In the year 1671, the director of the Paris Observatory, Italian-French mathematician & astronomer “Giovanni Domenico Cassini” argued to the “French Royal Academy of Sciences” that the longitude of Uraniborg (Tycho Brahe’s observatory) was the missing link to synchronize decades old observational logs & interpretations of Tycho Brahe (published & unpublished) with his recent observations which he had published in the year 1668 titled “Ephemerides Bononienses Mediceorum Siderum ex Hypothesibus et Tabulis” (Ephemerides of the Medicean Stars from Bologna, also called Cassini’s Tables) while he was still working in Bologna, Italy at the prestigious “University of Bologna” as the “Professor of Astronomy and Mathematics”. The “French Royal Academy of Sciences” would eventually agree to Cassini’s argument based on below 3 reasons :
1} The Longitude Problem: Even though Cassini’s published 1668 astronomical chart was considered to be highly accurate at that time, he knew that his “Tables of the Ephemerides of Jupiter’s Satellites (Galilean satellites : Io, Europa, Ganymede, and Callisto)” is not precise & only by knowing the exact coordinates of “Uraniborg” relative to Paris, can his ephemeris tables be completed with precision. In those times, many ships at sea would get lost due to the longitude problem. Knowing or measuring the latitude was relatively easy when compared to measuring longitude, as latitude could be known or measured by simply using the Sun’s altitude at noon, or the Pole star’s altitude at night. Sailors & Navigators would use a “Cross-staff” or “Backstaff (Davis quadrant)” or an “Astrolabe” for latitude measurement but knowing the longitude required “Comparison of Time”, so one need to know the true local time on ship along with an exact time at a reference place, but the problem was, there weren’t any accurate sea-worthy clocks invented until the 18th century, also pendulum clocks would fail badly on ships & there was no any universal prime meridian back then, & every nation would use their own zero-longitude. A few of the common references for the prime meridian used by European kingdoms in the 17th century were “Canary Islands (especially El Hierro), Paris, Lisbon, Toledo, Rome”. Cassini understood this longitude problem long back & by publishing his astronomical ephemeris tables, he wanted to provide a universal clock in the sky that would be visible from any place on Earth & would assist the sailors, navigators & even the cartographers–making maps & charts to calculate their longitude in the seas & on land. But Cassini also realized that his tables logs were still out of sync with decades old Tycho Brahe’s star catalog data as Tycho Brahe wrote almost all his observations of his 1000 star catalog, from his Uraniborg & Stjerneborg observatories on the island of Hven in Denmark, but since no one knew of the longitude coordinate of these observatories, all of the star positions in the catalog were offset. To fix this & improve ship navigation, along with map-making, it was necessary to calculate the exact longitude difference between Paris & Uraniborg/Stjerneborg (Tycho Brahe’s on-ground & underground observatories, respectively), both located on the island of Hven. Astronomers & navigators had a decent idea of Hven’s latitude, but the longitude coordinate was essentially a “scientific ghost”.
Note : On June 21st, 1667, which was the summer solstice day, at the exact moment of solar noon, with the Sun crossing the meridian, the members of the newly founded “Royal Academy of Sciences” would trace a precise “North-South” line on the ground using the shadow of the Sun, thus establishing the Zero-Point or the Paris Meridian. Once this North-South line was marked, the first stone, or the foundation stone, was laid by Jean-Baptiste Colbert, the first minister of the French crown, marking the beginning of the establishment of the Paris Observatory. After the foundation stone was laid, both the team of architects & the members of the “French Royal Academy of Sciences” unanimously agreed that the entire observatory building must be perfectly symmetrical around this meridian line, thus making the observatory act as a scientific instrument. Key dignitaries who were present at this foundation ceremony were “Jean-Baptiste Colbert, King’s 1st minister”, “Claude Perrault, the assigned chief architect for this project”, & the Scientific Academy’s astronomers & scientists along with French astronomer, priest & geodesy expert “Jean Picard”, who was the main scientist among the members who used his instruments for the precise measurement of the North-South axis of the observatory site on that day. King Louis XIV decided to give a miss to this foundation ceremony even though he provided all the necessary funding for the observatory. Also, chief architect Claude Perrault, while designing the Paris Observatory made sure that NO IRON is used in the construction of the building as scientists & astronomers of that time feared that using iron in construction like large iron bars & clamps may create local magnetic noise which may lead to loosing precision or inaccurate readings from the scientific instruments which were incredibly sensitive in those days. To nullify this fear of Magnetism Perrault used the method of “Stereotomy” which is the art of precise stone cutting & locking these stones together by their own weight and geometry.
