Legacy of the Chinese Astronomers

How the Babylonian and Chinese have left an astronomical legacy to the world

By , 21st May 2014 in Astronomy

Three thousand years ago, two great civilisations existed, both of which made major contributions to the development of astronomy.

One was the Babylonian empire, and the other China. Their astronomical contributions were in many ways vastly different, with the astronomy of ancient Babylon being the one brought to us by the Greeks, the astronomy of China grew up independently, steeped in tradition, its full glories coming to be known in the west only in the 20th century.

1675 image of a Chinese astronomer with an elaborate armillary sphere.
Image Credit: Wikipedia
1675 image of a Chinese astronomer with an elaborate armillary sphere.

The ancient civilisations of China and Babylon were geographically distinct, but they had several features in common, perhaps the most significant being that they were both great river cultures. In China's case, this meant the Huang Ho (Yellow River) and the Yangtze Rivers. Could it be that the challenges involved in controlling, and farming alongside, mighty rivers gave an early stimulus to that complex and organised mode of living that we call civilisation? Great rivers were also natural arteries for trade and communication, and the vast skies that went with these flat landscapes must have made the ancients constantly aware of the motion of the heavenly bodies. As cultures became more complex and used written records, people began to realise that celestial rhythms could be used to organise public and religious life.

As far back as 2000 BC, astronomy was having an enduring impact on Chinese culture. The rock carvings at Jiangjunya, in modern Jiangsu, clearly depict the Sun, Moon, bright stars and the Milky Way, and probably constitute the oldest surviving star maps in the World. By the time of the Shang Kingdom in l500 BC, the shoulder blades of oxen and deer were being used for purposes of divination. Chinese archaeologists have found many such bones carrying ancient inscriptions recording astronomical events.

Science of the Kings

China is unique in astronomical history for the run of surviving observations its civilisation has produced, spanning thousands of years. This derives from the way in which traditional Chinese culture saw the heavens, the Earth and political stability as being interconnected. In this culture the King, and later the Emperor, were seen as a divine personage who carried upon his shoulders the heavy burden of the peaceful ordering of the World.

If an Emperor was wise and just, then all went well, the rivers did not burst their banks, there were no epidemics, and harvests were plentiful. And what's more, the heavenly patterns proceeded with an ordered regularity. But if he was unjust, then even the heavens went out of kilter, as 'guest stars', such as comets or supernovae, disrupted the nightly peace, the Moon turned blood-red, and the Sun was attacked by evil dragons.

It was because of its obviously political and religious role in giving guidance to the Emperor that serious astronomy in China was kept very much within a particular organisation in the Mandarin class system: the Astronomy Bureau. Just as today the CIA would not welcome outsiders picking up, decoding and broadcasting "top secret" intelligence, so Imperial China discouraged non-Mandarins from interfering with "sensitive" astronomical knowledge. It was because the Chinese had been keeping sky records for so long that they became aware of emerging patterns.

Chinese Skymap
Image Credit: Unknown source, please advise
Chinese Skymap

By 500 BC they had recognised the presence of a 19-year pattern in what we would later come to call the Metonic cycle, and by AD 600 had developed empirical methods enabling them to make fairly reliable lunar and solar eclipse predictions. They were, however, very much aware of an incompatibility between two necessary methods of reckoning time - the daily convenience of a Moon phase calendar and the agricultural year of ?xed seasons governed by the Sun's motion through the solstices and equinoxes.

It was calendrical astronomy, with its aim of advising the Emperor in the ordering of creation that provided the great and continuing stimulus to Chinese astronomy. From remote antiquity to fairly modern times, records were compiled, studied and analysed, and great arithmetic cycles discerned within them. It was by this method, for instance, that the imperial astronomers had, by AD 500 established the length of the solar year as 365.24 days, which is within a tiny fraction of the value accepted by modern astronomers.

But how did Chinese court astronomers go about the study of the heavens in practical terms? Firstly, they divided the sky into ?ve "palaces" or zones, consisting of the "purple palace" of the circumpolar stars and four more representing the east, west, south, and north places respectively.

As the astronomical bodies moved through these palaces, their positions and any abnormality was assiduously recorded. Secondly, they divided the celestial equator into 12 zones determined by the places at which Jupiter stood at different points in its 12-year orbit. The Chinese also recognised around 284 separate star groups or constellations, although Chinese asterisms were made up of much smaller groupings than those we inherited from the ancient Middle East.

