The effects of Greek acculturation on the Julian solar calendar

Introduction

In 46 B.C. Julius Caesar proclaimed the Julian calendar, a massive reform of the previous Roman calendar. One of the most remarkable things about this remarkable calendar was the Greek influence on its design. It was devised by a Greek, used the leap year first proposed by a Greek, and had months named after gods based on the Greek pantheon. Yet the influence of the Greeks on the Julian calendar is often ignored by the literature. By examining some of the available primary and secondary sources on calendars and chronology, this paper outlines the steps in the acculturation between Greeks and Romans that resulted in the Greek influence on the Julian calendar, and in the process explains how the ancient Greek and Greek-influenced calendars worked.

Calendar Basics

There are two basic events that people use calendars to measure: the cycle of the moon and the cycle of the seasons. The earliest calendars followed the moon because it is so obvious and so regular, even to the primitive eye, and because its period is short enough to be easily counted. Most early cultures based their religious festivals around lunar phases. The length of the solar year, which determines the flow of the seasons, is much longer and harder to determine. However, for an agricultural society, anticipating the seasons is vital to schedule crop plantings and harvests at the right time. Therefore, the history of the calendar has been a struggle to build an accurate calendar to measure the solar year, while still maintaining some relationship to the moon's movement for religious purposes.

Some astronomical information will be useful to understand the problem. The moon reappears in the same phase every 29.53059 (mean solar) days, called the synodic month. The sun passes back and forth between the mean vernal equinox in about 365.2422 (mean solar) days, called the tropical (mean solar) year.[1]

The central difficulty is that the synodic month and the tropical year do not divide evenly. Further causing problems is the fact that neither the lunar period (synodic month) nor the year divide into an even number of days! Ancient calendar designers thus confronted the problem of devising a calendar that could incorporate both periods without introducing an error, which over time would bring the calendar out of sync with the real movements of the heavens.

Hesiod's Awareness of the Solar Year

The earth's rotation around the sun also matches with changes in the constellations, and from this fact arose the earliest useful solar calendars in many cultures. The first record of a Greek solar calendar comes to us from Hesiod in his Works and Days. He wrote his text to an audience of farmers, and among other things explains to them when to plow their fields and harvest their crops. The time, he says, is when the constellation of Pleiades, containing the star Sirius, brightest in the sky, first rises above the horizon:

Pleiades rising in the dawning sky,
Harvest is nigh.
Pleiades setting in the waning night,
Plowing is right. [2]

This means that when Pleiades first appears above the horizon just before dawn, it is time to harvest, and when Pleiades sets below the horizon as night falls, it is time to plow.

The Greek Lunisolar Calendar

Over time, the Greeks combined these calendars together into a lunisolar calendar that attempted to satisfy both the lunar and solar periods. Records show that many of the cities used different but similar calendars. The Athenian calendar is the one typically used as an example by modern texts.

The Athenian calendar by the 5th century B.C. had twelve months to a year. The months were named Hekatombion, Metageitnion, Boedromion, Pyanepsion, Maimakterion, Poseidon, Gamelion, Anthesterion, Elaphebolion, Munychion, Thargelion, and Skirophorion.[3] Each month alternated between 29 and 30 days, giving a total count of 354 days. The Athenians evidently knew that the actual year is longer that that, since they added an extra month every other year, an action known as intercalation. Still, over two years this brought the total to about 737 days, while two solar years are really about 730 days. To deal with this the Athenians seemed to intercalate months more or less as they needed them in order to synchronize their calendar with actual astronomical observations. They may have been guided by the introduction by Meton of Athens in 432 B.C.[4] of the Metonic Cycle, which showed that 235 lunar months equaled almost exactly 19 years.

On the other hand, the available evidence[5] from the chronological research based on a variety of tablets and other documents fails to follow any set pattern with total consistency. The ancient Greek cities most commonly used eponymous names for their years, for example the names of the archons in Athens. Despite the lack of total consistency, though, the Greeks did maintain the basic synchronicity of their calendar with the actually occurring seasons.

The Egyptian Calendar under the Greeks

The Egyptians developed their own, far more consistent set of calendars. One can easily imagine that they had a rather easy time of it. The Nile floods on a regular, solar cycle, with fours months of flood, four months of receding water when the crops are sown, and four months of dry season when the crops are harvested. Their astronomers noticed that Sirius rose just before the floods began and based their solar agricultural calendar on that event. Dutka, in Mathematical Intelligencer, explains to us that by the fifth millennium B.C. the Egyptians were using a calendar with 12 months of 30 days.[6] They then added five intercalary days called the epagomenes at the end of the year. The Egyptians simultaneously maintained a lunar calendar for religious purposes. Although the Egyptians developed a consistent and more accurate calendar long before the remainder of the ancient world, it is not intended as denigrating, merely practical, to note that the evidence was literally washing around their feet.

Although this calendar was more accurate, it undercounted the year by about a quarter of a day. Yet, when they put this calendar into use for civil purposes, they stuck to it with a kind of blind faith, despite the fact that it slid through the seasons slowly over time. Though the Egyptian civil calendar failed to mark the seasons, it did mark time with great regularity, a feature that has been of great use to modern chronologists in dating the events in ancient Egypt and neighboring regions.

