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  According to astral sources such as circular and tabular astrolabes and various observational reports to the kings in astronomical diaries, Babylonian astronomy goes back at least as far as 1800 BC. It appears to be focused mainly on the problem of establishing an accurate calendar, with an emphasis on recording and calculating the motions of the Sun and Moon.

  Another motive for the compulsive observation of these two heavenly bodies did exist, namely the belief in the fateful meaning of certain alignments in the heavens, and in particular, solar eclipses. The extensive continuity of the Babylonian civilization enabled records to be kept over very long periods of time such that features like the gradual precession of the equinox and the regular cycles of solar eclipses could be recognized and studied. The Babylonians divided the sky into zones, the most important being those which lay along the ecliptic, the apparent path followed by the Sun, Moon and planets. These zones were limited to the area called the Zodiac. Latin names for the signs of the Zodiac as we know them today are translations of the old Babylonian constellations.

  Babylonians appear to have been motivated by religious-philosophical reasons to take note only of isolated events. A planet’s first and last appearances in the sky, for example, were noted rather than the systematic paths of the planets. Such occurrences were taken to have astrological significance because of the chance that they might foretell human fate. Though no extant evidence suggests that the Babylonians, unlike the Greeks, came up with any geometrical model of the cosmos, at the height of its creativity (around 600 BC) Babylonian astronomy could predict planetary motions with surprising accuracy.

  The Babylonian Period

  When Hammurabi, the Semitic king conquered the Sumerians, completing the unification of the region ‘between the two rivers’, Babylon became the capital city of his kingdom. Located on the left bank of the Euphrates, some 70 miles (113 kilometres) south of modern Baghdad, Babylon was ruled by the Hammurabi dynasty during what is referred to as the ‘Old Babylonian Period’ of 2000–1600 BC.

  Following this ‘Golden Age’, when Babylon became the leading centre and capital, the whole region became known as Babylonia. Thus exists the convention of calling the mathematics and astronomy of this region ‘Babylonian’ even if they were not always originated or developed in the city of Babylon.

  The rich heritage of literature, religion and astronomy from this period, found in the ruins of the ancient cities, would never have endured without the existence of a durable recording medium. The cuneiform clay tablets handed down from the Sumerians were perfect. These tablets were made from soft clay and written upon with the wedge-shaped stylus from which the name cuneiform is derived. The Latin word cuneus means ‘wedge’.

  A completed tablet was dried or baked until hard and usually protected by a clay case or envelope. Practically indestructible when dried, these tablets have given a wealth of information to modern scholars from this period, including thousands of astronomical and mathematical records. The ancient site of Nippur, once the site of an astronomical observatory in the Assyrian kingdom, for example, has alone produced 50,000 tablets.

  The Old Babylonian period was a time of great advancement for the region. During this era most of the religious beliefs that developed encouraged the growth of a sophisticated astrology. Astrologer-priests made predictions of pending disasters based on celestial omens. By the beginning of the first millennium BC, the Babylonians had a highly developed writing tradition, sky-watching skills that had been applied in the creation of a calendar and a system of mathematics that was used to track the motion of the Sun and Moon.

  Other studies of calendar making by ancient civilizations such as the Egyptians and the Chinese show impressive schemes of constellation maps and Sun and Moon tracking, all designed to solve the problem of the synchronization of the Moon’s motion with the Sun. Though the Moon provides a convenient time cycle for dividing the year, it has no bearing on the all-important seasons, which depend on the Sun. But the Babylonians went beyond others in their tenacity to use the Moon’s cycle as a time keeping device. To do this they systematically approached the Moon’s motion in a matter not unlike the way natural science is carried out today.

  Babylonian astronomers started with careful observations of the Moon’s motion. These observations were accurately recorded over long periods of time. Next, they searched for repeating patterns in their records. Finally, they simulated these patterns using mathematical models to predict future positions. This may seem like a description of modern applied mathematical science, but is in fact how the Babylonians studied the motion of the Sun and the Moon during the first millennium BC.

  Developing a lunar-solar calendar was relatively simple compared to their more ambitious goal of describing the complete movements of the primary heavenly bodies. By so doing, the scribes wished to anticipate as much as possible the occurrence of a lunar or solar eclipse, one of the most feared omens to appear in the sky. An eclipse of the Sun or the Moon was an awesome sight for the ancients. There is much evidence from early societies that they were shaken by the darkening and disappearance of the two celestial bodies which seemed to govern and sustain their existence. The sky was a dominating feature of that world, a fact since obscured by the prevalence of artificial lighting and different modes of time keeping. The regularity of celestial events provided order to early understanding of the cosmos.

  Careful observations of the heavens allowed early stargazers to establish a division of time that enabled the development of calendars. Calendars, in turn, allowed for the planning of increasingly complex activities. Predicting the recurrence of the seasons (for agriculture) and reference points in the sky (for more extensive navigation) was essential in the development of a broader world view. An eclipse jeopardized this order and regularity.

