In which Roman basilica does a meridian line mark the summer solstice?

The meridian line marking the summer solstice is in Rome’s Basilica of Santa Maria degli Angeli e dei Martiri.

Astronomer Francesco Bianchini built the basilica’s meridian line at the request of Pope Clement XI. Completed in 1702, it functioned as a giant indoor sundial: sunlight entering through a small aperture projected onto a bronze line set into the floor. The installation helped test the accuracy of the Gregorian calendar and improve calculations connected with Easter.

At the summer solstice, the Sun reaches its highest apparent position in Rome, so the projected beam strikes the meridian line nearest the wall. At the winter solstice, it reaches the farthest point; at the equinoxes, it falls between those extremes. The line is about 45 metres long, and the basilica’s former Roman bath walls provided the height, orientation and stability needed for precise observations. It is sometimes confused with Bologna’s meridian in the Basilica of San Petronio, an important predecessor but not the Roman location described here.