The calendar reform and introduction of leap years
In the beginning, all calendars were lunar and guided by the monthly lunar cycle. When Julius Caesar visited Egypt, he fell in love with Cleopatra and immersed himself in Egyptian culture and scientific teachings.
When he returned to Rome, he brought with him the desire to correct the then outdated Roman Calendar and replace it with a model that followed the path of the sun. In this calendar the Roman astronomers inserted leap years, but they occured on average every 3 years.
His new Roman calendar, also called the Julian Calendar was introduced in Rome on the 1st of January 45B.C. and for centuries this model stood its test of time. But during the Middle Ages, it became apparent that the Julian Calendar was no longer totally accurate.
In the early 13th century A.D. an English friar named Roger Bacon calculated that the solar year was actually 11 minutes longer than Julius Caesar’s astronomers had calculated. By 1267 this had amounted to around nine days. Bacon wrote to Pope Clement IV and asked that the church should correct the calendar. If this issue was left unfixed the months would shift to such an extent that the holy days of Easter and Christmas would fall into a completely different seasons.
Pope Clement IV was very interested and asked Bacon to send his findings to the Vatican. Over the next year Bacon set off to compose his theories in a manuscript he called Opus Maius (Major Work). In this work he set out (amongst many other topics) to prove that the current way solstices and equinoxes were calculated had gone completely out of sync with the true seasons and a calendar reform was badly needed.
Sadly for Bacon Pope Clement IV died in 1268 before he could read or evaluate this important document. His successor Gregory X was less sympathetic and Bacon’s book became largely forgotten. But the friar did not rest and kept publishing his demands for calendar reform. In 1277 these calls landed him in prison and his ‘dangerous teachings’ were deemed to be heretic and were largely suppressed by the Catholic church.
It took almost another 300 years before Roger Bacon’s demand finally got taken seriously. Around 1573 Pope Gregory XIII appointed a calendar commission to look into the lost days. By this point it had amounted to 11.5 days.
This commission was led by the mathematician Christopher Clavius who directed work on the commission, but the main author was a physician called Aloysius Lilius (sometimes referred to as Luigi Lilio) who comprehended the papal bull that Gregory would issue on 24 February 1582. Unfortunately Lilio would not see his work to fruition and passed away in 1576, leaving Clavius to credit his work and drive the commission to completion.
The final product – the still used Gregorian Calendar – has now the correct length of year which corresponds exactly with the solistices and equinoxes and the year stays in complete sync with the season. The leap year correction added a day to the shortest month of the year – February – so every 4 years now February counts for 29days instead of the customary 28.