Delphinus

Delphinus's compact shape made it one of the more securely ancient figures Aratus described in the third century BCE, long before Ptolemy's catalogue fixed it in place. It is also, less obviously from its own myth alone, a charter member of the ancient sky's watery quarter.

The stretch of sky running from Aquarius and Capricornus through Piscis Austrinus, Eridanus, Equuleus and the ship Argo before its division carries a single old theme, water, that predates the individual Greek stories attached to each figure within it. Babylonian tradition, inherited by Greek astronomers along with the zodiac, already read this region as a sea because the Sun crossed it during the ancient Near East's rainy season; the dolphin sent by Poseidon or the dolphin that rescued the poet Arion are stories laid onto an existing watery reading rather than the reason for it.

Historians of astronomy, Richard Hinckley Allen among them, use the label Heavenly Waters for this loose cluster, a modern convenience rather than an ancient formal boundary, since Ptolemy himself organised his forty-eight only into zodiacal, northern and southern lists with no thematic sub-groups named in the text.

Delphinus is a minor member by size alone, its whole figure fitting into a patch of sky smaller than several single stars elsewhere carry as their own designation, but its membership in the theme is as old as the theme itself.

Delphinus, the Dolphin, is a small northern constellation, one of Ptolemy's forty-eight, its compact group of stars forming a pleasing figure of a leaping dolphin near the celestial equator, beside the Milky Way. In Greek myth it is the dolphin sent by Poseidon to persuade the sea-nymph Amphitrite to become his queen, rewarded with a place in the sky, or the dolphin that rescued the poet Arion. Two of its stars bear the curious names Sualocin and Rotanev, a nineteenth-century astronomer's Latinised name spelled backwards. In astronomical tradition the constellation is a pattern of stars rather than a zodiac sign, and its watery nature groups it among the water-associated figures. These are the features of Delphinus as the classical and modern traditions describe it, distinct from any astrological reading of the small region of sky it occupies.

Delphinus is generally reckoned the more securely ancient of the sky's two small water-creature figures near Aquila and Pegasus, distinguishing it from later seventeenth-century additions in the surrounding sky; it appears already in Aratus's third-century-BCE Phaenomena, one of the oldest surviving descriptions of the constellations, centuries before Ptolemy's own systematic catalogue.