Pope Clement XI (1700-1721) ordered Francesco Bianchini (1662-1729) to build a Meridian Line. Bianchini was the Secretary of the Commission for the Calendar. He chose the Basilica of Santa Maria degli Angeli because of the stability of its roman walls and foundations and its suitable dimension. The Line built by Giandomenico Cassini (1625-1712) in 1655 in San Petronio, Bologna , was the model for Bianchini, who improved it by allowing the observation of stellar transits.
Pope Clement XI inaugurated the Great Sundial on October 6, 1702 , the annual
Feast of St. Bruno, founder of the Carthusian Order, whose statue is in the entrance of the Church. Carthusian monks ruled this Basilica for three centuries until 1884.
It is not the shadow which shows the time, but the light. At the astronomical noon a sunbeam, entering the church from a coat of arms near the ceiling, crosses a meridian line drawn on the floor. The intersection point changes according to the various periods of the year; these are indicated by inscriptions and by inlays showing the signs of the zodiac and the stars of the related constellation. The sundial allows also the calculation of Easter and the position of the Pole-star.