2} Map Correction: In relation to the longitude problem, there was the issue of not having the right map. Back then France had its maps which were artistic, for example; the famous “Postes de France” (Posts of France) prepared by renowned French cartographer “Nicolas Sanson” (also called “Father of French cartography”) published in the year 1632 by French engraver, printmaker & publisher “Melchior Tavernier” under the kingship of “King Louis XIII”. But these artistic maps were all based on “dead reckoning”, meaning estimating distance & direction by how long it took a horse or a ship to travel rather than using any astronomical observations. Also, pre 1671, the many artistic maps of France were outrageously stretched & distorted, with the Mediterranean coast and the Atlantic coast placed wider apart than they actually were. This was because the French cartographers before 1671 were unaware of the true width of the country, which resulted in cities being placed 20 to 50 miles away from their true location. This was also a nightmare for the French Navy; many times, sailors would believe they were miles out into the sea but were dangerously close to the shore. To put an end to this misery, Cassini & his associates in the “French Royal Academy of Sciences” wanted not just to fix the French map but build a global master key by which any country map can be made with precision & thus was planned this expedition to Denmark, Uraniborg by Cassini & approved by the Academy in 1671. The recipe to make an accurate map includes only two ingredients, 1st is to have a “Zero Point” which was the “Paris Observatory” itself & 2nd to have an “Accurate Star Log”. Tycho Brahe’s decade-old star catalogue, which was still the most popular & most referred star catalogue, had the issue of not knowing the longitude of the source position from where the observations were made in preparing the star catalogue. As a result, all the stars’ positions in Tycho Brahe’s star catalogue were now offset & needed correction. And so, correction of maps or making new precise maps again came down to knowing the longitude difference between Paris & Denmark,Uraniborg/Stjerneborg (Tycho Brahe’s observatory), such that exact distance between these two places can be measured & with that, Tycho Brahe’s old star catalogue data indexed at his observatories Uraniborg & Stjerneborg as the local zero-point can be dragged back to the “Paris observatory Zero Point”, thus achieving the missing sync between Tycho Brahe’s star catalogue & Cassini’s astronomical charts.
3} Calibrating Cassini’s Jupiter’s Clock: Cassini wanted to officially test his published 1668 ephemeris table by measuring the accurate distance between Paris & Uraniborg/Stjerneborg by 1st measuring the longitude difference between these places, as latitude was already known. To measure the longitude difference, Cassini had planned to use the event of eclipses of Jupiter’s innermost moon, Io. The academy agreed to Cassini’s argument & saw this “Jupiter clock” method as the perfect beta test towards preparing an “universal clock” that can be used by the French Navy & merchant ships in the future.
4} King’s Minister Jean-Baptiste Colbert’s Objective: The minister wanted France to become the global leader in scientific research in all fields, especially astronomy, physics, mathematics, navigation & cartography. He knew, if the scientists & astronomers of the “French Royal Academy of Sciences” were able to calculate the longitude coordinate of Uraniborg/Stjerneborg (Tycho Brahe’s observatories) & sync Cassini’s astronomical charts with Tycho Brahe’s decades’ old star catalog & remove any errors of these logs & astronomy tables, then that would give France the edge over other European kingdoms in the race of scientific research with printing press & exploration with accurate navigation. Colbert specifically desired the shift of the scientific “centre of gravity” from Denmark to Paris, France.
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