Cassini’s astronomical charts were meant to be used to predict the precise positions and eclipse times of Jupiter’s four largest moons (the Galilean satellites : Io, Europa, Ganymede, and Callisto). More precisely, Cassini designed these charts to solve the problem of determining terrestrial longitude. Cassini understood that providing a “universal clock” in the sky would allow explorers & cartographers to compare their local eclipse times with the times listed in his tables & be able to calculate their exact east-west position. While preparing these tables, Cassini made use of high-quality lenses which he acquired from the most famous & celebrated lens maker in Rome at that time, Italian astronomer & optician Giuseppe Campani, which allowed him to observe the moons of Jupiter (Io, Europa, Ganymede, and Callisto) with far greater detail than his predecessors, including Galileo. Soon after publication, Cassini’s Tables gained traction among astronomers & many scientific communities within Europe & in no time, these tables were considered the most accurate astronomical charts of the time, predicting the movements and eclipses of Jupiter’s four largest moons, as previously mentioned. The success of these tables was so significant that in the same year 1668, King Louis XIV, the then King of France, at the suggestion of his minister Jean-Baptiste Colbert (1st Minister of State), sent invitation to Cassini, wanting him to recruit to lead the newly founded “Académie Royale des Sciences” (French Royal Academy of Sciences, founded in 1666) & the observatory project (Paris Observatory, founded in 1667). Cassini was offered a generous annual salary of 9,000 livres ($500k to over $1M+ USD at present-day value) with free accommodation & a significant travel allowance to oversee the design and construction of the Paris Observatory, which was still under construction. Cassini was more than happy to accept this offer. Still, being an influential person in Italy, before accepting the offer, Cassini needed formal concurrence from Pope Clement IX and the Senate of Bologna. Both the Pope & the Senate gave their consent under the belief that Cassini would be gone for not more than 2 years. Having received the necessary approvals, Cassini departed from Bologna on February 25, 1669, & after 40 days arrived at the French capital Paris on April 4, 1669. Two days later, on the 6th of April, 1669, Cassini was formally presented to King Louis XIV at the “Tuileries Palace”, who received him with great distinction. Now, since the Paris Observatory was still under construction at the time of his arrival, he was initially lodged in the “Palace of the Louvre”. Later on, upon the completion of the building, Cassini moved into the Paris Observatory as its de facto director on September 14th 1671, before which he was already leading the “Académie Royale des Sciences” (French Royal Academy of Sciences) since 1669.
Since his arrival in 1669, while Paris Observatory was still being constructed, Cassini took upon himself to convince King Louis XIV to employ Giuseppe Campani,” the renowned optician from Italy, for lens & telescopes for the Paris Observatory. Cassini convinced the French crown that Campani’s instruments were superior to all other optical instrument makers in the whole of Europe & so Campani was commissioned as a supplier of the finest lenses, telescopes & other optical instruments to France. Campani’s primary work was to make long telescopes & varying large focal length lenses specifically for the Paris Observatory. Lenses, some reaching focal lengths of 100, 136, and 200 palms (up to 45 meters), became the standard equipment for the Paris Observatory. After the opening of the Paris Observatory in 1671, King Louis XIV tested the telescopes himself & was so impressed with Campani’s lenses that he offered him a hefty compensation if he would “communicate his secret”, meaning his lens-grinding techniques while making the lenses & send it to France, but Campani courteously refused the King’s offer & chose to keep his methods private. Staying in Rome, Campani was handsomely paid for his exceptional work in optics for the Paris Observatory. Giuseppe Campani remained the primary supplier of lenses and telescopes for the Paris Observatory & other prominent figures for nearly his entire life till his death in 1715. Below is a list of the different lenses & telescopes made by “Giuseppe Campani” for the Paris Observatory & used by “Giovanni Domenico Cassini” while both were employed by the French crown
1} “Giovanni Domenico Cassini” carried his favourite “17-foot refractive telescope”, meaning a telescope with a focal length of 17 feet (approx. 5.2 meters) while coming to Paris from Bologna in April 1669. The telescope was made by Campani even before 1665 & Cassini used it to discover the eclipses of the Galilean satellites (Io, Europa, Ganymede, and Callisto), and the Great Red Spot on Jupiter in the year 1665. In the same year, Cassini calculated Jupiter’s rotational period & in the following year, 1666, Mars rotational period. Since his arrival in Paris in 1669, as the Paris Observatory building was under construction then, Cassini used this telescope in the gardens of the “Abbey of St. Germain” & later at the “Palace of the Louvre”, where he stayed till 1671 until the observatory building was fully constructed. Cassini also discovered Saturn’s moon Lapetus on October 25, 1671, using this “17-foot refractive telescope”
2} Once the construction of the Paris Observatory was completed, the 1st Minister of State, “Jean-Baptiste Colbert”, ordered more lenses with larger focal lengths & within a period of just over a year, Campani had made a 34/35 foot (approx. 11 meters) lens which the Paris Observatory received in December 1672. This 34/35 foot lens was mounted on a 34-foot tube (11 meters), which then could be moved using a pulley system to point stars, planets & other celestial objects in the sky. Using this 34/35 foot lens, Cassini was able to discover the moon Rhea (the second-largest moon of Saturn) in the same month & year, December 1672 & the Cassini Division in Saturn’s rings (a 4,800-kilometre-wide {2,980-mile} gap separating Saturn’s A and B rings) in the year 1675.
