43. Everyone knows the provisions of Popes St. Pius and St. Victor and of the Council of Nicea concerning the proper celebration of Easter. All likewise are aware that the Council of Trent reserved to the Pope the question of revising the calendar, and that finally the matter was resolved in all its details in the time of Pope Gregory XIII. For this reason Bucherius writes in the preface to his Commentario de doctrina temporum: "At the bidding of Pope Gregory XIII, Clavius has fully seen to the definite reckoning of Easter in future years." Clavius was a Jesuit priest with scientific talents who contributed greatly to the correction of the calendar. The Pope also studied the calculations of Aloysius Lilius, which had taken years to complete. Finally, after weighing all aspects of the difficulty during many meetings and after consulting with many experts, he published in 1582 his constitution Inter gravissimas which established the regular calendar (in veteri Bullario, vol. 1, cons. 74).