- A small southern counterpart of the great water-snake, Hydra, with which it is not to be confused. This is one of several examples of the repetition of constellation figures in the sky, as in the Great and Little Bear, the Great and Little Dog, the two lions, the horses Pegasus and Equuleus, the Northern and Southern Crown, and the Northern and Southern Triangle.
- Star Tales - Hydra (ianridpath.com)
Hydra is the largest of the 88 constellations, winding over a quarter of the way around the sky. Its head is south of the constellation Cancer, the crab, while the tip of its tail lies between Libra, the scales, and Centaurus, the centaur. The total length from its westernmost boundary to the easternmost one is 102°.5. Yet for all its size there is nothing prominent about Hydra. Its only star of note is second-magnitude Alphard, a name that comes from the Arabic al-fard appropriately meaning ‘the solitary one’. Bode on his Uranographia atlas gave it the alternative name Unuk es Schudscha, from the Arabic unuk al-shujā, neck of the serpent. Both names were originally given by al-Ṣūfī in his Book of the Fixed Stars (AD 964).
- Star Tales - Cancer (ianridpath.com)
The crab is a minor character in one of the labours of Heracles (the Greek name for Hercules). While Heracles was fighting the multi-headed monster called the Hydra in the swamp near Lerna, the crab emerged from the swamp and added its own attack by biting Heracles on the foot. Heracles angrily stamped on the crab, crushing it. For this modest contribution to history, we are told that the goddess Hera, the enemy of Heracles, put the crab among the stars of the zodiac. Its name in Greek was Καρκίνος (Karkinos), or Carcinus in Latin transliteration.