The oldest known map of the stars will go on display in Britain this week in an exhibition that will demonstrate the supremacy of early Chinese astronomy. A fresh analysis of the star chart, which was found buried in a desert cave on the ancient Silk Road between China and the West, has dated the manuscript to as early as the 7th century AD.
This makes the chart several centuries older than the first star maps produced in Europe during the height of the Renaissance when astronomy benefited from the invention of the telescope. The chart is written on paper – a Chinese invention – and is a representation of the complete sky with some 1,585 stars grouped into 257 clusters or “asterisms”.
Although the ancient Babylonians and Greeks were avid observers of the night sky and catalogued stars, they did not produce such a complete star chart, or at least one that has survived intact. The Dunhuang star map is named after the site on the Silk Road where it was discovered in 1907 by the British archaeologist Sir Marc Aurel Stein who recognised its importance and brought it back to the British Museum in London.
In 1959, the chart was dated by the science historian Joseph Needham to about AD940, but it was later re-evaluated by a Chinese scholar who dated it to between AD705 and AD710.
Two French academics now believe the star chart is even older and may have been drawn as early as the start of the Tang period, AD618. Jean-Marc Bonnet-Bidaud of the French Atomic Energy Agency and Françoise Praderie of the Observatoire de Paris also believe that the Dunhuang manuscript is a copy of a much older map that has since been lost to antiquity.
In a description for the British Library’s exhibition on the Silk Road, which opens this week, the scientists say: “The overall quality of the document clearly demonstrates a mature technique so the chart was probably produced as a copy of an earlier existing document.”
M. Bonnet-Bidaud said: “Curiously, the information in the texts accompanying the charts is extremely similar in style and content to the notations given in (a) much earlier astronomical text, the Yue Ling or ‘Monthly Ordinances’, dated approximately 300BC.
The chart is split into the 12 divisions of the Chinese year and includes faint stars that are difficult to see with the naked eye. It highlights the expertise of early Chinese astronomers who were the first to make detailed descriptions of astronomical phenomena, such as eclipses and comets, which are still used by modern astronomers.
One of the technical problems of producing a complete star chart of the sky is the difficulty of converting the three-dimensional sphere of space into a two-dimensional plan, M. Bonnet-Bidaud said. “A freehand drawing based on direct vision will be highly distorted since the eyes see only a limited portion of the sky at a time,” he said.
One possibility is that the early Chinese astronomers used a method of projecting the sky onto a cylinder using a Mercator-like projection system – the traditional way of making a two-dimensional map of the spherical Earth by sacrificing accuracy at the poles for the sake of accuracy nearer the equator.
Mme Praderie said it is likely that the stars nearer to the poles were drawn separately to overcome the distortions that would otherwise be introduced using such a projection method. “The composition of the chart and its presentation are modern,” she said. “They are similar to our modern geographical maps of the Earth.”
The origin and use of the star chart remains a mystery, although it is likely to have had a military purpose or perhaps to have been a guide to travellers, Mme Praderie said. It was almost certainly used for uranomancy, the divination of events by consulting the heavens, which is supported by texts on cloud divination preceding the star charts, she said.
“The long-tradition in China of searching the sky for celestial omens has, therefore, led to an early and unsurpassed precision in star catalogues,” she said.