Meridian Line of Basilica di San Petronio in Bologna, Italy

The Basilica di San Petronio in Bologna, Italy, houses a massive sundial that stands out among its Gothic architecture. Created by astronomer Giovanni Domenico Cassini, the sundial is a 67-meter-long meridian line that stretches through the left aisle of the church. Cassini's sundial was built in 1655, replacing a smaller one created by Egnazio Danti in the 16th century. The sundial uses a small hole in the fourth vault above the left aisle to project the Sun's image onto the marble floor at noon every day. Throughout the year, the Sun's image moves along the meridian line, appearing closer to the north end of the church in winter and the south side in summer. Cassini used this sundial to measure the Sun's position and size, providing evidence for the elliptical shape of the Earth's orbit around the Sun and the variation in the Earth's speed based on its distance from the Sun. This discovery supported Kepler's second law of planetary motion. Today, the sundial is a popular attraction within the church, with markings on the marble floor indicating the expected position of the Sun during different days and months. Visitors often gather around the sundial at midday to wait for the Sun's image to appear.