Calendars and Chronology in the Ancient World

Introduction

Calendars are devices used to track the passage of time. Even in cultures in which there are no “clocks” of any kind (whether mechanical or simpler devices such as sundials), the passing of days can be noted to monitor natural phenomenon and religious and civic activities. How days are grouped and organized is the basis of all calendrical activity. The kind of calendar a society uses is the basis for its sense of chronology—the sequence of dates and arrangements of events that are important to cultural continuity. Just as individuals cannot really know themselves without remembering the events of their lives, a society cannot really endure without knowing where it came from, how long it has been in existence, and what is coming next. In other words, a calendar is vital to a culture’s sense of history and identity; indeed, it could not exist without one.

Obviously, calendars help people keep track of what day it is, but they do much more than that. The first use is agriculture. It was necessary for ancient societies—especially those with little or no writing—to know when to plant, when to harvest, when the rivers would rise or fall, when the rainy season would come, and when to expect other changes in the environment. Indeed, complex farming, supporting large-scale societies, could not have existed without such knowledge.

Likewise, calendars are necessary for politics and complex governments. Taxes and other financial transactions are measured in time. Elections must occur periodically. Kings need to document the lengths of their reign. Judges must pass sanctions such as prison sentences, and calendars and units of time are required for all this.

Calendars are also necessary for economics. It would be impossible, say, to develop a modern capitalist economy—based on the time-dependent exchange of money—without a calendar. Loans and interest, for example, could not be calculated unless there was a simple and public means to know just how long the money was held.

Finally, there are religious and spiritual aspects to calendars, these being perhaps the most obvious uses of all. Days must be set for spiritual observances. Decisions must be made on whether to base these determinations on the motions of the Sun or the Moon. That is, should a celebration occur after a definite number of days into the year (such as Christmas in the West)? Or should it occur after a certain number of lunar months, falling on, or next to, a seasonal moment such as an equinox (such as Jewish Passover)? Questions such as these have always reinforced the close connections between astronomy and religion in the ancient world.

First observations of the heavens

No one knows exactly when the first devices to measure time or motion in the heavens were developed. However, there are remnants of instruments that many think were probably at least incipient calendars. For example, the scratches and notches on bones of the Ishango culture—a Neolithic fishing society living near Lake Edward in Africa about 10,000 b.c.e.—appear to track lunar months (from the appearance of one full Moon to the next). Also, all over the world there are hundreds of pictures or petrogylphs (images or diagrams chiseled in stone) that show astronomical phenomena and calendarlike images. Astronomers (and now even amateurs with readily available software packages) can use computers to show how the sky appeared hundreds or thousands of years ago. Much research shows that early rock art or cave paintings were actually depictions of astronomical events. One of the most famous cases is of the supernova explosion in the Crab Nebula on July 5, 1054 c.e. Japanese and Chinese written records describe the event in exact detail, but much evidence suggests that many Native American cultures in the American Southwest, for example, noted this incident with rock drawings and carvings showing what the sky would have looked like on this date. All this demonstrates that ancient cultures were astute observers of the sky and could make accurate calculations based on these observations.

Motion of the Sun and Moon

Although calendars are present in every culture, they are by no means simple devices to construct nor simple devices to use. The are several reasons for this, as is evident by examining what goes into the construction of a calendar. All calendar makers face the same questions as they begin: when to start; what named cycles of what length into which to group days (though Westerners feel that seven-day weeks are natural, other cultures present many other possibilities, often cycles longer than a single year); how the named cycles articulate with the solstices (the longest and shortest day of the solar year) and the equinoxes (the two days in each solar year that are of equal length); and whether to add any intercalations. The calendar maker must decide whether to periodically add a unit of time—such as a day every four years during leap year in the Western Gregorian calendar, or a whole extra month after several years in some other system—to fix time lost (or gained) during the year. The Gregorian calendar rounds up or down to a whole number of days to deal with the fact that a year is about 365.25 days. Intercalation also has the advantage of keeping solar time and lunar time synchronized.