3} Following the success of the 34/35 foot lens, 80, 90, 100 & 136 foot lenses were commissioned post 1680 by the French crown. These large-sized objectives could not be mounted on a telescope tube & so Cassini had to prepare an Aeriel Telescope apparatus using a connecting string & a pole, in order to make use of these lenses as telescope objectives. The 80 & 90-foot lenses were received at the Paris Observatory somewhere between 1682 & 1683. Installing & mounting these large-sized lenses on a tube was never an option, instead Cassini used a “Great Mast”, meaning a massive wooden pole & connecting strings to hold the 80 & 90-foot lenses. Cassini & his team had to face many technical difficulties in setting up this aerial telescope apparatus, especially with alignment. To be able to see anything, the eyepiece held by Cassini & the objective lens 80/90 feet away on the Great Mast had to be perfectly aligned along the same optical axis. But in total darkness, if the lens was tilted even a fraction of a degree, then light from the planet would miss the eyepiece entirely. Cassini, with his team, would mount the 80 & 90-foot lens on a ball-and-socket joint on the wooden pole. Cassini would then pull the connecting string to tilt the lens 80/90 feet away until light from the pointed celestial object “hit” his eyepiece. Another problem that arose with the Aeriel Telescope setup was with “Keeping the lens steady & minimising vibrations”, since the objective lens was perched on a tall, thin mast; even a slight breeze would cause the lens to sway. As a result, the image of the celestial object (Saturn specifically, as Cassini then was primarily focused on exploring the Saturnian system) would dance wildly or disappear entirely from view. To counter this, Cassini & his team developed complex pulley systems and heavy weights to stabilise the masts, but observations were often impossible on anything but the calmest nights. To add more, adjusting the height of the lens for a planet rising in the sky required a team of assistants manually cranking winches and pulleys in the dark, often following Cassini’s shouted commands from 80 feet away. Adding more to this precarious setup, on cold nights, moisture would condense on this massive Campani lens (80/90 foot lens) & since the lens was 80 feet in the air, the team couldn’t just wipe the moisture off. As a result, Cassini’s team often had to wait for the temperature to stabilize or seldom would they lower the mast & clean the lens & again redo the entire setup to realign the lens until light from the concerned distant celestial object falls perfectly on the eyepiece.
Note : Cassini, along with his team, would use dim lanterns or candle lights to see their astronomy charts & adjust the pulleys, as if the light was too bright, then their eyes wouldn’t be “dark-adapted” enough to see the faint moons of Saturn.
4} Spring of 1684, Cassini received the 100 & 136-foot lenses from Rome, made by Campani. Cassini & his team had the lenses mounted on the Great Mast (a massive wooden pole) located on the grounds of the Paris Observatory. Using this risky, perilous setup, balancing a lens over 100 feet in the air on a single pole, Cassini was successfully able to spot the two moons of Saturn, “Tethys and Dione” in March 1684. The larger 136-foot lens was used immediately afterwards on the same mast setup to verify the observations made with the 100-foot lens & thus Cassini’s discovery of 2 new moons of Saturn, “Tethys and Dione” was confirmed. After this discovery, Cassini did a royal dedication by naming his 4 discoveries of Saturn’s satellites (Lapetus-1671, Rhea-1672, Tethys & Dione-1684) as the “Sidera Lodoicea” (Stars of Louis) to honour King Louis XIV. Since observation using the Great Mast was becoming more challenging each day, in 1685, on Cassini’s request, King Louis XIV made arrangements to have a large wooden tower moved from the village of Marly onto the Observatory grounds. Originally, this tower was a critical component of the “Machine de Marly”, a massive complex system consisting of 14 giant paddle wheels & over 250 pumps designed to lift water from the River Seine, where this wooden tower served as a summit reservoir tower at the top of the mountain. Water was then pumped into the reservoir at the top of the tower, from where it flowed by gravity into the “Louveciennes Aqueduct” & eventually fed the fountains & gardens of the “Palace of Versailles” (the then de facto capital & seat of power of the Kingdom of France) & the smaller “Château de Marly”. After reaching the observatory grounds, the Marly Tower was repurposed to provide a more stable platform for the “aerial” observations using the longest & largest lenses (100-foot, 136-foot & later even 150+-foot lenses). Once the Marly Tower was fully operational in the late 1680s, the 80-foot & 90-foot lenses had largely been superseded by these even more powerful 100-foot, 136-foot, & more larger lenses. Also, the Marly Tower could be controlled with much more ease with manual control via a system of pulleys and internal stairs.
5} Following the 1684 success with the 100-foot and 136-foot lenses, the French crown’s demand for Giuseppe Campani’s optics reached its peak. In the late 1680s and 1690s, King Louis XIV commissioned even larger “objective” lenses. The 150-Palm Lens (~110-foot lens) was commissioned to supplement the 100-foot lens. The 150-Palm lens was primarily used for high-resolution studies of the Cassini Division in Saturn’s rings & also for detailed mapping of the moon’s surface.
6} The 200-Palm Lens (~146–150 foot lens) was the “giant” of the entire collection. Cassini used it on the Marly Tower to push the boundaries of the known solar system. While it offered incredible magnification, the “aerial” alignment (connecting an eyepiece to a lens 150 feet away via a string) was so difficult that it was only used on the clearest, most windless nights.
7} In the 1690’s, Campani also sent several “screw-barrel” microscopes (Compound Microscopes) to Paris. Cassini, with his assistants of the observatory & other members of the “French Royal Academy of Sciences”, used these to examine the micrometre wires used in their telescope eyepieces. To measure the diameter of planets accurately, they needed perfectly spaced silk threads, which could only be verified and calibrated under this Campani Compound Microscope.
Links :
Giovanni Domenico Cassini “Ephemerides Bononienses Mediceorum Siderum ex Hypothesibus et Tabulis” ==> https://historica.unibo.it/entities/publication/96397806-0c63-43f1-9a64-a5eaf2172736/viewer/iiif?canvasId=5a0be72b-2057-4991-88bb-4c6dbbf56a1b&canvasIndex=56 (in Italian)
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