Rome's Meridian

In the Church of S. Maria degli Angeli e dei Martiri, Piazza della Repubblica, Rome.

Santa Maria degli Angeli was the official state church during the Kingdom of Italy (1870-1946). One important scientific function given to the church was establishing there the Meridian of Rome, the zero-degree longitude for Rome and a type of solar calculator that was used to predict the time of Easter. The meridian was used until 1884 when Greenwich, England, was established by international conference as the universal zero longitude.

The brass bar laid in the floor of the church, seen above with numbers along its side, mark the position of the sun as it passes directly overheard at solar noon; a small optical telescope high in the wall of the church is positioned so that an image of the sun is projected across the meridian as the sun passes its zenith each day. Completed in 1702 the observatory still functions today.

The two side-bars labeled "Terminus Paschae" mark the earliest possible date for Easter.

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