A meridian line does not indicate the time: instead, with its length of it is one of the largest astronomical instruments in the world, allowing measurements that were for the time uniquely precise. The sun light, entering through a hole placed at a height in the church wall, projects an elliptical image of the sun, which at local noon falls exactly on the meridian line and every day is different as to position and size. The position of the projected image along the line allows to determine accurately the daily altitude of the sun at noon, from which Cassini was able to calculate with unprecedented precision astronomical parameters such as the obliquity of the ecliptic, the duration of the tropical year and the timing of equinoxes and solstices.