This insignificant constellation, second-smallest in the sky, was one of the 48 constellations listed by the Greek astronomer Ptolemy in the second century AD. It was unknown to Aratus 400 years earlier, and its invention is often attributed to Ptolemy. He called it Protomé hippou, the forepart of a horse; Equuleus is a later Latin name.
However, that is not the whole story. The Greek writer Geminus, who pre-dated Ptolemy by a century or two, tells us that Hipparchus introduced a constellation called Protomé hippou in the second century BC. We do not have Hipparchus’s original to check, but presumably it was the same constellation that Ptolemy adopted for his own catalogue. Hence the true inventor of Equuleus seems to have been Hipparchus rather than Ptolemy.
Equuleus, the foal, seen next to the head of Pegasus
in the Uranographia of Johann Bode (1801).
Equuleus consists merely of a few stars of fourth magnitude and fainter forming the head of a horse, next to the head of the much better-known horse Pegasus. The early mythologists such as Eratosthenes and Hyginus never mentioned this little horse, but perhaps Ptolemy (or Hipparchus) had in mind the story of Hippe and her daughter Melanippe, sometimes told for Pegasus but which seems more appropriate for Equuleus.
Hippe, daughter of Chiron the centaur, one day was seduced by Aeolus, grandson of Deucalion. To hide the secret of her pregnancy from Chiron she fled into the mountains, where she gave birth to Melanippe. When her father came looking for her, Hippe appealed to the gods who changed her into a mare. Artemis placed the image of Hippe among the stars, where she still hides from Chiron (represented by the constellation Centaurus), with only her head showing.
The fourth-magnitude star Alpha Equulei is called Kitalpha from the Arabic meaning ‘the section of the horse’, in reference to the whole constellation.
In the Chinese constellation system, Alpha Equulei was joined with Beta Aquarii to form Xu (‘emptiness’), a place connected with death and mourning, after which the 11th Chinese lunar mansion was named. Between Xu and the 12th mansion, Wēi, were four pairs of stars representing judges or arbiters of different kinds. Delta and Gamma Equulei were Sifei (‘deified judge of right and wrong’), while Beta and 9 Equulei were Siwei (‘deified judge of disaster and good fortune’). The other two, Silu and Siming, judges of rank and life, were over the border in adjoining Pegasus.
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