Topic: From the Old Roman Calendar to the Julian Calendar






Julius Caesar was the man who decided and ordered the introduction of a new calendar to be used in the vast areas of his great Empire, the Roman Empire. After the fulfillment of his operations in Egypt and his acquaintance with the great astronomers of Alexandria (mainly Sosigenes), he decided to make an overturn in the history of time recording. He was already informed that the calendar then used was based on the arbitrary intercalations of days after some defined years and it was dependent on the will of the high priests, who were affected by the will of political persons about the duration of the year, adapting it so as to satisfy their interests.

What is the result in the everyday life of the people if a calendar is not aligned with the tropical year? What is the tropical year? There are 2 kinds of years: the sidereal year and the tropical year. Sidereal year is the duration of the revolution of the earth around the sun, after which an observer of a certain place in the earth views exactly the same point of the heavenly sphere. Tropical year is the duration of the apparent revolution of the sun around the earth, between two consecutive passes of the sun (revolving in the ecliptic) through the earthly equator plane, going downwards (up and down are set in relation to the north and south pole of the earth). This pass defines the start of the spring. Therefore if the calendar is not aligned with the real tropical year the dates shift with respect to the seasons.

The solution was simple. Since the duration of the tropical year was not 365 days but nearly 365 days and 6 hours, the things would be better if every 4th year a whole day was added. The criterion for a year y to be characterized as leap is to be divisible by 4, or y modulo 4 = 0. For the years B.C. the year must be transformed into algebraic arithmetic by subtracting 1, and putting minus, due to the fact that year 0 was never recorded and used and after 1 BC is 1 AD. In this way, year 1 BC becomes year 0, year 2 BC becomes -1 etc. Thus, years: 1 BC =0, 5 BC=-4, 9 BC=-8 are considered as leap years.

The new calendar was established on January 1st of 45 BC. To repair the problems of the previous calendars, Caesar ordered that the year 46 BC should have extra days resulting to a 445 days year. They called it the ‘last year of confusion’. The extra days in 46 BC caused disruptions throughout the Roman world in everything from contracts to shipping schedules. After Caesar’s death in 44 BC, a college of pontiffs began counting leap years every 3 years. This was repaired by Emperor August in 8 BC, who ordered the next 3 leap years to be skipped (David Ewing Duncan, The Calendar, Publ. by Fourth Estate, London, 1998).

The extra day every 4 years was added between February 23rd and 24th. February 24th was called according to the roman terminology: 6th day before the Kalends of March (March 1st) or in Latin: ante diem sextum Kalendas Martias (in brief a.d. VI Kal. Mart). So the extra day before February 24th was called one more 6th day before the Kalends of March or in Latin: ante diem bis sextum Kalendas Martias (in brief a.d. bis VI Kal. Mart). For this reason the leap years are called also ‘bis-sextum’ years, that is ‘twice sixth’ years.

The Julian Calendar is extended for the reference of historical events that occurred before its introduction by Julius Caesar, that is before January 1st 45 BC. This extended calendar is called proleptic Julian Calendar. In this way the date of the first recorded eclipse of the ancient times, which had been predicted by the philosopher Thales, is estimated on May 28th, 585 B.C.

The Julian calendar was a very good calendar in the frame of those times, as it repaired the great misalignment between the calendar dates and the seasons, defined by the equinoxes and the solstices (real dates). The deviation before the Julian reform was so intense that the solar eclipse occured in 14 March, 190 BC, in terms of proleptic Julian Calendar, occured in the 11th of July, in terms of the inaccurate pre-Julian calendar. Then the pontiffs tried to repair the situation by a series of successive intercalary months in the following years. This irrational use of the method of intercalation led to a great delay of the calendar dates to the real dates. Thus the assignment of the consuls in 153 BC, which was set in the the 15th of March (after 222 BC), had ocurred in terms of the used calendar dates in the 1st of January. This wrong date, coincided later with the start of the year, was since then the established date for the assignment of the consuls.

Even so the duration of the Julian Calendar was not exactly equal to the real duration of the tropical year, which is 365 days, 5 hours, 48 minutes and 45 seconds, resulting to a delay of one day every nearly 128 years. This led to the need of a new reformation from the Old Style (Julian) Calendar to the New Style (Gregorian) Calendar in 1582 in the Catholic countries. This reformation will be described in the next ‘Info of the day’.