  By the third millennium BC, the Babylonians had become obsessed with celestial omens. Unlike the Egyptians, who had absolutely no interest in the dozens of eclipses that crossed the Nile during this same period, the Babylonians seemed so concerned about eclipses of the Sun and the Moon that they developed elaborate schemes to record these occultations over very long periods of time. This kind of record keeping suited them well. As J.J. Finkelstein of Berkeley has explained in his paper on Mesopotamian historiography (1963):

  To the Mesopotamian, the crucial and urgent study was the entire objective universe, without any interposition of the self between the observer and the observed. There probably has never been another civilization so single-mindedly bent on the accumulation of information, and on eschewing any generalization or enunciation of principles.

  The Babylonians thus had compelling reasons for looking to the heavens. As the heavens were generally thought to be the home of the gods, Babylonians tried to read their destiny in unusual celestial happenings. For example, a letter from a diviner from the time of Hammurabi (about 1780 BC) reports on an eclipse of the Moon, which he suspected was a bad omen. During the same Hammurabi period a short manual of celestial omens appeared with the following instruction: ‘If, on the day of its disappearance, the god Sin (the Moon) slows down in the sky (instead of disappearing suddenly), there will be drought and famine.’

  Although celestial omens from the Old Babylonian period are known, more substantial development only came in the first millennium BC. Indeed, it was the thousands of artefacts of astronomical divinations that were found at the famous library at Nineveh that then produced the thousands of clay tablets referred to above.

  In 536 BC, after seventy years of supremacy, the Babylonian empire came to an end when it fell to the Persians.

  Assyrians: Warriors and Astrologers

  The Assyrians were an extremely war-like people living around Assur in the Tigris valley in about 1100 BC. These people destroyed the first Babylonian state and extended their boundaries towards Asia Minor and Armenia. The new capital Nineveh was the political centre of a large military empire and as such was adorned with magnificent buildings made of the ubiquitous c
lay. Babylon, the great and rich commercial centre whose wealthy citizens largely governed themselves, retained its rank as a venerable seat of ancient culture. The Assyrian kings recognized the importance of Babylon at first, taking their oaths of office there, but in 689 BC the Assyrians turned against the great city and had it destroyed.

  Yet they did not wipe out the Babylonian’s fascination with stargazing. Having already adopted the ancient and quasi-religious practice of ‘divination’, they also absorbed the mathematical methods carried over from the Old Babylonian Period. Their rulers employed specialists in divination to continue the tradition of recording and interpreting eclipses and conjunctions of the Moon with planets, planetary movements, meteors and comets. A superstitious fear of calamity coupled with the belief in negotiating with the gods led to the intense interest in predicting eclipses. This in turn gave birth to the development of a programme of stargazing not unlike that practised by the Babylonians before them.

  The Assyrians applied their skills of organization and discipline, building astronomical observatories with temple towers throughout the region. Over the period dating from 709–649 BC, reports were prepared which indicate not only detailed observations, but in the case of unfavourable eclipses, attempts at prediction. As the divination cult decreed, a successful prediction provided an opportunity to make supplication against any anticipated danger to come.

  In time, however, the Assyrians were conquered by the Chaldeans, the last dynasty to rule in Babylon before Cyrus the Great’s conquest by Persia. According to ancient historical writings, Persians were also known for their predictive sky-science and their obsession with celestial observations. There are conservative estimates that these people observed 373 solar eclipses and 832 lunar eclipses during their history, an impressive record given the rarity of this phenomenon.

  As legend has it, Nabonassar destroyed all the records of the previous kings of Babylon so that the reckoning of the Chaldean dynasty would begin with him. This new beginning was so effective that, centuries later, the Egyptian astronomer Ptolemy used Nabonassar’s reign to fix the beginning of an era (‘the reign of the Babylonian kings’). This was because he felt that the earliest usable observations began at this time. He went so far as to suggest that the era began at midday on 26 February 747 BC.

  The date of 26 February 747 BC also marked an important beginning in the history of astronomy, because from this date, highly accurate astronomical observations by the chaldeans were kept on a regular basis until after the birth of Christ. Although the motive for these reports was still mainly astrological, these observations became increasingly what can only be described as scientific. Astronomical texts reveal that through centuries of pre-eminence under the Chaldean dynasties and later even during periods of decline, the celestial observations continued at Babylon on a regular basis with little change of pattern.

  Modern scholars estimate the programme lasted almost eight hundred years. The most recent surviving astronomical text dates from AD 75, an almanac prepared from contemporary observations. Thus, from 750 BC to AD 75, the watch keepers at Babylon recorded what they saw in the heavens onto clay tablets. These tablets, which may be the most remarkable extant archive in history, are now stacked in the British Museum in the UK.

