The basilica is dedicated to the
Christian martyrs, known and unknown. By a brief dated 27 July 1561, Pius IV ordered the church "built", to be dedicated to the
Beatissimae Virgini et omnium Angelorum et Martyrum ("the Most Blessed Virgin of all the Angels and Martyrs"). Campaign managers to accomplish this were St. Philip Neri and St. Charles Borromeo. Impetus for this dedication had been generated by the account of a purported mystical vision experienced in 1541 at
Santa Maria di Loreto, Rome of the ruins of the Baths by a Sicilian monk,
Antonio del Duca, who had been lobbying for decades for papal authorization of a more formal veneration of the Angels. It was also a personal monument of
Pope Pius IV, whose tomb is in the
apsidal tribune. The
thermae of Diocletian dominated the
Viminal Hill with their ruined mass.
Michelangelo Buonarroti worked from 1563 to 1564 to adapt a section of the remaining structure of the baths to enclose a church. Upon Michelangelo's death in 1564, the work was carried on by his pupil,
Jacopo Del Duca. Some later construction was directed by
Luigi Vanvitelli in 1749. In 1911, the Vanvitellian façade on Piazza Esedra was demolished to restore the suggestive niche of the calidarium with Roman bricks. This intervention, however, made the church less visible, and often mistaken for a ruin. At Santa Maria degli Angeli, Michelangelo achieved a sequence of shaped architectural spaces, developed from a
Greek cross, with a dominant
transept, with cubical chapels at each end, and the effect of a transverse nave. There is no true
facade; the simple entrance, with its unique concave brick shape, is one of the ancient exedras of the calidarium of the
thermae. 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 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 used the window behind the pope's coat of arms and a movable telescope to observe the passage of several stars such as
Arcturus and
Sirius 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. == Cardinal Protectors since 1687 ==