At the beginning of the 18th century, Pope Clement XI commissioned the astronomer, mathematician, archaeologist, historian and philosopher Francesco Bianchini to build a meridian line, within the basilica. Completed in 1702 with the assistance of the astronomer Giacomo Filippo Maraldi. Built along the meridian that crosses Rome, at longitude 12° 30' E., to check the accuracy of the Gregorian reformation of the calendar, and to produce a tool to exactly predict Easter.