The church of San Petronio in Bologna was the site of a solar observatory as early as 1576 when Egnazio Danti, cosmographer to Cosimo I de’ Medici, installed the first meridian line there. Unfortunately it did not fulfill its purpose, which was to provide an accurate date for the spring equinox, thence Easter. In spite of uncertainties about the precise length of the solar year, the Gregorian calendar was promulgated anyway, in 1582. We still use it today.

Almost 75 years later, the opportunity arose to reconstruct the meridian. Enter a 29-year-old astronomy professor named Giovanni Domenico Cassini. Cassini increased the height of Danti’s solar peephole—or gnomon hole—to 1000 inches (based on the French foot) or 27.07 meters above the church floor. The length of the meridian line was increased by 2.5 times to 66.71 meters, or 1/600,000 of the Earth’s circumference, per Cassini’s calculation. The line had to run on the floor between the aisles and columns of the church on a north-south axis without obstruction. The instrument was tested with great fanfare at the summer solstice of 1655 and proved fully successful.

Cassini’s illustrated account of his heliometer was published 40 years later in 1695. The image shown here is taken from a large foldout plate depicting the design and details of installation. Caltech’s copy of the book is currently shown in the exhibit “On the Map” in Parsons-Gates Hall of Administration.

Reference: John Heilbron, The Sun in the Church. Cambridge: Harvard University Press, 1999.