ALOYSIUS LILIUS AUTHOR OF THE GREGORIAN REFORM OF THE CALENDAR

During the 16th century the disagreement between the dates of the Julian calendar, that had been in use since 46 BC and the vernal equinox, necessitated a correction to the computational rules used to regulate the flow of time. This was a very difficult task as it was necessary to resynchronize the civil time with celestial indicators, maintaining a lien adamant: the date of the vernal equinox, conventionally fixed perennially on March 21. In fact, during the Nicaea Council (325) the celebration of Easter was fixed on the first Sunday following the XIV Moon (Full Moon) belonging to the first month after the vernal equinox. Luigi Lilio, a physician, astronomer and mathematician, using imprecise astronomical data contained in tables from three centuries before, was able to elaborate a calendar that has stood the test of time. By the use of two equations he was able to synchronize the solar and lunar cycles and to develop a useful tool, named the epact cycle, to determine without uncertainty the Easter date. Furthermore, the Lilian method offers the possibility to correct the calendar according to the variation of the tropic year during time. Unfortunately, only a few details of his personal life are known. Indeed, he has left only a few faint traces in public or private archives, so that today his name is almost unknown. In this paper are reported the few details known of his life and a reconstruction of his plan for the calendar reform