The Calendar Takes Shape in Mesopotamia

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The Calendar Takes Shape in Mesopotamia


The calendar used today in the West has its roots in the system developed by the astronomers of Mesopotamia—and particularly the Mesopotamian civilization of Babylonia—during the period from the third to first millennium before the Christian era. Other civilizations created their own calendars with varying degrees of accuracy, but it is from Mesopotamia that the concept of the year, month, and day each gained their most consistent and lasting definition. A fourth means of marking time, the week, may also be traced (if perhaps indirectly) to Babylonia.


Practically from the beginning of recorded time, men recognized that a year lasts about 360 days, a number still reflected in the use of a 360° circle among mathematicians and astronomers today. This may also have influenced the Babylonian adaptation of a sexagesimal, or base-60, number system (as opposed to the base-10 system used by Westerners today) in c. 2700 b.c.

The figure of 360 represented a mean, or close to one, between the length of the lunar and solar calendars. As its name implies, a lunar calendar is based on the Moon's revolutions around Earth, of which there are 12 during a solar year. With an average of 29.53 days per synodic month—synodic being a term that refers to the conjunction of celestial bodies, in this case the Moon and the Sun—the lunar calendar lasts about 354.37 days. The adoption of a lunar calendar makes sense on a short-term basis, but over a longer period it soon gets out of phases with the seasons. This explains why today the only major lunar calendar in use is that of the Muslim Middle East, a region that experiences little seasonal variation in climate.

Problems with the lunar calendar influenced the development of a second form, the luni-solar calendar. According to this system, most years consist of 12 months, but every few years it becomes necessary to insert a thirteenth month—a process known as intercalation—to keep the calendar in phase with the seasons. The Chinese calendar in use throughout much of East Asia today, as well as the Jewish or Hebrew calendar, are surviving examples of the luni-solar method. The remainder of the world, however, uses a solar calendar that has its origins in Rome. Yet elements of the Roman, Jewish, and Islamic calendars can be traced to foundations established by the astronomers of Babylonia and other Mesopotamian civilizations.


Babylonian achievements in astronomy resulted from an interest in astrology, and indeed Mesopotamian star-gazing always had a religious component. These astrologers were perhaps the first to equate heavenly bodies with deities: thus the Moon was called Sin, a goddess first worshipped by the Sumerians. In this vein it is interesting to note the close relation between early Babylonian nomenclature for heavenly bodies, and the terms later used by the Romans.

The latter, for instance, associated the Sun with Apollo, who drove across the sky in a fiery chariot; in Babylon it was Shamash who performed the same function, and his became the Babylonian name for the Sun. This similarity may also be found in the names for planets still used today: Venus was a fertility goddess, as was Ishtar, the name used by the Babylonians for that planet. Likewise Marduk was king of the gods, and the Babylonians applied this name to the largest of the solar system's planets, known today as Jupiter.

These facts establish two key points: later astronomers' indebtedness to the Babylonians, and the close relationship between religion and the beginnings of astronomy. But the calendar also had numerous down-to-earth applications, of course, reflected in Mesopotamian methods for dividing the year. Early astronomers of the region simply divided the solar year into two seasons, roughly equivalent to spring-summer and fall-winter. Because Assyria was further north, it became natural to recognize a third season, and still further north, in the Hittite civilization of Anatolia (modern Turkey), astronomers divided the year into four seasons that reflected the cycles of planting and harvesting.

Then there was the idea of the month, which began at the first sign of the new Moon. This practice of reckoning months had already become common by the third millennium b.c., but month names were not standardized. Each city had its own name for months, and sometimes several names, and by the twenty-seventh century b.c. Sumerians began reckoning dates according to the ruler in power. This is a practice familiar to anyone who has read the Old Testament, which is full of passages that begin "On the ____ day of the ____ month in the ____ year of the reign of ____."

