacher for Cassini, who discovered how flexible this noble man was and how open his opinions were, an attitude and an approach so much different to that of the Jesuits, that, however, Cassini always remembered with pleasure during all his life. 

The virtues of Cassini became known and the support of many friends opened to him the doors of the prestigious University of Bologna, where he was given the chair of Astronomy in 1651. 

Cassini – observation of a comet ... 

In 1652, Cassini was able to observe a comet that passed over Bologna. Its position was nearly vertical over the city, an event that favoured the quality of the observation. 

He assumed the orbit was too much eccentric and far away to compute the parallax. Moreover, he assumed it had the same type of motion of other celestial bodies [confirming some of Tycho’s assumptions]. 
 
 
 
 
 
 
 
 

The Heliometer and the motion of the Sun  

Urgent and important problems were related to the accuracy of the motion of the Sun, i.e. the dates of the Liturgical Calendar. 

An unexpected opportunity arose all of a sudden: in Bologna, San Petronio, was undergoing great restoration works and the old Danti’s Sun-dial was to be done away with.. 

Cassini aware of this happening, understood there was enough space for a new instrument, which he called "heliometrum": it had gigantic proportions. The local meridian line was traced on the floor. The opening cerimony took place on 21 June 1655, the Summer solstice. 

With the Heliometer he proved Kepler was right 

Measuring the diameter of the Sun at the two solstices, Cassini could measure the eccentricity of the Solar orbit around the Earth, coinciding with that "bisected", foreseen by Kepler. Undoubtedly, the validity of Kepler’s theories, at times so much criticized, was "scientifically" confirmed. 

The obliquity of the ecliptic was measured with the Heliometer  

The obliquity of the ecliptic is given by the angles the Sun traces at the two solstices. Its measurement was not difficult to take, but the nearly 67 metres long heliometer allowed an accuracy never reached before. 
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Cassini established the period of revolution of the Medicean’s planets and determined the rotation periods of Jupiter and Mars  

Cassini measured with extreme accuracy the apparition and the disappearance of the Medicean planets. The observation of their reciprocal eclipses allowed him to precisely determine the longitude, thus solving one of the biggest problems of his time. 

The Medicean planets are an "unparalleled watch", visible from Earth, from different positions that can also be far away from each other: indeed, we can use them as an astronomical Harrison chronometer.  

Cassini, also, observed that the transit of the shadow of the satellite "Io" is delayed when the Earth is moving away from Jupiter and anticipated when the Earth is approaching Jupiter. 

At first, Cassini assumed this phenomenon was due to the "finite" velocity of light, but abandoned this hypothesis, because with the other three satellites he was not able to observe a similar situation. Cassini’s assumption was not correct, as he did not understand that the reason was not the absence of the phenomenon, but the intrinsic difficulty required by the measurement, due to the slowness at which these satellites passed by. 

His assistant Roemer, on the contrary, in no time, availed himself of this opportunity and determined the speed of light with good accuracy. 

These careful observations allowed Cassini to discover Jupiter’s superficial formations and to determine Jupiter’s rotation period. Cassini also determined the rotation of Mars. The picture shows some of the Cassini’s drawings of the Martian formations. 
    
 
 
 
 

Blood transfusions   

" In 1667 Cassini gave news of his experiments on blood transfusion and on the observations he had carried out on insects. 
In two letters, both published in the Summer of year 1667, one addressed to Berlingiero Gessi, Ambassador in Rome of the Bolognese Government, and the other one addressed to Michele Giustiniani, a historian, Cassini announced the success of his experiments on two lambs and described in detail all the steps of the experiment and the innovative techniques he had followed, i.e. the use he made of the hen’s feather quill, in place of the metal cannula , which allowed, due to its transparence, to visualize the flow of the blood, and to regulate it accordingly" (*). 

The zodiacal light 

At the beginning of Spring, in 1668, he observed a luminescence, triangular in shape, on the ecliptic plane, very much alike to the Milky Way. He called it "zodiacal light". He could not understand the phenomenon. Today we know it is due to residual interplanetary dust stagnating on the ecliptic, that, in particular conditions, becomes visible from Earth, when illuminated by the Sun. 