This last question on intercalations involves what the natural units of a calendar should be. It is commonly assumed that a “day” is defined as one rotation of the earth about its axis (from one sunrise to another); a “year” is one orbit of the earth about the Sun; and a “month” is one orbit of the Moon about the earth. However, none of these lengths of time are integral multiples of each other. That is, a lunar month is about 29.5 days rather than a simple number such as 30. As mentioned above, a year is really about 365.25 days long rather than 365 days; this is why there is an additional leap day every four years. Another problem is, if “day” is defined as successive appearances of the Sun, this unit will vary from season to season: As everyone knows, days in summer are longer than days in winter. Add to this the lack of a writing system for many ancient cultures, and it is apparent that great human ingenuity was required to make the first calendars several millennia ago.

Calendrical systems

Depending on the starting point and assumptions, many different kinds of calendars are possible. Some of the calendars listed in the table of calendrical systems on the following page are representative of other systems related geographically or culturally (for example, systems similar to the Maya calendar were found throughout much of Mesoamerica). Even within the same culture area, such as Rome, different calendars were invented for varying purposes. Many are solar calendars, based on the motion of the Sun, but others are lunar, based on the orbit of the Moon. Some are combinations of both (lunisolar), and a few are based on the motions of the stars or constellations (stellar). The length of year under each system, given in the right column of the table, often reflects the desired unit of measure rather than any scientific accuracy. The following are descriptions of some of the ancient world’s calendars.

Classical Middle East

The earliest calendars of classical antiquity were lunar. The Sumerians as far back as 2000 b.c.e. used a hypothetical year of twelve months of 30 days (giving a year of 360 days). In 1800 b.c.e., the Babylonian Empire adopted the Sumerian calendar (and the Persians later adopted the Babylonian calendar). The Babylonians added leap months periodically to make the solar and lunar years coincide.

Early on, however, the Egyptians, as far back as 2000 b.c.e., discovered the solar calendar of 365 days. It is likely that the extreme predictability of the Nile floods—and the repeated measuring of their high-water marks—helped them to establish these dates. Also, the Egyptians discovered that Sirius (the Dog Star) rises once a year in direct line with the Sun (which also coincided with the Nile floods). By timing Sirius’s appearance from year to year, the Egyptians calculated that the solar year was actually a quarter-day longer than 365 days (though this was not often incorporated into their calendrical calculations). This was almost two thousand years before Julius Caesar established his Julian calendar in Rome.

Greek and Roman calendars

The most famous calendars in the West are those developed by the Greeks and Romans. Originally, the ancient Greek and Roman calendars were lunar, based on twelve months of about 29.5 days (giving a year of 354 days). However, these soon proved inadequate, being 11 days short of the solar year. For example, using this shorter lunar calendar, within a mere sixteen years, the summer and winter solstices would switch, causing this lunar calendar to be totally out of synchrony with the seasons (and making it completely useless for agricultural planning).

Julius Caesar, after dallying with Cleopatra VII in Egypt, returned to Rome in 47 b.c.e. with knowledge he had gained of Egyptian mathematics and astronomy. One of his first decrees was to reform the calendar throughout the whole of the vast Roman Empire. As one writer argues, such action was not just a practical convenience but a demonstration that the empire and the emperor had the authority and symbolic power to reorder time. Caesar called the best minds of the era together, from as far away as Alexandria, to fix the inadequacies of the old calendar. Basically, Caesar adopted the system proposed almost two hundred years earlier by Ptolemy III: Assume the year is 365.25 days long, count each year as 365 days, and have a leap year of 366 days every four years. Certain adjustments had to be made, such as where the leap day should go and how to get the current year back in alignment with the equinox, but the so-called Julian calendar was proclaimed on January 1, 45 b.c.e. The word kalendus, meaning the first day of the month of March (New Year) in the Roman calendar, is the origin of the English word “calendar.”

