Sosigenes

Egyptian or Greek astronomer

• Born: c. 90 b.c.e.
• Birthplace: Unknown
• Died: First century b.c.e.
• Place of death: Unknown

Sosigenes advised Julius Caesar on the development of the Julian calendar, which, with only slight modification, is still in use today.

Early Life

Virtually nothing is known about the life of Sosigenes (soh-SIHJ-eh-neez), an Alexandrian astronomer and mathematician who flourished in the first century b.c.e. Even the place of his birth is disputed. Some sources maintain that he was born in the Roman-controlled Egyptian city of Alexandria; others say that he came to Alexandria from Greece.

Since the ancient days of the pharaohs, Egypt had been besieged by foreign powers. Its people had been conquered by the Assyrians, then the Persians, and finally, in the fourth century b.c.e., by Alexander the Great, for whom the city of Alexandria is named. In the years just before Sosigenes’ life, the Ptolemies, who ruled following the collapse of Alexander’s realm, had made Alexandria into a world center for commerce and cultural development. Trading vessels arrived from lands as diverse as Britain and China. Science and art flourished. The city had several parks, a university, and a library with 750,000 volumes. It became a mecca for philosophers and scientists, producing such great scholars as Hipparchus, Ptolemy, Euclid, and Hero of Alexandria. It is no wonder, then, that Sosigenes ended up in Alexandria.

Interest in Alexandria was not limited, however, to intellectuals. Foreign leaders continued to see Egypt as a prize. During the century before the birth of Sosigenes, the ruler of Macedon, Philip V, conspired with the king of Syria to conquer and divide Egypt. This move attracted the attention of the great and expanding Roman Empire. Though the Romans were embroiled in the Second Punic War with Carthage, they managed to send an army east to punish the two rulers. After both countries had been conquered, the Romans set up a protectorate in Egypt. Thus, the Alexandria of Sosigenes’ time was under profound Roman influence. The Romans did not, however, interfere with the growth and development of the city, which now held more than half a million inhabitants—more than Rome itself.

After ending a bloody civil war in 48 b.c.e., Julius Caesar rested a year in Egypt before returning to Rome to become dictator. Perhaps it was during this visit that he became acquainted with Sosigenes, who by that time had come to be considered an authority on astronomy. One of Caesar’s goals, as ruler of the Roman Empire, was to make radical reforms to the calendar in use at that time. The Roman republican calendar was so out of synchronization with the natural year that the vernal equinox, the springtime event when the sun’s path crosses the celestial equator, had occurred months later, in early summer. Caesar called on Sosigenes to advise him on this matter and to develop a new calendar to replace the problem-ridden one of old.

Life’s Work

The earliest calendars were lunar in that they followed the phases of the moon. Each month was designed to chart a complete cycle of lunar phases, from new moon to full moon and then on to the next new moon. Lunar calendars were easy to use, especially because they were tied to readily observable astronomical events. Their main problem was that they were independent of important phenomena on Earth. Most notably, they did not follow the seasons. The progression of seasons follows the solar year, the time it takes Earth to complete a revolution around the sun, roughly 365 days. The phases of the moon, however, follow the synodic period (the time between successive new moons) of about thirty days. A lunar year might consist of twelve such cycles (twelve months), or 360 days. The five-day discrepancy meant that seasonal events (monsoons, river flooding, snowfall, and the like) would drift five days forward each lunar year. Thus, by the time a calendar had been in use for two decades, the cold-weather days of winter would occur three months later, in the “spring” months.

Agricultural concerns dictated a need for a calendar that would closely follow the seasons. Farmers would then know when to plant and when to harvest. Such seasonal, or solar, calendars would be based on the observed motion of the sun through the constellations of the sky (ancient astronomers did not realize that this drifting of the sun through the constellations was actually caused by the revolution of Earth). The difficulty with solar calendars was that they did not follow the phases of the moon—which were important for setting the dates of religious feasts and events.

