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Speaker |
Sigismondi, Costantino |
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| Talk Title |
Meridian Service in Rome at S. Maria degli Angeli with Bianchini and at Collegio Romano with Secchi |
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Abstract |
The father of Italian geodesy is Francesco Bianchini (1662-1729) who built the meridian line in the Basilica of S. Maria degli Angeli in Rome with the purpose of measuring accurately the variation of the obliquity of Earth's orbit and the tropical year duration. He compared the observations of eclipses made in Rome and in Bononia, at the meridian line (1655) made by Giandomenico Cassini and found that the meridian of the Pontifical State was from Rome to Rimini. While Bianchini published many details of the meridian line in 1703, the presence of two decorations near the Summer solstice position has remained unexplained until 2018. Only one of them receives the image of the Sun nowadays and allows immediately to evaluate the secular shift of the solstice's position. The position of the red marble strip under this decoration is the materialization of the solstitial center of the solar image in 1702. For 1.5 centuries the signal was given by that meridian line, Secchi was able to automatize the procedure of the ball-drop, as in Greenwich observatory. A signal was spread to the city, through the fall of a sphere visible from far on top of the Flamsteed house,to give the instant of the local meridian transit. This was established in 1833 and automated in 1852 by Airy. Secchi realized a similar device upon the roof of St. Ignatius, where his Observatory was located. This ball-drop gave the signal to the cannon on Gianicolo hill. Nowadays the tradition of the cannon continues, without the intervention of astronomers. Secchi measured carefully the meridian of Rome, and paved the way to the modern geodesy and the fundamental meridian of Italy at 12° 27' 08" from Greenwich. |
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