The Basilica of St. Mary of the Angels and the Martyrs is a titular basilica church in Rome, built inside the frigidarium of the Baths of Diocletian in the Piazza della Repubblica. I stumbled upon this church by chance and although it looks fairly ordinary from the outside the inside of the church holds a most fascinating feature which I have not seen before. On the floor of the church is a long meridian line and along with a unique small hole high on the internal wall of the church, which allows the sun to shine, this forms a kind of sundial to measure the sun’s meridian crossing as well as the passage of the stars.
At the beginning of the eighteenth century, 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. Completed in 1702, the object had a threefold purpose: the pope wanted to check the accuracy of the Gregorian reformation of the calendar, to produce a tool to predict Easter exactly, and, not least, to give Rome a meridian line as important as the one Giovanni Domenico Cassini had recently built in Bologna’s cathedral, San Petronio. Alan Cook remarked, “The disposition, the stability and the precision are much better than those of the famous meridian… in Bologna”.
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 measure the sun’s progress through the year more precisely; (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.
Bianchini’s sundial was built along the meridian that crosses Rome, at longitude 12° 30′ E. At solar noon, which varies according to the equation of time from around 10:54 a.m. UTC in late October to 11.24 a.m. UTC in February (11:54 to 12:24 CET), 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 between the these two extremes. The longer the meridian line, the more accurately the observer can calculate the length of the year. The meridian line built here is 45 meters long and is composed of bronze, enclosed in yellow-white marble.
In addition to using the line to measure the sun’s meridian crossing, Bianchini also added holes in the ceiling to mark the passage of stars. Inside the interior, darkened by covering the windows, Polaris, Arcturus and Sirius were observed through these holes with the aid of a telescope to determine their right ascensions and declinations. The meridian line was restored in 2002 for the tricentenary of its construction, and it is still operational today.