Wednesday, 6 June 2012

Meridiana, Bologna

Following his comment on the post of 30th May 'Recent Italian earthquakes', Richard has now sent further information:-
"The church is the Basilica of San Petronio in Bologna, and the optical arrangement is known as "The Meridiana".  It is admirably described in a book "The Meridiana in the Basilica of San Petronio", written by Giovanni Paltrinieri and translated into English by John Heilbron, and is published by Basilica di San Petronio, Bologna.
The pinhole image of the sun, through a hole in the roof projects on to the nave floor and the instrument was built in 1655 by Gian Domenico Cassini, who was a well-known astronomer of the time.  Its purpose was to measure several things, principally the moment the sun was due south.  This moment was used to set the area's clocks for the ringing of the hours etc and for general astronomy too.  It also was so accurate (for its day) that it detected the diminution or convergence of the Obliquity of the Ecliptic, which is the 22,000-year cycle forming one of the three orbital forcing mechanisms effecting climate change, especially in the Quaternary Ice Age, but also seen eg in chalk/clay couplets within the Cenomanian Chalk."

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