Builders putting in foundations for a primary school on the campus of Jiaotong University in the Chinese city of Xian in 1987 called in archaeologists: they had stumbled across an ancient tomb. No Terracotta Army greeted them - the tomb had been looted twice already in antiquity. But the tomb's unknown owner had nevertheless left an interesting legacy which is only just being made known in the West. On the domed ceiling was an ancient star map depicting a whole series of constellations recognisable in Chinese astronomy - the oldest such map of any accuracy ever found.
The tomb seems to have belonged to an important government official who was about 60 years old when he died. From its contents - 200 or so coins scattered on the floor and several vases - and construction, archaeologists could date it to the end of the Western Han dynasty, around 25 BC.
A red line around the walls of the tomb marks the division between sky and earth. The map of the sky on its ceiling looks quite different from a Western star chart. From ancient times, Chinese astronomers divided the night sky into several hundred constellations, most of which would not be familiar to us. The Western system of astral mapping did not become widespread in China until this century. Only a few well-known formations such as the Plough (Beidou - the Northern Dipper) and Orion (Shen - the Triad) are common to both schemes. The difference is not surprising. Of the 6000 or so stars estimated to be visible to the unaided eye over the entire sky, many are very faint, so any attempt to divide them into groups must necessarily be arbitrary.
The Western constellations have a long history, perhaps originating in Sumeria (now Iraq) around 2000 BC. Star groups recognised by Babylonian astronomers formed the basis of Greek lists such as those of Hipparchus (129 BC) and Ptolemy (about AD 130). Ptolemy charted more than 1000 stars in 48 constellations. Copies of his catalogue are still preserved in his Almagest, a compendium of ancient Greek astronomy which had a profound impact on the medieval Islamic world. Today, 88 separate constellations are recognised by the International Astronomical Union, including several formations in the south polar regions which were beyond Ptolemy's horizon.
Ancient Chinese astronomers divided the night sky into many more constellations than the Greeks. They identified roughly 300 small star groups or 'asterisms', most of which contained no more than five stars. They saw the celestial vault as a macrocosm of the Chinese empire, and almost every aspect of life was represented there. Asterisms and stars in the Purple Palace region - which always remains above the horizon - include the Emperor, Empress and Crown Prince. Also appearing in the sky are Minister, Keeper of the Law, Commander-in-Chief, Celestial Prison, Celestial Granary, Row of Shops and Celestial Stable. A planet or comet entering a constellation was an omen for the terrestrial equivalent. As an astrological treatise compiled in the 7th century AD remarks, 'When a comet leaves (the Celestial market), alterations in the sites of market places or a change of capital may be expected.'
The most important features of the Chinese divisions of the sky were the xiu or lunar lodges - 28 constellations circling the equatorial regions. They had some parallels with the Western zodiac, playing a major role in astrology, and were used to pinpoint the positions of stars in other asterisms. In 1978, a list of the names of all 28 lunar lodges was discovered in a tomb in Hubei province, inscribed on the lid of a lacquer chest dating from 433 BC. It is these same star groups that are portrayed on the Jiaotong star map.
The map's colours - mainly reds, blues and greens - are still quite vivid, although some of the paint has flaked off. The lunar lodge star groups are displayed in a ring 2.9 metres in diameter. Inside the ring are typically Taoist representations of the Sun (containing a flying crow) and Moon (complete with hare and toad), and also cranes flying among clouds. The ring arrangement means that the orientation of the constellations relative to one another is not accurate, but individual constellations are clearly recognisable. About half the lunar lodges depicted on the tomb ceiling are still well preserved. Several of these resemble those found on later star charts. Some 80 surviving stars are depicted by white dots of roughly equal size, joined into groups by short lines, both features which appear on later Chinese star maps. In general, such maps make little or no attempt to distinguish between stars of different brightness. It was the 17th century before the Western system of stellar magnitudes, developed by Ptolemy, was first introduced to China.