  To give this achievement some perspective, consider an equivalent project to obtain similar observations at Windsor Castle starting at about the time of its construction in the early thirteenth century, during the time of Richard I and the Magna Carta. If the time depth of the Windsor ‘archives’ were to match Babylon’s, sky watching would still be going on today – having continued through the reign of the Plantagenets and the War of the Roses, the marriage celebrations of Henry VIII, Elizabeth and the Spanish Armada, the Civil War, the Interregnum and the Restoration. Perhaps by the late seventeenth century observations would have been taken over by the Astronomer Royal and visited by Newton and Halley during the Glorious Revolution. During Queen Victoria’s reign, her husband Albert, the overseer of great civic works, would no doubt have supervised the project. In the twentieth century, scribes would get deferments from the Great War, survive the blitz of the Luftwaffe and even the celebrations of the end of the Millennium.

  The priests and scholars responsible for this remarkable programme of observations recorded continuously for over eight hundred years could be called ‘Babylonian watch keepers’. As the centuries passed, mathematical models were applied to reproduce past observations and predict future movement of heavenly bodies, and the cult of astrology became more and more like what we now know as astronomy.

  Though the greatest concentration of these observatories was in Babylon and the towns near it, the Assyrian records are the most complete as a result of the sacking of Assurbanipal’s great library at Nineveh in 612 BC. This era was recognized by later historians as a turning point in the history of science. In the centuries to follow, increased accuracy in observations and the applications of mathematics turned the work of the scribes in Babylon into a science.

  The Zodiac and the Celestial Sphere

  With a strong mathematical tradition dating back to the Old Babylonian Period, Chaldean astronomers began to develop mathematical theory, relegating observations to a more a minor role. Analysis of the records of ancient observations suggests a model of mathematical simulation based on the celestial sphere shown in the figure overleaf. This model made the prediction of current and future astronomical phenomena possible.

  However, more accuracy was demanded. As early as 1000 BC the scribes had recognized 18 constellations through which the Moon, the Sun and the planets always appeared to move. By 500 BC these constellations were systematized in such a way that they were distributed among the twelve months, individually or sometimes in pairs. For example, the second month of the Babylonian year (which corresponds to mid-April to mid-May), had symbols of both Taurus and the Plaeides; the third month Gemini and Orion; and the twelfth month Pisces and Pegasus. This ring of constellations that lines the ecliptic was called the zodiac.

  The Celestial Sphere Model.

  In an attempt to gain precise markings for the astronomical diaries and observations, the ecliptic path on which the Sun moved was divided into 12 equal parts of 30 degrees, making a total of 360 degrees, a complete rotation. This scheme was adopted after the Persians conquered Babylon in 538 BC but only used for the first time in a diary in 464 BC.

  By about 400 BC the zodiac constellations had become the clearly defined zodiac that is used today. Beginning with Aries (which corresponds to the time period of mid-March to mid-April), each covers 30 degrees of the sky. The zodiac constellations used on the celestial sphere model of the heavens to locate the Sun, Moon and planets is a scheme that has essentially lasted to the present day.

  Constellations of the Zodiac on the ecliptic.

  Up until the seventeenth century, the main motivation for the study of the movements of the heavenly bodies was the need of astrologers to have tables listing future positions of the Sun, Moon and planets. As we will see, some of the greatest figures in the history of astronomy had to pander to their patron’s requests for better accuracy and improved astrological prognostications. For the Babylonians, this was a matter of life or death and the scribes learned to predict positions by using their sophisticated numerical system to take full advantage of the cycles revealed by their observational records. Columns of numbers were used to investigate special configurations of heavenly bodies such as repeating cycles of the alignments of the Earth, Moon and Sun.

  The most important achievement of this entire early period from the standpoint of the history of science was the Babylonian solution to the problem of the motion of the Sun and the Moon. It was their custom to designate the day after the new Moon as the beginning of each month, that is, when the lunar crescent first appeared after sunset. Originally, this day was determined by observation, but later they wanted to calculate it in advance – that is to say, to make a calendar.

  By about 4
00 BC, astrologers realized that the Sun and the Moon’s apparent motions around the ecliptic did not have a constant speed. These bodies appeared to move with increasing speed for half of each revolution to a definite maximum and then to decrease in speed to the former minimum. The Babylonians attempted to represent this cycle arithmetically by giving the Moon a fixed speed for its motion during one half of its cycle and a different fixed speed for the other half. Later they refined the mathematical method by representing the speed of the Moon as a factor that increases linearly from the minimum to maximum during half the revolution and then decreases to the minimum at the end of the cycle.

  With these calculations of the lunar and solar months, Babylonian astronomers could predict the time of the new Moon and the day on which the month would end. They could accordingly predict the daily positions of Moon and Sun for every day during the month.

  This mathematical science reached the height of its creativity during the Seleucid period. Predictions were essentially reduced to arithmetic, and though this led to highly precise predictions, the Babylonians never considered any geometrical models of the cosmos, (like the celestial sphere) which may have further supported their calculations. Problems were ultimately solved arithmetically without recourse to the cosmology that was to follow.

  The Decline of Babylon

  Inevitably, the Chaldeans did not maintain political power in Babylon. After less than one hundred years, their empire was overthrown by a powerful alliance of Medes and Persians in 538 BC. Babylon then became part of the Persian Empire under Cyrus the Great. Though the days of independent Mesopotamian kingdoms were over, astronomical observations still continued.