It seemed impossible to reconcile these two demands. Egypt, at the time of Sosigenes, had no fewer than three calendars in use. The oldest calendar was actually a very good one, by modern standards. It was a lunar calendar but was corrected each year by the rising of the star Sirius (the day on which a given star rises at the moment the sun sets is a seasonal year constant). Thus, this calendar—lunar, but regulated by the solar year—accurately predicted seasonal events such as the flooding of the Nile River, an important consideration for farmers.

Governmental and administrative personnel, however, wanted something more: a calendar that would not vary from year to year, so that they could set predictable dates for treaties and business contracts. Therefore, a true seasonal calendar was developed. It consisted of twelve months, each of which contained exactly thirty numbered days. As this worked out to 360 days, the Egyptians then intercalated five extra days at the end of each year. At first, this calendar worked as well as the lunar calendar. As the decades passed, however, the seasonal calendar grew out of synchronization with the seasons. Farmers went back to the Sirius-regulated lunar calendar. Astronomers tried to determine the reasons for the failure of the seasonal calendar. They developed a new lunar calendar. This one, instead of being corrected by Sirius, was tied to the civil year (the seasonal calendar). This helped the religious leaders set their events but was of no use to the farmers, who continued to use the old calendar.

Sosigenes realized that the reason for the problems that developed in the civil calendar was that the solar year did not consist of exactly 365 days. His calculations revealed that the year actually consisted of 365.25 days, so that any calendar based on a 365-day year would lose a whole day every four years. He decided that the way to solve the problem would be to intercalate an extra day every fourth year. Though the Egyptian government did not listen to his proposals, his work attracted the attention of Julius Caesar, who sought his advice on amending the Roman calendar.

This task proved quite a challenge, for the calendar of the Roman Republic was in a shambles. It consisted of twelve months, each having either twenty-nine or thirty-one days except for February, which had twenty-eight. The year ended up having 355 days, far too few to be in step with the seasons. A Roman administrative office known as the Pontifices was assigned to intercalate whole months when necessary to reconcile the calendar with the seasons. Sosigenes must have seen this solution as a rather messy one—and it was not made any better by the actual practice of the Pontifices. It seems that these officials chose to add extra months not as needed by the solar year but instead to increase the time in office of their favorite politicians. As a result, when Sosigenes took the job, the Roman calendar was several months off the solar year.

Sosigenes’ task was twofold. First, he had to correct the current year, 46 b.c.e., so that it would align itself with the seasons. Second, he was to develop a new calendar that would keep synchronization with the solar year. A fixed system of intercalation would also be helpful, to prevent the Pontifices from changing the calendar according to their whims. Sosigenes accomplished the first task by intercalating a full ninety days into the year 46, making that year have 445 days. Then he designed a new calendar, which was to start on the first of January in 45 b.c.e. For this calendar, which came to be called the Julian calendar after Caesar, Sosigenes used his knowledge of the problems with the Egyptian calendars. He made each standard year consist of 365 days, with each of the twelve Roman months having either thirty or thirty-one days, except for February, which he left at twenty-eight days. In order to keep the calendar in precise synchronization with the solar year, he required that every fourth year an extra day should be intercalated in February.

This calendar, with its extremely simple and fixed method for intercalation, should have finally ended all the confusion and discrepancies caused by the old calendar and the meddling Pontifices. Sosigenes and Caesar seemed to have considered everything in their new calendar. They even prescribed that the intercalary day, the punctum temporis, should be inserted between the twenty-third and the twenty-fourth of February (Roman custom for adding days in the past) and that persons born on the intercalary day would, for legal purposes, be considered to have been born on the twenty-fourth.

Unfortunately, both men completely overestimated the capabilities of an ignorant bureaucracy. The Pontifices managed to misinterpret the command to add the extra day every fourth year. They counted the year in which they added the day as the first year of the cycle and thus managed to insert the extra day every three years. Julius Caesar was assassinated in 44 b.c.e., and Sosigenes had no authority over the Pontifices. It was not until 8 b.c.e. that Augustus remedied the problem and enforced the correct observance of the Julian calendar. It is probable, however, that Sosigenes did not live to see this accomplished.