Some of the constellations on the Jiaotong map overlay illustrations of men and animals. Few of these depict the subjects the constellations represent - on Chinese star maps pictorial symbols for asterisms are rare. However, the Jiaotong painting displays the symbols of the four cardinal directions: the dragon (denoting the eastern sky), tortoise (north), tiger (west) and bird (south). The figures of the dragon and tiger seem to have been deliberately defaced by robbers, perhaps because they were considered to be guardians of the tomb.
By September last year, the tomb had already suffered some damage from fungi and from visitors' breath, even though it is not open to the public. Zhang Ji-zhen, a professor of electrical engineering at Jiaotong University, is acting as keeper of the tomb and aims to install a periscope system so that the star map may be viewed from outside.
Previously, only illustrations of a few isolated asterisms (notably Beidou) were known to have survived from the 6th century AD or before. Yet many other, perhaps more impressive, ancient star maps may await discovery. At least since the time of Qin Shihuang (the emperor responsible for the production of the now famous Terracotta Army), who died in 210 BC, it appears to have been the custom to depict the stars on the ceiling of a tomb. Chinese writers record that in Qin Shihuang's burial chamber the stars were represented by pearls. But only an annexe of his mausoleum at Lintong, some 30 kilometres to the east of Xian, has been excavated. Under the name Chang'an (meaning 'Everlasting peace'), Xian was the capital of China during the Western Han dynasty (202 BC to AD 9) and it assumed this role again in medieval times. Many unexplored imperial tombs are nearby but there is little money available to investigate them.
The next earliest extensive star map in China, dating from AD 526, is at Luoyang in Henan province and is also painted on a tomb ceiling. At that time Luoyang was the capital of the Northern Wei dynasty. This crudely executed chart, which was discovered in 1973, portrays the stars as red circles on a buff background. Few constellations apart from Beidou are recognisable, but the Milky Way stands out, bisecting the night sky. It does not appear in the Jiaotong painting.
These star charts are just a hint of the skills of astronomers in China at this early period. Later copies of one partly preserved star catalogue, originally dating from about 70 BC, list the celestial coordinates of 120 stars - many to better than the nearest degree. Chinese writings indicate that detailed star maps and celestial globes were produced from the 1st century AD onwards, though none survive. One chart constructed around AD 270 was said to depict 1464 stars in 283 groups, and these numbers influenced the medieval maps of the constellations.
But apart from a colourful but inaccurate chart of the night sky from around AD 800, and originating from Dunhuang in the Sinjiang region, which is held in the British Library, there is no extensive map of the constellations from China before the 13th century. The most important surviving medieval star chart is a circular map, engraved on stone in 1247, at Suzhou in Jiangsu province. It shows the whole of the sky visible from central China, displaying 1434 stars in about 300 constellations as well as the Milky Way. An earlier map, originally printed in AD 1094, only exists in copies from the 18th century onwards. In five sections, one of which shows the south polar regions of the sky as a circular void, it depicts 1455 stars in 283 groups.
When the Jesuit Matteo Ricci reached China's capital Peking in AD 1601, he was followed by a succession of Jesuit priests, many of whom were excellent astronomers. Several attained the position of Astronomer Royal at the Qing court. It was the Jesuits who introduced to China Western techniques of mapping the sky, including the division of the stars into six classes of brightness and the first accurate knowledge of the far southern stars. Jesuit star charts in the Chinese style include a bronze celestial globe 1.5 metres in diameter produced by Ferdinand Verbiest in 1670, and an extensive map displaying more than 3000 stars produced by a team of Jesuits in 1752. The globe, which weighs nearly 4 tonnes, is at the Old Observatory in Peking and is still in excellent condition after more than 300 years in the open air.
Traditional Chinese mapping of the sky came to an end in 1903, with a celestial globe about a metre in diameter which shows 1440 stars. This replaced the globe cast by Verbiest which had been taken to Germany in 1900 and was not returned to China until 1921. Formal Western astronomy was adopted in 1912 with the establishment of the Chinese Republic, but the star maps remain a unique record of the heavens of antiquity.
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18:00 23 July 2008
