From Smith's Dictionary of Greek and Roman Biography and Mythology
Cetus
Cassiopeia boasted of her daughter Andromeda's beauty, and said that she surpassed the Nereids. The latter prevailed on Poseidon to visit the country by an inundation, and a sea-monster was sent into the land. The oracle of Ammon promised that the people should be delivered from these calamities, if Andromeda was given up to the monster; and Cepheus, being obliged to yield to the wishes of his people, chained Andromeda to a rock. Here she was found and saved by Perseus, who slew the monster and obtained her as his wife. ( Apollodorus ii. Metamorphoses by Ovid iv)
Athena placed her among the stars, in the form of a maiden with her arms stretched out and chained to a rock, to commemorate her delivery by Perseus. Conon gives a wretched attempt at an historical interpretation of this mythus. The scene where Andromeda was fastened to the rock is placed by some of the ancients in the neighbourhood of Iope in Phoenicia, while others assign to it a place of the same name in Aethiopia. The tragic poets often made the story of Andromeda the subject of dramas, which are now lost.