When visiting Bologna I entered the Basilica San Petronia and remembered a presentation by Patrick from Helios on the famous meridian line.
One of the largest meridian line is located in this Basilica San Petronia in Bologna (Italy). It was constructed by (Gian Dominica) Cassini and the meridian line measures 66,8m. The sun light, is entering through a 27,07mm pinhole located at 27,07m height in the church's wall.
The meridian line was constructed to continue checking the dates of the Gregorian calendar, but Cassini also used it to verify Kepler’s second law by observing if the apparent diameter of the Sun decreased in the same way as its apparent speed. Cassini could thus observe the diameter variations of the Sun image which were projected on the floor of the church with the incredible precision and derive from them the variations of the Earth to Sun distance. He verified also that the Sun diameter (and hence its distance) did not decrease in the same way as its speed decreased: the Sun velocity variations during the year proved that the Earth was faster when it was closer to the Sun and slower when it was further away. That was the first observational confirmation of Kepler’s second law proving that celestial bodies were not uniformly moving along circular orbits as astronomers had believed and tried to describe for over twenty centuries. Moreover, Cassini’s measurements preceded of more than three decades the publication of Isaac Newton's Philosophiæ naturalis Principia Mathematica, which would have given a physical framework, the law of universal gravitation, to Kepler’s empirical relations, experimentally proved by Cassini with his meridian.
Even today Cassini's observations are used for scientifical studies eg. analysis of Solar Diameter Measurements Made at the Basilica of San Petronio during
and after the Maunder Minimum (The Astrophysical Journal, 912:122 (6pp), 2021 May 10)