The Greek pharaoh Ptolemy III Euergetes tried to correct the 1/4 day error by adding another intercalary day every four years, the first instance of a regular leap year. This we have from the trilingual Decree of Canopus from 238 B.C.:

And that the seasons of the year may coincide wholly with the present settlement (or, constitution of the world), and that it may not happen that some of the popular festivals which ought to be held in the winter come to be celebrated in the summer, [owing to] the Star (i.e. the Sun) changing one day in the course of four years, and that festivals which are now kept in the summer come to be celebrated in the winter in times to come, even as hath formerly happened, and would happen at the present time if the year continued to consist of three hundred and sixty days, and the five additional days which it is customary to add thereto; from this time onward one day, a festival of the Well-Doing Gods, shall be added every four years to the five additional days, before the New Year...[7]

In this text Ptolemy seems eager for the natural year and calendar year to agree, an enthusiasm apparently not shared by the Egyptians, who did not accept the change. However, the leap year was remembered and later incorporated in the Julian calendar by Sosigenes of Alexandria.

The Roman Republican Calendar

The origin of the Roman calendar is unknown. It may have been copied from the Greek calendar, or may have been invented by Romulus in the eighth or seventh century B.C. Originally, the months were Martius, Aprilis, Maius, Junius, Quintilis, Sextilis, September, October, November, and December. The last six months are taken from the roman words for the numerals five through ten. Supposedly Numa Pompilius, second king of Rome, added January and February to the preexisting months in order to rectify the deficiency of days, bringing the total to 355 days with months of length 28-31 days. Not only was this new total not enough, but later when January became the first month it also rendered the numbered months nonsensical.

This Roman republican calendar suffered from a deficit of days, so the Romans added an intercalary month "Intercalans" every few years to bring their calendar back in line with reality. The pontifices were charged with intercalating the years, a duty that they carried out irregularly. They may have erred out of confusion, but they also had the opportunity to add the months for self-serving political ends, such as to give their political allies longer terms of office. In any case, they did not serve the needs of farmers, because by the time of Julius Caesar, the calendar was out of synchronization with the seasons by 90 days.

The Question of Greek Influence on the Roman Calendar

Any speculation on the effect of the Greek calendar on the early Roman calendar must necessarily be guesswork. The first four of the ten months were named after Roman gods, based on the Greek pantheon. But to assume since the Roman pantheon derived from the Greek pantheon, that also the Roman calendar derived from the Greek, is attractive but unsubstantiated by any of the ancient documents and contemporary works examined.

However it seems clear that the Romans relied on the Greek interest in science and astronomy over developing their own talents. In this we may trust the Cambridge Ancient History:

On the whole, the standard picture of the Romans as uninterested in theory for its own sake can stand. It was the Greek Sosigenes who wrote the astronomical treatises and did the calculations on which Caesar's reformed calendar was based...[8]

Although indirectly, this supports the idea that the general trend of Roman reliance on the Greeks in areas of science and philosophy may have extended not only to the Julian calendar, but also to the earliest Roman calendars as well, where little reliable evidence survives to determine the issue conclusively.

The Julian Calendar

Caesar campaigned in Egypt where he met the Greek philosopher and astronomer Sosigenes of Alexandria. Sosigenes returned to Rome with Julius Caesar and among other things undertook a redesign of the Roman calendar. Using the Greek and Egyptian knowledge of calendars, he undertook to reform the republican calendar into a solar calendar with 365 1/4 days that incorporated the leap year first proposed in the Decree of Canopus and rejected in Egypt. In this he was successful.

By this time the republican calendar was out of synchronization with the seasons by 90 days and the feeling in Rome was that the pontifices were abusing their power over the calendar. Julius Caesar brought the year back into line with the seasons in 46 B.C. (called the "Year of Confusion" or the "end of the years of confusion" by virtue of its great length).

The new calendar, dubbed the Julian calendar, had the same twelve months as the republican calendar with additional days spread around in sufficient number to bring the total to 365. In addition, Caesar decreed that every four years, an extra day be added in February, bringing the mean average number of days to 365 1/4. The actual number of days per year is 365.2422 so this was a very accurate approximation, being out by only about 1 day per century. The month Quintilis was renamed July in his honour.

Following Caesar's death soon afterward, the priests erred by adding the leap day every three years instead of every four (possibly because the Romans counted starting from one, not zero, so that they would consider the first year leap year "I", then declare the next leap year when the count reached "IV" thus intercalating every three years). Caesar Augustus recognized the mistake and corrected it, took the opportunity to rename Sextilis to August, and fiddled with the number of days in the months so that "his" month would have as many days as that of Julius.

The resulting Julian calendar was used for some fifteen centuries before it was reformed by Pope Gregory XIII to correct the one day per century error. The Gregorian calendar omits the extra day of the leap year on years divisible by 100, except years divisible by 400. Thus 1700, 1800, and 1900 did not have leap years, but 2000 did. Aside from that change, the modern calendar is identical to the Julian version.

Conclusion

Given the evidence at hand, thin as it is in terms of documenting the origins of the earliest Greek and Roman calendars, we see that clear patterns of acculturation are present between the Greeks and Romans and through the Egyptians. In their direct contact, we suggest that the Roman assumption of Greek gods to name their months may have accompanied other calendar influences. In Egypt we suppose that the Greeks may have been impressed with the regular Egyptian calendar and attempted to improve it with the leap year. Finally we see that Sosigenes carried that idea to Rome and used his scientific talents to design the Julian calendar which is still with us today. The movement of ideas through these acculturative events is what demonstrates the Greek influence on the Julian calendar.

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