Julian calendar — history, structure, and legacy

Previous calendar

Censorinus describes a Roman calendar as a twelve-month lunar calendar. This was adjusted to the solar year at irregular intervals as needed.

Caesar's calendar reform

Appian, Cassius Dio and Macrobius report in their writings that Julius Caesar became acquainted with the switching cycle of the later Julian calendar in Hellenized Egypt in Alexandria in 47 BC. The supplementary information of Macrobius therefore allows the possibility that Julius Caesar traveled to Egypt to discuss the new calendar form of the Julian calendar with the experts of the Egyptian calendar, probably among others with the Egyptian astronomer Sosigenes, after Julius Caesar had become more familiar with the Egyptian calendar through Acoreus.

This new calendar - later called "Julian" in his honour - came into force in 45 BC. It consisted of eleven months with 30 or 31 days each and one month with 28 days. The old designations from the Roman calendar were initially retained. The confused year 708 a. u. c. was extended to 445 days and began on October 14, 47 B.C. In the old Roman calendar February was first shortened to 23 days in the leap years and the leap month Mensis intercalaris was inserted, which contained in addition the shortened remaining days of February. This leap month was omitted by the reform.

Subsequent changes

The switching rule was interpreted literally by the pontifices after Caesar's death, but this led to incorrect switching. Caesar had ordered the switching every fourth year, but the priests understood this to mean switching every three years, according to the inclusive count. This is probably the oldest known example of a fencepost error. The over-counting of leap years was corrected by the Emperor Augustus, who suspended the circuits in 5 BC, 1 BC, and 4 AD, and did not resume them until 8 AD. As a result of this initial year of regular intercalation of a leap year, it is the years with a year number divisible by 4 that have been leap years ever since.

Contrary to medieval interpretations, Augustus did not have the distribution of days among the months changed.

Start of the year

The Julian calendar itself was recognized throughout the Roman Empire, but the beginnings of the year were handled differently from region to region. According to the Roman calendar, the beginning of the year was on March 1 until 153 BC. This day was in the old Rome originally the calendrical yearly beginning, on which in the temple of the Vesta the holy fire was lighted. In 153 B.C. the Roman consuls had brought forward their reign to January 1, and with the beginning of their term of office this also became the new beginning of the year.

In Egypt the turn of the year was on 29 August, in Constantinople and later also in Russia on 1 September, in the western Mediterranean as well as widely in England, Germany and Switzerland on 25 December, later in Great Britain on 25 March and in other countries on still other days. It was not until the early modern period that January 1 became more or less universal in the West, and not until the early 18th century in the East.

Annual Census

Also the counting of the years was different in the different parts of the Roman Empire; in the west the years were usually not counted at all, but were named after the two consuls who were in office for one year. In addition, the counting "from the foundation of the city (Rome)" and later the Diocletian era was used. In the East, the Seleucid era was common, counting 312 BC as year one. Later, in the West, the Christian era, which is still in use today, prevailed; in the East, the era "from the creation of the world" was still in use for a long time; this was set by the Byzantines at the year 5509 BC.

Month names

Roman Empire

In 44 BC the Quintilis (originally "fifth month", since 153 BC the seventh) was renamed Julius in honour of Julius Caesar by the Lex Antonia de mense Quintili. Later, the Sextilis (originally "sixth month", since 153 BC the eighth) received its new name in honor of Emperor Augustus.

The calendar at the time of the Roman Empire knew the following twelve or thirteen months:

• Ianuarius
• Februarius
• Mensis intercalaris (omitted by the introduction of the Julian calendar)
• Martius
• Aprilis
• Maius
• Iunius
• Iulius (originally Quintilis)
• Augustus (originally Sextilis)
• September
• October
• November
• December

Other months were also at times named after Roman rulers, but apparently none of these changes survived their deaths. Caligula named September (seventh month) Germanicus; Nero named Aprilis (second month) Neroneus, Maius (third month) Claudius, and Iunius (June) Germanicus; Domitian named September Germanicus and October (eighth month) Domitianus. September was also renamed Antoninus and Tacticus; November (ninth month) was also given the names Faustina and Romanus. Commodus was unique in that he named all twelve months after his adopted names (January to December): Amazonius, Invictus, Felix, Pius, Lucius, Aelius, Aurelius, Commodus, Augustus, Herculeus, Romanus and Exsuperatorius.

Frankish kingdom

Charlemagne later named all months with mostly agricultural terms of the vernacular language of the time, Old High German. The German names were developed regionally differently and continued to be used until the 15th century and, with some changes, partly until the late 19th century.