However, the new system was not without its flaws nor was it immune to meddling from later pontiffs and emperors, well-intended or merely vain. For example, the Roman senate decided to honor the emperor Augustus by renaming a month after him. To prevent his month having fewer days than the month named after Julius Caesar (July), the senate voted to give the month of “August” 31 days. Days, then, had to be borrowed from someplace else (February). Later emperors tried similar modifications. However, the biggest problem was that the Julian calendar was still slightly too long (about eleven minutes per year, meaning it would gain about a week every millennium). Pope Gregory XIII in 1582 c.e. issued a papal bull correcting these problems with a more accurate value of the solar year, declaring that three out of every four centennial years would not be leap years (for example, 1700, 1800, and 1900 would not be leap years but the year 2000 was). A complex formula for calculating Easter was also established. These reforms became known as the Gregorian calendar, which is the one used in most Western countries today (and is the de facto universal calendar throughout the world). The traditional Julian calendar continues to be used in many rites of the Eastern Orthodox Church, especially for calculating Easter. It currently runs 13 days behind the Gregorian calendar.

By the first century c.e., the seven-day week had become commonplace throughout the Roman world (perhaps being taken from Jewish custom). The origins of the seven-day week in the West are obscure; other cultures have weeks of varying lengths (four days in Africa, five days in Central Asia, six days in ancient Assyria, or ten days in ancient Egypt, to name only a few). It is likely that the approximately four seven-day phases of the Moon in each month gave rise to this division. Each of the days was named after a Roman or Scandinavian god (“Thor’s Day” becoming “Thursday,” “Saturn” becoming “Saturday,” and so on).

The Jewish calendar

The first Jewish calendars may have been written as early as in the time of Solomon around 1000 b.c.e. They are similar to older Roman and Middle Eastern calendars in that the year is solar and the month is lunar. However, Jewish months do not necessary coincide with full moons, and the number of days in the year varies. The supposed date of creation is 3761 b.c.e. Because of the importance of the Sabbath, a seven-day week and the divisions of the day were important to early calendar makers. Part of the complexity of the Jewish calendar is caused by the way of creating cycles to make the solar year and lunar year line up. This is crucial, as most of the important Jewish observances, such as Rosh Hashana (New Year), Yom Kippur (Day of Atonement), and Passover, are month-dependent.

Hijrah Muslim calendar

Most countries in the Muslim ancient world used the hijrah calendar. Hijrah, or “flight,” refers to the Prophet Muḥammad’s exodus from Mecca to begin teaching in Medina on July 16, 622 c.e. (hence the day the calendar begins). This was the time when Muḥammad began his public preaching. The Islamic year is 354 days long, consisting of twelve lunar months of usually 29 or 30 days. Because of the shortness of the lunar year of 354 days compared with the 365 days of the solar year—and because no intercalations are added, the months make a complete cycle every thirty years. Ramadan, the ninth month (of 30 days), is the most important time in the Islamic year, and devout Muslims abstain from all food and drink from dawn until dark. As with all Islamic months, Ramadan moves through the seasons.

Today, most Muslim countries still use the Hijrah calendar, though some countries, such as Turkey in 1677 c.e., have adopted the solar Western calendar. As the calendar is consistently lunar, it is relatively easy to convert dates to the Gregorian calendar: Take the Hijrah year and multiply it by 0.97, then add 625.5. To get a Hijrah date from a Gregorian date, subtract 625.5 from the Western date, multiply by 1.0307, and add 0.46 (rounding up).

Calendars of sub-Saharan Africa

Compared with Muslim, Egyptian, and North African astronomical and calendrical systems, much less is known about those in ancient sub-Saharan Africa. Many, however, as with the dynasties of the west African Yoruba kings, were quite complex. Apparently most were seasonal and often depended on stellar observations as much as those of the Moon or the Sun. Among hunting and gathering foraging bands such as the San (formerly Bushmen), for example, the Moon is the epitome of the cycle of life and death. Even the curved draw of the hunting bow is associated with the crescent Moon.

In many places, calendar systems of other areas are incorporated into the local practices. For example, the Swahili cultures on the east coast of Africa also use the Muslim calendar for many important Islamic religious activities. However, the older Swahili calendar is used for spirit festivals and many agricultural events. Also, New Year’s in the traditional Swahili calendar was supposed to coincide with the Persian New Year.

Maya calendars

Many of the Native American cultures of Latin America were accomplished observers of the heavens. The Aztecs of Mexico and the Inca of Peru all had very complex astronomical and calendrical systems. However, it was the Maya of Mesoamerica who had the most evolved and accurate calendars, and for their time, they were one of the most advanced cultures of the ancient world. They were probably better astronomers than their European contemporaries, and their measurements even rival those of modern times. For example, Maya values for the lunar month varied from 29.5302 to 29.53086 days (compared with the value of 29.53059 days as reckoned in modern times).

