The interior of the Basilica di San Petronio, Bologna; Cassini’s Meridian Line


Cassini’s Meridian Line

The church has a meridian line inlaid in 1655 the paving of the left aisle; it was calculated and designed by the astronomer Giovanni Domenico Cassini, who was teaching astronomy at the University.

A meridian line does not indicate the time: instead, with its length of 66.8 metres (219 ft) it is one of the largest astronomical instruments in the world, allowing measurements that were for the time uniquely precise.

The sunlight, entering through a 27.07 mm (1.066 in) hole placed at a 27.07 m (88.8 ft) 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. On the other hand, the size of the projected sun’s image, and in particular its rate of variation during the year, allowed Cassini the first experimental verification of Kepler’s laws of planetary motion.

The winter solstice end of the meridian line

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