In 1569, after the subjection and definitive annexation of Siena, Cosimo de’ Medici (1519-1574) persuaded Pope Pius V (1504-1572) to change his title from "Duke of Florence and Siena" to "Grand Duke of Tuscany". It may be that during the negotiations Cosimo offered to, or was requested to, sponsor the work needed for a definitive calendar reform that would eliminate the ten days’ delay with which the spring equinox, established as March 21, occurred in respect to the true astronomical equinox. The liturgically exact date of Easter and the related movable feasts depended, in fact, on the correct date of the equinox. Cosimo’s commitment consisted of transforming the church of Santa Maria Novella into a sort of astronomical observatory, with the cosmographer Egnazio Danti (1536-1586) commissioned to construct the instruments required for the observations and for measuring the movements of the Sun. Between 1572 and 1575, Danti installed on the façade of the church three instruments specially constructed for observing the equinoxes and solstices: an astronomical quadrant bearing six sundials, an equinoctial armilla and a gnomonic hole for a pinhole sundial which, however, he did not manage to complete.