No doubt they stand there because Paul began the construction, and Urban accomplished the consecration, of the basilica. But they also offer an epitome of the Church's relation to science. There are other arresting reminders of astronomy in and around St. Peter's. The sarcophagus of Gregory XIII portrays Clavius and other presenting the new calendar to the Pope. Shafts from the sun fall through the dome and windows of the cathedral to make puddles of light on the marble floor.
Shadows cast by the obelisk in the square outside serve as a gigantic solar clock.
At sunset, rays shining through the stained-glass window over the western altar dramatize the presence, and indicate the aptness, of the sun in the church. Through much of the Scientific Revolution, between 1650 and 1750, Catholic churches were the best solar observatories in the world. Constructed initially to solve the pressing problem of providing an unquestionable date for Easter, the instruments that made the churches' observatories also threw light on the disputed geometry of the solar system. Within sight of the altar, they subverted Church doctrine about mankind's place in the universe. Measurements made in the oldest cathedral observatory, San Petronio in Bologna, in the heart of the Papal States, supported Kepler's revolutionary discovery that neither the sun's orbit, nor the earth's, can be a circle, and thus indirectly favored ideas condemned at the trial of Galileo.