Italian Masters: Cassini – Expert Engineer and Astronomer

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Figure 1: Giovanni Domenico Cassini

We’ve been taking a break from hard-hitting mechanical engineering and materials science blogs with some pieces on the Italian masters of science, mathematics and engineering in the 16-19th centuries.  In previous blogs, I explored the lives and contributions of mathematician/physicist Evangelista Torricelli and physicist/engineer Giovanni Venturi.  For this blog, I’m focusing on Giovanni Domenico Cassini (Figure 1), a structural engineer and astronomer extraordinaire in the late 17th century.  Cassini started small, studying astrology (not astronomy!) early on in his career, but his fascination with the actual scientific properties of our celestial neighbors and his lifelong dedication to watching the night sky led him to a number of great discoveries.

Giovanni Domenico Cassini (1625-1712)

Giovanni Domenico Cassini was born in 1625 in Perinaldo, on the Mediterranean coast near the border with France.  In his youth, he studied mathematics, astronomy and astrology at a series of Jesuit colleges.  His initial passion was astrology, an interest which earned him important positions later in life even as he became more convinced it had no scientific backing.  Cassini was appointed as professor of mathematics and astronomy at the University of Bologna in 1650, and in 1669 was chosen by Louis XIV to set up and run the Paris Observatory.  Cassini remained the director of the Paris Observatory, as well as Louis XIV’s personal astronomer and astrologer, until his death in 1712.

A Dedicated Astronomer

Saturn_from_Cassini_Orbiter_2004-10-06Figure 2: Saturn, taken by the Cassini Orbiter in 2004 (click the image for breathtaking full-size)
NASA/JPL/Space Science Institute

Cassini spent over 50 years watching the night sky, starting with his assignment as an observer at the small Panzano Observatory in 1648.  Cassini made a number of significant astronomical discoveries during his tenure at Panzano and Paris.  He discovered four of Saturn’s moons, Iapetus (1671), Rhea (1672), Tethys (1684), and Dione (1684), and made the first observations of the gap in Saturn’s rings, which is now called the Cassini Division.  He also studies the paths of comets, proposing that they traveled in orbits around the sun.  His work extended to Mars as well, creating a detailed map and calculating its period of rotation.  Cassini sent a colleage to French Guiana, and the two of them took measurements on the position of Mars in the sky at precisely the same time one night.  By determining the distance between the two observers on Earth and the anglular difference in their lines of sight to Mars, Cassini was able to use trigonometric parallax to calculate the distance from Earth to Mars.  This was one of the first calculations that gave a true sense of the scale of the solar system.

Cassini also spent significant time observing Jupiter, discovering the Great Red Spot and the banded weather patterns and calculating its period or rotation.  His careful observations of the Jovian satellites let him accurately utilize a method of determining longitude, invented but untested by Galileo Galilei decades earlier; (the methods of calculating longitude at the time relied on knowing precisely what the time was, and the Jovian moons’ predictable orbits meant they could be used as a universal clock).  In a prescient observation, Cassini actual hypothesized that some discrepancies in his Jovial observations were the result of light traveling at a finite speed.  He came very close to calculating the correct value, estimating 10-11 minutes for light to travel half of the Earth’s orbit; we now know it takes 8 minutes and 20 seconds.  Although Cassini ultimately dismissed the idea, seeking another explanation that fit conventional wisdom, his data was later used in 1676 by Danish astronomer Ole Rømer when he made the first accurate calculations of the speed of light.

Hydraulic Engineering, Structural Engineering, and Cartography

Cassini-topo-mapFigure 3: Cassini’s completed topographic map of France

Cassini was also well-respected for his work as an expert engineer in structural and hydraulic matters.  The same precision and attention to details that served him so well in astronomy also lent itself to engineering endeavors.  He served as Pope Clement IX’s superintendent of fortifications in 1663, and later as his superintendent of waters in 1665.  The same meticulousness, combined with his longitude-measuring system, also led Cassini to practice and perfect cartography.  He spent the latter part of his life working on an exquisitely-detailed topographical map of the entirety of France.  The project was ultimately passed on to his son and grandson, but when they finally published it in the late 1700s it was the first accurate topographical map of a whole country, shown in Figure 3.

Lasting Impact

Cassini’s name has reappeared on the public’s radar due to the resounding success of the Cassini-Huygens spaceprobe mission.  Cassini-Huygens is a twin probe that entered orbit around Saturn in 2004 after a sojourn past Jupiter.  Huygens landed on the surface of Saturn’s moon Titan and broadcast data for 90 minutes, but Cassini has remained in orbit for 12 years now, sending astounding and beautiful images of Saturn and its moons, like Figure 2 above.

Although Cassini doesn’t have any amazing inventions to his name, nor a unit of measurement named after him, he did help propel the field of astronomy at its astounding pace of development in the 17th century.  Considering the celestial bodies he discovered and mapped and the calculations of their behaviors and distances, Cassini helped give the world a better sense of their place in the cosmos (or even just their place in France, with the help of his family’s map).  Cassini was one of the first to think beyond our little ball of dirt and try to figure out just how much larger our universe might be.


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