Measuring the height of the Sun at different seasons is always essential to the assembling of calendrical data. At first, the Chinese astronomers did this with a bamboo pole about 2.5m (8ft) long, noting exactly how long the shadow was for a given day in the year. With a sufficiently long run of observations - over decades or centuries perhaps - such shadow observations could be averaged out to give some pretty accurate results. This could be done even better when bamboo poles were replaced by great stone instruments, where a ray of sunlight passing through a small hole in a high tower was made to fall upon a water-levelled scale set in the exact plane of the meridian. One example is the 40ft-high tower built by the astronomer Guo Shoujing in AD 1276, which still survives virtually intact at Gaocheng.

The armillary sphere in the courtyard of the Ancient Observatory in Beijing
Image Credit: Wikipedia
The armillary sphere in the courtyard of the Ancient Observatory in Beijing

Like their Greek counterparts, yet wholly independently of them, the Chinese erected great bronze armillary rings as reference points to delineate the celestial equator, meridian and ecliptic, one of the earliest of which was set up by Luoxia Hong in 12O BC. These "armillary spheres" could be used for out-of-the-meridian angular sightings of astronomical bodies, giving the equivalents of what modern astronomers call right ascension and declination angles. Several of these great armillaries survive, and in addition to being instruments of considerable accuracy for the time (working on equatorial coordinates); they were pieces of exquisite beauty.

Chinese craftsmanship has long been legendary, not just in the more familiar porcelain and jade working, but in the casting of large bronze artefacts. Chinese geometry Like all Chinese circular graduated instruments, armillary spheres were divided not into 360 degrees, but into 365 and a quarter digits deriving from the daily units of the Sun's path around the sky. This astronomically precise, but geometrically clumsy number (365 and a quarter is impossible to subdivide into holes like 90°) may have been one reason why China never developed a geometrical astronomy and cosmology based upon spherical triangles and points within a sphere. Indeed, Chinese scholars never formulated a 'philosophical geometry' as a means of envisaging space in the way the Greeks did, with their inheritance of the Babylonian 12 zodiac divisions, each broken into 30 daily parts. This is, perhaps, the origin of our 360° circles.

Patterns in the stars

Western astronomy was built around Ptolemy's 48 constellations, but Chinese astronomers recognised nearly 300. These were made of smaller groupings, often of no more than six or seven stars. By the time the definitive Chinese planisphere was complied, as a teaching aid to the future emperor Nan Tsung in AD 1193, there were 1,565 individually named stars. Most Chinese constellations are very old, and few corresponded to Western groupings. The stars of the Greek constellation Hydra (the Water Snake), for example, are divided into more than three groups in the Chinese system.

East and West do share certain conspicuous asterisms, such as Orion's Belt - known as Shen, or the 'Union of Three' to the Chinese, and the Great Bear, or Northern Bushel. Modern scholars have noted that China lacked 'maritime' constellations, such as Cetus the Whale or Cancer the Crab, preferring agricultural or military names, such as Thein Lecch'ng, or the 'Celestial Ramparts', near Aquarius. And there are some delightful star myths: Altair and Vega were the herd boy and the spinning girl; while Antares seems to have been Orion's brother, with whom he quarrelled.

The Chinese and cosmology

The Chinese never had a geometrically-based cosmology like the Greeks, and they believed China and her Emperors to occupy the centre of Creation. But Chinese cosmological ideas nonetheless passed through several major conceptual developments. Central to all three cosmological systems that developed between circa 500 BC and AD 500 was the importance of the polar axis of the Universe, around which the sky rotated. The most ancient cosmological scheme was known as the Kai Thein. Here the sky was seen as a canopy arching over a curved Earth somewhat resembling an upturned bowl, and at the bottom was an ocean. Then there was the Hun Thein, promulgated by Loxia Hong and Zhang Heng between 130 BC and AD 100. This understood Earth as a sphere, probably floating in a great ocean, and surrounded by a spherical sky. This idea had its critics, though, who asked how the fiery Sun could pass from west to east at night if it was necessary to go beneath the ocean on which the Earth floated. But the Xuan Ye theory was the most remarkable of all: space was seen as an infinite realm, through which astronomical bodies moved under the direction of a kind of wind.

It is fascinating how different cultures develop what might be termed different 'intellectual styles'. While the Chinese had a practical geometry for surveying, map-reading, and architecture, they never developed a science of proofs and axioms. However, Chinese mathematics made great progress in complex arithmetic and algebra in a way that the Greeks did not. Chinese astronomy remained a closed book to the West until very recent times. Marco Polo, the medieval traveller to China, said little about it, and it was not until Jesuit missionary scientists to China wrote their books in the late seventeenth century that even its most basic facts were disclosed to European scholars. It was in the twentieth century that Chinese astronomical culture came to be studied by outsiders, largely in the wake of pioneering research into Chinese scienti?c history by Dr Joseph Needham of Cambridge.

Now, both Chinese and Western scholars are working together to make the richness of China's astronomical legacy available to the world.

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