The Dunhuang Star Chart
The British Library, London Until 18 August 2009.
Along the ancient trade route of the Silk Road connecting China and the West, the Mogao Caves honeycomb the Mingsha Hill some 25 kilometres southeast of Dunhuang, a desert town in Gansu province. Excavated between the fourth and fourteenth centuries, the caves were Buddhist shrines and temples where travellers prayed for the success of their journeys.
In 1900, the Taoist priest Wang Yuanlu propelled the Mogao Caves to the status of an archaeological crown jewel when he stumbled upon a hidden library in Cave 17. It contained more than 40,000 manuscripts on a myriad of subjects, from religion, history, art and literature to mathematics, medicine and economics. The documents had been sealed in the cave by Buddhist monks in the eleventh century.
Among the manuscripts was an exquisite star chart. It shows the entire sky as visible from China, skilfully drawn by hand in red and black inks onto a fine, four-metre-long paper scroll. In 1907, archaeologist Marc Aurel Stein took the chart and more than 7,000 other cave manuscripts to the British Museum in London.
Dated to between 649 and 684 ad, the chart is the oldest extant graphical star atlas in the world, explains Susan Whitfield, director of the British Library's International Dunhuang Project, which aims to make information and images about the artefacts available on the Internet. The atlas is on display at the British Library in London this summer to celebrate the International Year of Astronomy.
The atlas is divided into two sections. One shows 26 drawings of differently shaped clouds accompanied by text on cloud divination. The other section portrays 12 star maps, each depicting a 30 ° division of the sky in the east–west direction, plus a map of the circumpolar sky. The star positions are drawn as observed from a latitude of 34° N, possibly from the Imperial Observatory in Chang'an (present-day Xi'an) or another site in Luoyang.
The atlas shows 1,339 stars arranged in 257 groups, or asterisms, two of which resemble the constellations of the Big Dipper and Orion. It includes faint stars that are difficult to see with the naked eye, and several in the Southern Hemisphere. The styles of the dots differentiate the three schools of astronomical tradition established during the Warring States period (476–221 bc), each of which adopted alternative names and descriptions for the star groups.
The positions of the brightest stars are surprisingly accurate to within a few degrees, says astronomer Jean-Marc Bonnet-Bidaud of the CEA, the French Atomic Energy Commission, who has studied the atlas together with Whitfield and Françoise Praderie of the Paris Observatory (J.-M. Bonnet-Bidaud, F. Praderie and S. Whitfield J. Astron. Hist. Herit. 12, 39–59; 2009). Stars near the celestial horizon are drawn using a cylindrical projection, in which meridians are mapped to equally spaced vertical lines, and circles of latitude are mapped to horizontal lines. The circumpolar region uses an azimuthal projection, preserving the directions of the stars from a central point. These methods are still used in geographical mapping today.
Ancient Chinese astronomers divided the celestial circle into 12 sections to follow the orbit of Jupiter, known as the Year Star in China, which loops the Sun about every 12 years. The Jupiter cycle is also the basis for the 12 months of the year that make up the Chinese calendar. On the Dunhuang chart, the text accompanying each star map names that region of sky, the astrological predictions associated with it and the states of the Chinese empire thought to be influenced by that division.
The chart may have been reproduced from an earlier atlas by tracing it on to fine paper. It has no coordinate grid, and shares wording with another traditional astronomical text, Yue Ling, or Monthly Ordinances, which has been dated to around 300 bc. Yet it remains the earliest-surviving detailed map of the entire northern sky, pre-dating others by several centuries. Older star maps described only part of the sky. The Book of Fixed Stars, an Arabic work written by the Persian astronomer Abd al-Rahman al-Sufi (903–986 ad), displays individual constellations but gives no information on their relative positions. The oldest-known star chart in Europe is the Vienna manuscript. Dated to 1440 ad, it shows only a limited number of stars in northern constellations, plotted in an azimuthal projection from the ecliptic pole.
The chart may have been used to consult the heavens to predict earthly events. Astronomy was an imperial science in ancient China, and court astronomers and astrologers created star charts from at least the fifth century bc. Chinese emperors sought celestial clues for political and warfare decisions, and the importance of divination led to an early precision in star catalogues.
But why was the chart kept in the Mogao Caves rather than in the imperial archive? “It remains a mystery,” says Whitfield. A political and secret document, it may have served a military purpose rather than being a guide for travellers. When the Taoist priest discovered the hidden library, he could hardly have guessed that he was opening the door to a world of such fascinating antiquity.
See http://www.nature.com/astro09 for more on the International Year of Astronomy.
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Qiu, J. Charting the heavens from China. Nature 459, 778–779 (2009). https://doi.org/10.1038/459778a