The Maya actually used two calendars: the Tzolkin (count of days) religious calendar and the Haab civil calendar. The Tzolkin had a 260-day cycle derived from repeating thirteen numbered “weeks” with 20 named days (that is, 13 times 20, or 260 days). Any given day in the cycle, then, would be called by its name and the number: 12 Kan, 4 Ahau, and so on. Each of these 260 days had astrological and religious significance.

The Haab civil calendar was solar and had eighteen uinal “months” consisting of 20 days each. Five days were added at the end to give a year of 365 days (18 times 20, or 360 days, plus the 5 extra). In general, these days named occurrences of more secular significance (such as agricultural events).

The Maya used both systems simultaneously, with both cycles repeating within each other to produce fifty-two “civil” years. If the 365-day system (18 times 20, plus 5) and the 260-day lunar year (13 times 20) were started at the same point, it took fifty-two years to get back to the exact same spot in both cycles. This sacred cycle of fifty-two years was called the Calendar Round, and the end of each Calendar Round was thought to have auspicious, usually catastrophic, significance.

However, a fifty-two-year cycle would not allow the Maya to make the important connections to the past that they, as expert genealogists and historians, desired; therefore, another system also was developed. This Long Count began on August 12, 3113 b.c.e., apparently the date the Maya believed the world began. The Long Count, however, was neither solar or lunar but purely mathematical, based on nine cycles. A uinal “month” consisted of 20 days; eighteen uinal months was one tun, or “year”; twenty tun years gave one katun cycle; twenty katun gave one baktun, twenty baktun gave one pictun, and so on in multiples of twenty. This gave cycles of more than 63,000,000 (Western) years.

North America

Although much is known about the famous Maya and Aztec calendars, knowledge of other Native American systems is rather sparse. However, though the precise details are not known, ample evidence exists that at least various Pueblo cultures in the American Southwest—the Hopi, Zuni, the ancient Anasazi, and also the Navajo—were students of the heavens. Some based their calendars on the motions of the stars (which, in some ways, are simpler to follow than the motions of the Sun or Moon). For instance, according to traditional Navajo mythology, when the constellation Revolving Male (similar to part of the Big Dipper in the West) lies parallel to the horizon in early evening, it is time to plant. Its position also indicates when different animals will be mating or when winter has arrived. When the constellation known in the West as the Pleiades appears, this means it is too late to plant and still be able to harvest before the first frost.

However, there is also evidence that Eastern woodland Native Americans probably had complex calendrical systems. One of the most famous examples is from the Cahokia site, just across from St. Louis on the Illinois side of the Mississippi River. At this site stand more than a hundred mounds, including Monk’s Mound, a giant earthen pyramid that is the largest pre-Columbian artificial structure in the New World north of Mexico. The Cahokia civilization may have had as many as thirty thousand to forty thousand people living in a complex urban society when the culture reached its peak population about 1200 c.e.

Most striking, however, is the presence of the huge Woodhenge circles, named analogously with the famous Stonehenge structure in England, which many feel were used for calendrical observations. Numerous holes containing remains of cedar posts were first discovered in the 1960’s, though road construction crews obliterated some of them. Regardless, enough evidence still exists to lead many astronomers and archaeologists to conclude that the 200-foot (61-meter) diameter circles of Woodhenge (and there are at least four) were giant observatories. For example, the Sun rises behind the same poles on three overlapping circles on both the spring and autumn equinoxes. Likewise, the winter and summer solstices, which apparently can be predicted within a day or two, can also be readily observed by standing in the center of one of the circles. Such a solar calendar would make for a year of about 365 days, although, interestingly, it appears that the Cahokia astronomers had little interest in lunar activity. There is also ample evidence of other mathematical abilities: The city itself is well-planned, and the mounds appeared to be deliberately aligned along certain axes, oriented in nonrandom directions.

