Santa Maria degli Angeli e dei Martiri

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Santa Maria degli Angeli e dei Martiri's facade is the ancient tepidarium of the Baths of Diocletian.

Santa Maria degli Angeli e dei Martiri (English: St. Mary of the Angels and the Martyrs) is a basilica church in Rome, built inside the tepidarium of the baths of Diocletian. The basilica is dedicated to the Christian martyrs who, allegedly, worked to build the Roman baths.

Michelangelo Buonarroti worked to adapt the baths to a church up to 1561. Later construction was directed by Luigi Vanvitelli.

Santa Maria degli Angeli was the official state church during the Kingdom of Italy (1870-1946). More recently, national burials have been held in the church. The church hosts the tomb of Armando Diaz, the general who won World War I on the Italian front.

William Henry Cardinal Keeler is the Cardinal Priest of the Titulus S. Mariae Angelorum in Thermis.

[edit] The meridian line

Around 1700, Pope Clement XI commissioned the astronomer, mathematician, archaeologist, historian and philosopher Francesco Bianchini to build a meridian line, a sort of sundial, within the basilica. The object was threefold: the pope wanted to check the accuracy of the Gregorian reformation of the calendar, to produce a tool to exactly predict Easter, and to give Rome a meridian line as important as the one Giovanni Domenico Cassini had recently built in Bologna's cathedral, San Petronio. This church was chosen for several reasons: (1) Like other baths in Rome, the building was already naturally southerly oriented, so as to receive unobstructed exposure to the sun (2) The height of the walls allowed for a long line to more precisely measure the sun's progress through the year (3) The ancient walls had long since stopped settling into the ground, ensuring that carefully calibrated observational instruments set in them would not move out of place and (4) Because it was set in the former baths of Diocletian, it would symbolically represent a victory of the Christian calendar over the earlier pagan calendar.

Diagram of Bianchini's meridian in Santa Maria degli Angeli e dei Martiri. The ray on the right comes from the sun, and hits the line at solar noon through the year. The ray on the left is from Polaris.

Here Bianchini's gnomon projects the sun's image onto his line just before solar noon

Bianchini's sundial was built along the meridian that crosses Rome, at longitude 12° 50'. At solar noon, around 12.15 p.m. (1.15 p.m. in summer time), the sun shines through a small hole in the wall to cast its light on this line each day. At the summer solstice, the sun appears highest, and its ray hits the meridian line at the point closest to the wall. At the winter solstice, the ray crosses the line at the point furthest from the wall. At either equinox, the sun touches the line exactly halfway between the these two extremes. The longer the meridian line, the more accurately can the observer calculate the length of the year. The meridian line built here is 45 m long, and is composed of bronze, enclosed in yellow-white marble.

In addition to the line to mark the sun, Bianchini also added holes in the ceiling to mark the passage of stars. Inside the dark interior, Polaris, Arcturus and Sirius are visible through these holes, even in bright midday. The meridian line was restored in 2002 for the tricentenary of its construction, and it is still operational today.