The rediscovery of the obliquity meter in the meridian line of St. Maria degli Angeli in Rome

Francesco Bianchini (1662-1729) constructed the meridian line in the Basilica of S. Maria degli Angeli in Rome - after the will of pope Clement XI - with the purpose of accurately measuring the variation of the obliquity of Earth's orbit and the duration of the tropical year.

While Bianchini published many details of the meridian line in 1703, the presence of two decorations near the summer solstice position remained unexplained. These decorations are two "obliquity meters" used at 10 minutes of distance before the meridian transit. The second meter was obscured by the architect Luigi Vanvitelli when he renovated the Basilica for the 1750 jubilee year. He preserved all astronomical details on the floor, even a 6-stars arc of circle at 34.80 m from the pinhole vertical point that is related to Sirius - as well as a method of equal altitudes to measure geographical latitude, invented by the Jesuit Giovanni Battista Riccioli (1598-1671), but not found in Bianchini's documents.

Vanvitelli set up the ornament between squared pilasters that were properly cut for allowing the meridian to function at summer solstice, though not enough to illuminate both meters. This indicates that he had no understanding of the use and of the meaning of the obliquity meters, an issue that is now clear after precise measurements and geometrical calculations.

The meter that receives the image of the Sun nowadays allows direct evaluation of the secular shift of the position of the solstice since 1702.