The “Impossible” Circle

“More than 30 years ago Paolo Brenni went to the Astronomical Observatory in Palermo to check the Ramsden Circle and manage its shipment to Florence for a restoration treatment at the Science and Technology Foundation …” The first author of this poster had the honour of working with Paolo and wants to remember him here by presenting the restoration of the Ramsden Circle, a unique instrument in the world, now part of the collection of the Astronomical Observatory in Palermo. Unpublished details relating to the construction technique will be reported, revealing the perfection of an instrument that was highly admired in the late 18th-century scientific world. In consideration of the technological knowledge of the time, its construction was really challenging (Ramsden called it "titanic") and reached the top of the best available technology in those years. Giuseppe Piazzi (1746-1826), founder and first director of the Observatory in Palermo, discovered the first asteroid, Ceres Ferdinandea (1801), and carried out observations for the two editions of his famous Catalogues of stars (1803 and 1814) by using this instrument. Piazzi considered the Circle a masterpiece. Even today -two centuries later- the talent of a maker like Ramsden (admitted to conferences only as an auditor!) amazes anyone. By inserting a telescope in a circular scale to achieve higher accuracy in star position measurements he was able to deliver to Piazzi a brilliantly and originally designed instrument, created with the tools of the time. However, how did he do that? We will also try to clarify how Ramsden succeeded in the difficult construction of the Palermo Circle.