Yi, the Wings of the Vermilion Bird

Yi, the Wings, is the twenty-seventh of the twenty-eight xiu and the sixth mansion of the southern palace, the Vermilion Bird. Its name means wings, fittingly the largest mansion by the span of its stars, identified with stars in Crater and Hydra as the outstretched wings of the celestial bird. In the later astrological tradition its animal totem is the Snake. As one of the twenty-eight lunar mansions it marks a portion of the Moon's path. These are the conventions of the Chinese astronomical and calendrical tradition, in which the twenty-eight xiu divide the sky into four palaces of seven mansions, the mansion occupied by the Moon on a given night informing the almanac's reckoning of the day's character.

Yi being the largest of the twenty-eight mansions by angular span, spread across parts of modern Crater and Hydra, is a direct consequence of the determinative-star method: where two bright reference stars happen to fall close together, as at Zi, the intervening mansion is narrow, and where the nearest usable reference stars are widely separated, as here, the resulting mansion is correspondingly broad, an unevenness the equal-arc Vedic nakshatra scheme was specifically constructed to avoid by abandoning star-anchoring altogether in favour of fixed degree divisions.