Lilio's proposal: coordinating the Sun and the Moon

The expression "Lilian Calendar" refers to Luigi Lilio's project at the basis of the Gregorian reform. To understand it, two problems must be distinguished: keeping the calendar in step with the cycle of the seasons, and correcting the lunar computation used to determine Easter.

The correction of the solar year

The Julian calendar provided for a leap year every four years. This rule produced an average year slightly too long compared with the cycle of the seasons, causing the equinox to drift progressively through the dates of the calendar.

The rule adopted by the reform keeps leap years every four years, but excludes century years not divisible by 400. 1900 was therefore not a leap year; 2000 was; 2100 will not be.

The system of epacts

The lunar problem called for a different intervention. The traditional nineteen-year cycle offered a good approximation of the relationship between solar years and lunations, but accumulated a discrepancy over time.

Lilio's project used epacts to organise the calculation of the Moon and to allow it to be corrected. According to the conventions of the ecclesiastical computus, the epact expresses the age of the Moon at the beginning of the year: it is used to find the dates of the ecclesiastical moons in the tables.

The strength of the system lay in the possibility of applying corrections according to established rules, preserving a method that could be used over the long term.

Civil calendar and the calculation of Easter

The civil Gregorian calendar follows the solar cycle. The calculation of Easter, on the other hand, combines solar and lunar rules: it uses a conventional equinox fixed at 21 March and an ecclesiastical Moon calculated by means of tables, which does not necessarily coincide with the astronomical Moon as observed.