Julian Calendar

The Julian calendar, proposed by Roman consul Julius Caesar in 46 BC, was a reform of the Roman calendar. It took effect on , by edict. It was designed with the aid of Greek mathematicians and astronomers such as Sosigenes of Alexandria.

The calendar became the predominant calendar in the Roman Empire and subsequently most of the Western world for more than 1,600 years until 1582, when Pope Gregory XIII promulgated a minor modification to reduce the average length of the year from 365.25 days to 365.2425 days and thus corrected the Julian calendar's drift against the solar year. Worldwide adoption of this revised calendar, which became known as the Gregorian calendar, took place over the subsequent centuries, first in Catholic countries and subsequently in Protestant countries of the Western Christian world.

The Julian calendar is still used in parts of the Eastern Orthodox Church and in parts of Oriental Orthodoxy as well as by the Berbers.

The Julian calendar has two types of years: a normal year of 365 days and a leap year of 366 days. They follow a simple cycle of three normal years and one leap year, giving an average year that is 365.25 days long. That is more than the actual solar year value of 365.24219 days (the current value, which varies), which means the Julian calendar gains a day every 128 years. For any given event during the years from 1901 to 2099 inclusive, its date according to the Julian calendar is 13 days behind its corresponding Gregorian date.

Table of months

History

Motivation

The ordinary year in the previous Roman calendar consisted of 12 months, for a total of 355 days. In addition, a 27- or 28-day intercalary month, the Mensis Intercalaris, was sometimes inserted between February and March. This intercalary month was formed by inserting 22 or 23 days after the first 23 days of February; the last five days of February, which counted down toward the start of March, became the last five days of Intercalaris. The net effect was to add 22 or 23 days to the year, forming an intercalary year of 377 or 378 days. Some say the ''mensis intercalaris'' always had 27 days and began on either the first or the second day after the Terminalia (23 February).

According to the later writers Censorinus and Macrobius, the ideal intercalary cycle consisted of ordinary years of 355 days alternating with intercalary years, alternately 377 and 378 days long. In this system, the average Roman year would have had days over four years, giving it an average drift of one day per year relative to any solstice or equinox. Macrobius describes a further refinement whereby, in one eight-year period within a 24-year cycle, there were only three intercalary years, each of 377 days (thus 11 intercalary years out of 24). This refinement averages the length of the year to 365.25 days over 24 years.

In practice, intercalations did not occur systematically according to any of these ideal systems, but were determined by the pontifices. So far as can be determined from the historical evidence, they were much less regular than these ideal schemes suggest. They usually occurred every second or third year, but were sometimes omitted for much longer, and occasionally occurred in two consecutive years.

If managed correctly this system could have allowed the Roman year to stay roughly aligned to a tropical year. However, since the pontifices were often politicians, and because a Roman magistrate's term of office corresponded with a calendar year, this power was prone to abuse: a pontifex could lengthen a year in which he or one of his political allies was in office, or refuse to lengthen one in which his opponents were in power.

If too many intercalations were omitted, as happened after the Second Punic War and during the Civil Wars, the calendar would drift out of alignment with the tropical year. Moreover, because intercalations were often determined quite late, the average Roman citizen often did not know the date, particularly if he were some distance from the city. For these reasons, the last years of the pre-Julian calendar were later known as "years of confusion". The problems became particularly acute during the years of Julius Caesar's pontificate before the reform, 63–46 BC, when there were only five intercalary months (instead of eight), none of which were during the five Roman years before 46 BC.

Caesar's reform was intended to solve this problem permanently, by creating a calendar that remained aligned to the sun without any human intervention. This proved useful very soon after the new calendar came into effect. Varro used it in 37 BC to fix calendar dates for the start of the four seasons, which would have been impossible only 8 years earlier.Varro
''On Agriculture'' I.1.28.
/ref> A century later, when Pliny dated the winter solstice to 25 December because the sun entered the 8th degree of Capricorn on that date, this stability had become an ordinary fact of life.

Context of the reform

Although the approximation of days for the tropical year had been known for a long time, ancient solar calendars had used less precise periods, resulting in gradual misalignment of the calendar with the seasons.

The octaeteris, a cycle of eight lunar years popularised by Cleostratus (and also commonly attributed to Eudoxus) which was used in some early Greek calendars, notably in Athens, is 1.53 days longer than eight mean Julian years. The length of nineteen years in the cycle of Meton was 6,940 days, six hours longer than the mean Julian year. The mean Julian year was the basis of the 76-year cycle devised by Callippus (a student under Eudoxus) to improve the Metonic cycle.

In Persia (Iran) after the reform in the Persian calendar by introduction of the Persian Zoroastrian (i. e. Young Avestan) calendar in 503 BC and afterwards, the first day of the year (1 Farvardin=Nowruz) slipped against the vernal equinox at the rate of approximately one day every four years.

Likewise in the Egyptian calendar, a fixed year of 365 days was in use, drifting by one day against the sun in four years. An unsuccessful attempt to add an extra day every fourth year was made in 238 BC ( Decree of Canopus). Caesar probably experienced this "wandering" or "vague" calendar in that country. He landed in the Nile delta in October 48 BC and soon became embroiled in the Ptolemaic dynastic war, especially after Cleopatra managed to be "introduced" to him in Alexandria.

Caesar imposed a peace, and a banquet was held to celebrate the event.Lucan, '' Pharsalia:'
''Book 10''.<