In 45 BCE (709 AUC – since the founding of the City) Julius Caesar with the help of his astronomer Sozygenes of Alexandria introduced a reform of the Roman calendar. The reason for the calendar reform was that the previously used lunar Roman calendar had been disrupted, resulting in 46 BCE. December was in September. Therefore, in order to re-synchronize the calendar with the seasons, the year 46 BCE. extended by 90 days.
Sozyges calculated that each year is made up of 365.25 days. He specified the number of days for each month and made February a leap month every four years. The change, however, was not to add one day to February, as is done today, but to repeat the same day. So every four years, every Roman lived the same day twice. It was set for February 24. The repeated day was called bissextilis and the leap year was also called bissextilis or bissextum. The new calendar starts on January 1st. The months were as follows:
- Ianuarius (31) – dedicated to Janus,
- Februarius (28) – for the februa, or cleansing rites, to take place before the beginning of the new year.
- Martius (31) – dedicated to Mars,
- Aprilis (30) – consecrated to Venus,
- Maius (31) – dedicated to all gods,
- Iunius (30) – probably dedicated to Lucius Junius Brutus or Juno,
- Quintilis (31)=”fifth”,
- Sextilis (31)=”sixth” – later Augustus in honour of Emperor Augustus,
- September (31)=”seventh” – Emperor Caligula intended to change the name of the month to “Germanicus”, but the changes were not officially adopted,
- October (30)=”the eighth”,
- November (31)=”ninth”
- December (30)=”tenth”.
The previously occurring repeated month of Mercedonius (also known as Intercalaris) has been removed.
Shortly after the assassination of Caesar, priests charged a leap year not every four but every three years. It was probably a priest’s mistake. This caused another shift of days in the calendar. The first leap years were 45 BCE, 42 BCE and 39 BCE until 9 BCE, when the anomaly was discovered. This was remedied by Octavian Augustus, who banned a leap year for the next 12 years. It was related to the introduction of 12 leap days during the 36 years of the new calendar, and not 9 as required by the reform. Thus, it was not until 5 CE. the calendar reform was successful. The earlier 50 years (45 BCE to CE 5) are called “Julian years of error”.
The Julian calendar kept all the external features of the previous Roman lunisolar calendar, especially the insertion of an extra day in a leap year between February 24 and 25, and the division of the month into moon phases (which in the purely solar Julian calendar had no reflection in reality and was maintained the force of habit). In addition, despite the change in the length of the months, the dates: Nona and Ida remained the same as before Caesar’s reform – that is, Nona fell on the 7th, and Ida on the 15th day of those months, which in the royal calendar are 31 days each (it is March, May, July and October), and in the remaining ones – 5th (Nony) and 13th (Ida). These terms lost their connection with the phases of the moon in human consciousness long before the Julian Reform.
After the fall of Rome in 476 CE Caesar’s solar calendar was in force in Europe for many centuries, e.g. in Spain, Portugal, Poland and Italy until 1582, in Russia from 1700 to 1918 (previously the Byzantine calendar was used, in which the year began on September 1), and in Greece until 1923. Due to the fact that the Julian calendar was 1 day in 128 years late, it was replaced by the Gregorian calendar in 1582; to this day, however, some Churches still use this calendar to emphasize their individuality.