Just in time

In the 18th century a meridian sun dial had been built in a Spanish church that, while Christian, was not in Rome. To rectify this disparity of grandeur Pope Innocence XII had the meridian line clock built here. Among other practical reasons, the location was chosen because it was the site of Diocletians baths and therefore was seen as a fitting symbol of the victory of the infallible Gregorian calendar over Roman / pagan ones.
It is a double meridian made up of 2 gnomons, one oriented to the South and the other North. The former tells the precise time of the sun's zenith the date and the inclination of the sun's rays on that date while the latter traces the polar star's movements.
The way the austral gnomon or solar meridian works:
+ the sun's rays shine through the small hole in the south wall onto the floor of the basilica,
+ striking a long strip of copper plated brass which runs from the transept to the presbytery precisely at 12 noon.
Bianchini’s meridian tells when noon was throughout the year, the arrival of the solstices and equinox, as well as a being a calendar.
+ for some unexplained reason there is a large pendulum beside the sun dial. On the ball there is a map of the world and you can just see Australia.Read more