The mythical single-horned beast, the unicorn, is represented by this constellation which was unknown to the ancient Greeks. Monoceros was first depicted in 1612 under the name Monoceros Unicornis on a globe by the Dutch theologian and cartographer Petrus Plancius. This was the same globe on which Camelopardalis, another of his inventions, first appeared.
In 1624 the German astronomer Jacob Bartsch depicted it under the name Unicornu (sic) on a star chart in his book Usus Astronomicus Planisphaerii Stellati and as a result he was sometimes wrongly credited with its invention. In his book, Bartsch pointed to several passages in the Bible that supposedly mention unicorns. It is not clear whether this is why Plancius introduced the constellation, but the unicorn has long been regarded as a Christian symbol. The Polish astronomer Johannes Hevelius adopted Monoceros in his influential star atlas and catalogue published in 1690 which ensured its acceptance by other astronomers.
Monoceros prances between Canis Major (below it) and Canis Minor (above) on the Atlas Coelestis of John Flamsteed (1729).
Monoceros fills a large area between Hydra and Orion where there was no Greek constellation. It is not prominent (its brightest stars are of fourth magnitude) but it lies in the Milky Way and contains a host of fascinating objects, most notably the Rosette Nebula, a wreath-shaped mass of glowing gas with embedded stars.
There are no legends associated with the constellation, as it is a modern figure, and none of its stars have names.
Depictions of the unicorn
Plancius had already shown the unicorn and giraffe together in one corner of his world map of 1594 (below) which depicted animals from Africa; they appear with some elephants, plus what appears to be a fat-tailed sheep, and, further to the right, some dromedaries. The posture of the unicorn, dipping its horn into a stream as though drinking, is reminiscent of a scene from the Hunt of the Unicorn tapestries, woven in the southern Netherlands around a century earlier, which Plancius could well have seen; however, in the sky, the unicorn is imagined with head and horn held high.
Chinese astronomers were adept at creating constellations from faint stars, but even they struggled in Monoceros. A chain of four stars consisting of 8, 13 and 17 Monocerotis plus one in southern Gemini formed Sidu, representing the four major rivers of China (Yangtze, Yellow, Huai and Si). Delta Monocerotis and one other star, probably 18 Mon, formed Queqiu, representing two hillocks either side of a gateway to the palace. According to Sun and Kistemaker (1997) Alpha Monocerotis was part of Tiangou, a guard dog, most of which lay in northern Puppis; other sources, though, place Tiangou farther south. Beta and Gamma Monocerotis seem not to have featured in any Chinese constellation.
© Ian Ridpath. All rights reserved