South and Southeast Asia

The early Hindu calendar of around 1000 b.c.e. divided the solar year into twelve months of 28 or 30 days each, adding a leap month every five years. By 400 c.e., outside influences, especially Greek and Mesopotamian, brought a new fascination with astrology and, hence, increased interest in the Sun, the constellations, and the Zodiac. Dates in this classical calendar were given as lunar month, fortnight (depending on the waxing or waning of the Moon), day (a thirtieth part of a lunar month), and solar year of the era.

In 78 c.e., the Shaka (or Shakakāla) calendar was supposedly started by the legendary king Shālivāhana. This is the standard Hindu calendar that many places in India and Southeast Asia still follow. Almost all Hindu festivals follow the Shaka calendar. As it is generally a solar calendar, little more needs to done to obtain the Western Gregorian year than subtract seventy-eight.

Around 500 to 600 c.e., Indian astronomy underwent two transformations. First, technical observational knowledge increased, and this had impact on the scientific calendrical systems. For example, precise measurements of Jupiter showed that it had a twelve-year cyclical period with certain background stars, and new calendars were made with new months incorporating places in this twelve-year cycle. However, partly prompted by this increase in technical skill, people began to take a more intense interest in astrology and divination. This prompted the development of the Kaliyuga calendar, or Chronology of Fictitious Times. It begins on February 18, 3101 b.c.e., the supposed starting point of all celestial revolutions when the Sun, Moon, and all the planets were aligned in perfect conjunction. From this starting point, another such conjunction was predicted to occur 4,320,000 years later. Much of Indian philosophy of ancient times was an attempt to uncover what the meanings of these cycles might be.

China and East Asia

There is little doubt that the Chinese were also excellent astronomers; many say they were the most precise in the world until about 1200 c.e. They also built some of the world’s first observatories. The Chinese bureaucracy realized that calendrical knowledge not only represented royal authority but also political control: That is, knowledge of calendars allowed for control of knowledge about agricultural and water cycles; controlling the water system allowed the government to regulate the economy and the people.

The earliest Chinese written ideographic characters appear on Shang oracle bones from around 1400 b.c.e.; there is also evidence that the Chinese knew of a solar year of 365.24 days and a lunar month of 29.5 days. They also realized, a century or two before the Greeks, the necessity for intercalations—adding extra days to adjust for the differences between a real solar year and an assumed one of just 365 days. In the third century b.c.e., a meteorologically based calendar also became popular.

The basic Chinese lunar calendar had months of 29 or 30 days (giving a year of only 354 days), requiring extra months be added seven times during a twelve-year cycle. Each of these years in this cycle are named after one of the well-known twelve animals in the Chinese zodiac: rat, ox, tiger, rabbit, dragon, snake, horse, ram, monkey, rooster, dog, and pig (for example, the year 2001 is the year of the snake). Each of these signs has a personality, as do people born in those years.

Ancient Chinese calendars were closely tied to Chinese divination practices, and classical writings such as the Yijing (eighth to third century b.c.e.; English translation, 1876; also known as Book of Changes, 1986). However, the Chinese were great almanac compilers as well. In a lunar calendar, every month begins with a new Moon; therefore, it is always possible to tell what day of the month it is from the phases of the Moon. As most Chinese festive occasions are lunar, such a calendar is convenient. However, solar calendars are more beneficial for farmers, so the ancient Chinese put their (quite substantial) knowledge of the subject in various almanacs. Besides giving astronomical information, these almanacs also gave readers personal advice (much like newspaper horoscopes in the West). Some of these are still used today by traditional Chinese.

Stonehenge and other megalith calendars

Probably no discussion of ancient calendars could be complete without mentioning Stonehenge, the famous Druid Neolithic observatory built between 2000 to 1500 b.c.e. Hundreds of other such structures have been found throughout the British Isles. The stone archways set in a large circle could predict eclipses and summer and winter solstices; the rising and setting of the Sun and Moon could also be tracked and the lunar months observed.

Techniques for keeping track of time arose at many times and in many places. This is in stark contrast to other intellectual achievements over the course of history. For example, the invention of the alphabet or the number zero appear to have been discovered only a few times. Anthropologists and archaeologists, then, believe that calendars, or some way of keeping track of time, are something that all cultures, past and present, possess. In that sense, then, calendars take their place along with notions of family, marriage, and a belief in the supernatural as human universals.

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