Significance

The Julian calendar that Sosigenes developed has survived to the present time. The names of a few months have been changed, and the extra day is now inserted at the end of February rather than after the twenty-third. Still, the basic structure of the calendar has changed little through the centuries. In fact, the only significant difference between the modern calendar and the Julian one has to do with how often the extra day is intercalated. For the most part, the four-year rule is still followed; the years of 366 days are referred to as leap years. After several centuries of using the Julian calendar, however, it was noticed that the seasonal events were again out of synchronization with the dates. This problem was traced to Sosigenes’ figure of 365.25 for the length of the year. Advances in astronomy were able to determine the number more precisely, finding it to be 365.24219 days. Sosigenes was only off by eleven minutes per year, but over the centuries this error propagated into several days. It was finally corrected in 1582 by Pope Gregory XIII, who omitted ten days from the calendar that year to bring the dates back into alignment with the vernal equinox. Then he instituted the policy of making centurial years (1600, 1700, 1800, and so on) common years instead of leap years, unless they were evenly divisible by four hundred. Thus, 1900 was not a leap year, but 2000 was.

Besides his achievement with the Julian calendar, little is known of Sosigenes. The few bits of available information are intriguing. It is known that he wrote three treatises on astronomy. One of them, on “revolving spheres,” was likely a primary source for Pliny the Elder’s chapters on the sky in the second book of his massive Naturalis historia (77 c.e.; The Historie of the World, 1601; better known as Natural History). Unfortunately, none of Sosigenes’ texts is extant. All that has survived are a few isolated fragments. One of these fragments indicates that Sosigenes believed that the planet Mercury revolved about the sun—a truly remarkable insight. Hipparchus, the great Alexandrian astronomer who lived before Sosigenes, maintained that all celestial objects revolve about Earth, and Ptolemy, the great Alexandrian astronomer who lived after Sosigenes, developed a model of the solar system based on Hipparchus’s data and ideas. Sosigenes’ view was ignored. It would be some fourteen centuries before anyone would advance such a notion again.

Bibliography

Duncan, David Ewing. Calendar: Humanity’s Epic Struggle to Determine a True and Accurate Year. New York: Avon Books, 1999. A sweeping history of calendars, including Caesar’s scrapping of the Roman system in favor of one based on Alexandrian science. More Eurocentric than Richards’s book.

Michels, Agnes Kirsopp. The Calendar of the Roman Republic. Princeton, N.J.: Princeton University Press, 1967. The best work available on the pre-Julian calendar. It includes a discussion of the peculiarities of the Roman enumeration of dates, which continued into the Julian calendar of Sosigenes.

Mommsen, Theodor. The History of Rome. Translated by William P. Dickson. 4 vols. New York: Charles Scribner’s Sons, 1887. This work gives the political background for the calendar reform of Julius Caesar and Sosigenes.

Packer, George. Our Calendar. Williamsport, Pa.: Fred R. Miller Blank Book Co., 1892. This work describes the Julian calendar and Pope Gregory’s reform. Though out of date, the book is useful for anyone who seeks a mathematical examination of the calendar.

Philip, Alexander. The Calendar: Its History, Structure, and Improvement. Cambridge, England: Cambridge University Press, 1921. The best overarching discussion of Sosigenes’ work with the Julian calendar. It explains, in accessible terms, both the astronomical and the anthropological concerns that influenced the development of the modern calendar.

Pliny the Elder. Natural History. Translated by John Bostock. London: Henry G. Bohn, 1855. Has extensive annotations; one of the most useful translations of Pliny’s great work. In the second book, Pliny writes of Sosigenes’ work on the planet Mercury. He discusses the calendar reform in his eighteenth book.

Richards, E. G. Mapping Time: The Calendar and Its History. New York: Oxford University Press, 2000. A history of the development of calendars worldwide, more global in perspective than Duncan’s book.

Steel, Duncan. Marking Time: The Epic Quest to Invent the Perfect Calendar. New York: John Wiley, 2000. From the Sumerians to the present day, astronomer Steel traces the history of the calendar as it paralleled the growth of civilization.