Driven by the practical need for a luni-solar year that encompassed the entire agricultural cycle, Sumerian scribes in about 2400 b.c. first adopted a 360-day year consisting of 12 30-day months. According to this system, the financial (what modern people would call "fiscal") year began two months after barley-cutting, when it was time to begin settling accounts. The barley harvest itself marked the beginning of the agricultural year, and since the Sumerians associated good harvests with good leadership both in a political and religious sense, it was a natural step to make this the beginning of the royal year as well.

Thus at the beginning of the year, the ruler offered the first fruits of the harvest to the gods, ensuring their continued favor. Political considerations also influenced the naming of years, which were not numbered except—as noted above—in terms of the ruler in power. Thus if something notable happened during the "____ year of ____'s reign," it also became, for instance, "the year in which ____ built the temple of Inana." By the seventeenth century b.c., however, the Babylonians had standardized year nomenclature, counting regnal years in the form later adapted by the authors of the Bible.

Earlier, in the eighteenth century b.c., the Babylonians under Hammurabi (r. 1792-1750 b.c.) standardized the lunar calendar that had been in use among various Mesopotamian civilizations for about four centuries. The Babylonian year began in the spring on the first day of the month of Nisanu, and after the seventeenth century b.c., the period between the time when a ruler assumed power and Nisanu 1 was referred to as "the beginning of the kingship of ____."

The adoption of the lunar calendar eventually resulted in the need for an intercalated month. This, too, had been a feature of the earliest lunar calendars of the twenty-first century b.c., but its implementation had been rather erratic. Each of the various Sumerian city-states used their own intercalation systems, resulting in enormous confusion. The foundation of multinational empires by the Babylonians, Assyrians, and still later the Persians helped lead to standardization of this system through directives from above. By about 380 b.c., Persian emperors had ensured that the lunar and solar calendars were more or less aligned.

The Babylonian system that came to prevail throughout the Near East consisted of 12 basic months: Nisanu, Ayaru, Simanu, Du'uzu, Abu, Ululu, Tashritu, Arakhsamna, Kislimu, Tebetu, Shabatu, and Adaru. Every 19 solar years, or 235 lunar months, marked an entire luni-solar cycle, which required intercalation in years 3, 6, 8, 11, 14, 17, and 19. In all but the seventeenth year, the added month was called Adaru II, but on the seventeenth year it was Ululu II.

This system may seem highly complicated to a modern observer, but it worked for many centuries, and the Israelites' exposure to Babylonian culture during the Captivity (587-539 b.c.) ensured its lasting impact on the Hebrew calendar. The Bible in turn reflects the Mesopotamian and particularly Babylonian influence on a number of particulars—including, as perhaps shown in the opening chapters of Genesis, the concept of the week. It is not clear, however, whether the modern idea of a week has its roots in the Near East or in a separate system that evolved in Rome.

There, an eight-day market cycle had long prevailed, and by the second century b.c. this became a seven-day cycle with days referring to the gods and their ruling planets: Saturn, the Sun, the Moon, Mars, Mercury, Jupiter, and Venus. The week-day names used by the French and other speakers of Romance languages still reflect the Roman influence, whereas Germanic languages such as English use a combination of Roman and Norse terms. Thus Tiu, Woden, Thor, and Freya replaced Mars, Mercury, Jupiter, and Venus to lend their names to Tuesday, Wednesday, Thursday, and Friday respectively.

Likewise the modern form of the solar calendar more clearly shows the impact of Roman civilization, from whence comes the present system of months, as well as the designation of January 1 as the first day of the year. Yet long before Rome, the Babylonians established the basic idea of a 365-day year divided into 12 months of approximately 30 days apiece, and Roman calendar-makers built on a Mesopotamian foundation established centuries before.


Further Reading


Moss, Carol. Science in Ancient Mesopotamia. New York: F. Watts, 1998.

Neugebauer, Otto. The Exact Sciences in Antiquity. New York: Dover Press, 1968.

Internet Sites

"The Babylonian Calendar." (December 3, 2000).

"Calendars." (December 3, 2000).

Harper, David. A Brief History of the Calendar. (December 3, 2000).