Aloysius Lilius

Aloysius Lilius
Bust of Aloysius Lilius.
Born	c. 1510 Ciro, Kingdom of Naples
Died	1576 Rome, Papal States
Nationality	Italian
Fields	Mathematician, astronomer, medic, chronologist, philosopher
Known for	Gregorian calendar

Aloysius Lilius (c. 1510 – 1576), also variously referred to as Luigi Lilio, Luigi Giglio, was an Italian doctor, astronomer, philosopher and chronologist, and also the "primary author" who provided the proposal that (after modifications) became the basis of the Gregorian Calendar reform of 1582.

The crater Lilius on the Moon is named after him, as is the asteroid 2346 Lilio. In computer science, the Lilian date is the number of days since the adoption of the Gregorian calendar on 15 October 1582.

Not much is known about the early life of Lilius/Lilio/Giglio. It is known that he came from Calabria, Italy, from Ciro. He studied medicine and astronomy in Naples, after which he served Earl Carafa. He settled in Verona and died in 1576. Although he was still alive at the time when his proposal was presented at Rome, it does not seem that he made the presentation; it was handled by his brother Antonio, also a physician and astronomer.

He is primarily known as the "first author" of the Gregorian Calendar: he wrote the proposal on which (after modifications) the calendar reform was based. Lilio's brother Antonio presented the manuscript to Pope Gregory XIII; it was passed to the calendar reform commission in 1575. The commission issued a printed summary entitled Compendium novae rationis restituendi kalendarium (Compendium of a New Plan for the Restitution of the Calendar), printed in 1577 and circulated within the Roman Catholic world in early 1578 as a consultation document. Lilio's manuscript itself is not known to have survived; the printed 'Compendium' is the nearest known source for the details it contained.