Ian Ridpath’s Star Tales

THE FARNESE ATLAS

This sculpture of Atlas from the second century AD features a celestial globe engraved with the constellations known to the ancient Greeks. Unlike on later globes, no individual stars are shown, just the constellation pictures.

Everyone is familiar with maps of the Earth, but to most people a map of the sky is a mystery. Yet there are many similarities because the celestial cartographer faces the same problem as the terrestrial one: how to represent a curved surface on a flat sheet.

The earliest representations of the sky were actually globes, on which the constellations were shown as though viewed from a God-like position beyond the stars; this meant that the constellation shapes were represented back to front by comparison with the way we see them from Earth. In the Museo Nazionale, Naples, is a marble statue of Atlas holding on his shoulders not the Earth but a globe of the heavens on which the constellations are depicted in this mirror-image way (see picture at right). The sculpture is called the Farnese Atlas, after Cardinal Alessandro Farnese (later Pope Paul III) who acquired it in the early 16th century and exhibited it in the Farnese Palace in Rome.

It is the oldest known celestial globe, for historians think that the sculpture was probably made in Rome around the second century AD. Even more significantly, it is thought to be a copy of a Greek original from the second or third century BC, around the time that Aratus wrote his Phaenomena. Thus the globe held by the Farnese Atlas provides our only firsthand look at the star pictures that the ancient Greeks imagined in the sky. (Note: In 2005 the American astronomer Bradley Schaefer claimed that the star positions on the Farnese globe were based on the original catalogue of the ancient Greek astronomer Hipparchus, which would be of great interest if true. However, Dennis Duke of Florida State University, among others, has refuted this analysis.)

An early form of flat star chart was the astrolabe, popular with the medieval Arabs. Usually made of brass, the astrolabe was a disk on which the positions of bright stars were indicated; the principle lives on in the star-finding devices called planispheres used by present-day amateur astronomers and sailors. The earliest surviving astrolabes date from the tenth century AD, but written evidence shows that they were known much earlier, possibly even in the time of Ptolemy, c. AD 150. Astrolabes are a rich source of old star names.

Other than astrolabes, the oldest known flat sky map is a Chinese paper scroll, over 2 metres long, believed to date from the mid to late 7th century AD. It is known as the Dunhuang star chart after the place on the Silk Road trade route in north central China where it was found in

THE DUNHUANG MANUSCRIPT

Chinese constellations differed markedly from Western ones, being usually much smaller and incorporating fainter stars. This illustration shows the sky around the north celestial pole from a chart dating from 650 to 670 AD that was found in the early 20th century in caves at Dunhuang, China. Among the Chinese constellations shown on this view of the north polar region, only the familiar shape of the Plough or Big Dipper is recognizable. The Dunhuang manuscript is the oldest surviving star map in the world. (© British Library).

the early 20th century; it is now in the British Library, London. Since the Dunhuang chart depicts the Chinese constellation tradition, which was independent of that in Europe and Arabia, most of the constellations are unrecognizable to modern eyes (see, for example, the north polar region of the sky in the box at left).

Chinese constellations were smaller than Western ones, and hence more numerous, each constellation usually consisting of only a handful of stars. This made it easier to identify areas of sky without use of celestial coordinates. Chinese astronomy was flourishing as early as 240 BC, when they observed Halley’s Comet. By the end of the third century AD, Chinese astronomers had developed a system of 283 constellations consisting of 1464 stars (although the Dunhuang chart shows only 1340 stars in 257 groups). These constellations did not depict myths but facets of Chinese court and social life, such as Dizuo, the seat of the emperor; Huanzhe, the court eunuchs; and Tianshi, the celestial market. This system was still in use when Jesuit missionaries introduced Western constellations to the Chinese in the 17th century.

Albrecht Dürer, the great German artist, drew the first European printed chart of the heavens in 1515; it was a pair of woodcuts, one showing the zodiac and all constellations north of it, the other showing all known constellations south of the zodiac, based on the stars and constellations catalogued by the Greek astronomer Ptolemy in his Almagest. The southern map of the pair clearly shows the constellation-free zone around the south pole. The constellations are in mirror image, as on a celestial globe or an astrolabe, a tradition that most early maps were to follow. In addition, Dürer’s depictions of the constellation figures established an artistic style that was echoed on many later celestial maps.