Cassini, cartographer of France 

" It was a back-breaking task, that on which Cassini started to work since some years trying to improve the determination of longitude, so as to draw maps always more precise and detailed. His method made use of the eclipses of Jupiter’s satellites. This was the starting point onto which his great cartographical work was based. It was developed, day after day, with the contribution of scientific groups: many of them working far away from Paris; all of them, in a certain way, "co-ordinated" by the "Observatoire". The French King, and his Minister Colbert, attaching primary importance to this problem, subsidized the project with great generosity. Cassini was extremely meticulous and a ... great organizer ". (*) 
 
z  (*) Bibliography: Anna Cassini - "Un'avventura tra le stelle - Gio. Domenico Cassini da Perinaldo alla corte del Re Sole", 
                                 published by: Comune di Perinaldo 
 
 
 
 
 
 
 
 
 

Cassini - observation and drawings of the Solar spots 
  Cassini discovered some of Saturn’s satellites - the famous "division" between the rings  

The simultaneous observation of Mars, with respect to stars nearby, carried out by Cassini in Paris and by Jean Richer at Cayenne (French Guiana), allowed to compute the parallax.  
 
 
 
 
 
 
 
 
 
 

Cassini studied the motion and the orography of the Moon  

Cassini measured the oscillations of the Moon, called "librations" and explained their origin. He also analyzed the particular motion of the Moon in synchronous resonance with the Earth [the Moon always points the same face toward the Earth, because its motions of rotation and revolution are same]. These studies allowed the formulation of significant statements in celestial mechanics, known as "the three laws of Cassini". In 1679, Cassini completed the great Map of the Moon. 
 

z  z  A Map of the Moon was put down by Cassini between 14 Sept. 1671 (Barthalot, 1982, p 48) and the beginning of 1679   On Sat. 18 Feb. 1679 the imprinting was presented to the "Académie" members   (i.e. during the weekly meeting of the    Physics group).    source:   http://www.leidenuniv.nl/fsw/verduin/  stathist/huygens/acad1666/index.html

Map of the Moon by Cassini 2nd edition: the 1st edition is dated 1679. Imprinted by Jean Patigny.  The second edition is dated 1787.   20 'Pouces de Paris' (54 cm) - diameter.  This second edition is same as the first one, except for the text:  "Carte de la Lune ... de Jean    Dominique Cassini"  Imprinted on the back side.  source:   http://www.leidenuniv.nl/fsw/verduin/  stathist/huygens/acad1666/index.html

 
 
 
 

Cassini presents the four fundamental laws of celestial mechanics 

These laws make reference to a body in resonance (rotation – orbit) with its main body of reference [i.e. Earth and Moon or Sun and Mercury]. The laws are based on the hypothesis of a Keplerian orbit (reference is made to Mercury), with a constant precession of the nodes and of the pericentre. 

(a) The body has an uniform rotation about the principal axis of inertia and the rotational angular velocity is equal or very near to a 3/2 n. 
(b) The direction of the rotational axis of the body and the normal to the orbital plane form a constant angle. 
(c) The direction of the rotational axis of the body, the normal to the orbital plane and the axis of the orbital precession are coplanar.  
     This is trivial in the case of simple Keplerian motion. It is true with precession, and can be demonstrated algebraically. 
(d) Whenever a body passes at the pericentre of its orbit, the axe of minimum moment of inertia is facing the Sun.  

Cassini, today, would be a "top manager" 

The way he set up the "Observatoire", an astronomical centre in continuous contact with other excellent organizations around the world; the way he was able to co-ordinate simultaneous observations such as that of Mars, emphasize his extraordinary managerial qualities and his pragmatical approach to matters. His cartographical and hydrogeological commitment was an effort of titanic dimention: measurements were taken in very severe environmental conditions, both for men and instruments. For the latter, Cassini developed special engineering techniques, an anticipation of the so called "rugged", made instruments for military use, resistant to ambient and mechanical stress. A man of great intelligence that honoured his Country: Liguria ... 
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