According to a Greek legend, the sea god Poseidon placed the figure of Cassiopeia among the stars. It is said that Cassiopeia has a ridiculous upside-down position to punish her for having been pretentious. Cassiopeia was very proud of her beauty. She claimed that she and her daughter Andromeda were more beautiful than the sea-nymphs, the Nereids.

The nymphs complained to the sea god Poseidon, who threatened to send a sea monster and flood to destroy Cassiopeia's land. In despair, the king Cepheus consulted an oracle to prevent the destruction of his reign. The oracle predicted that only the sacrifice of Andromeda to the monster could appease the wrath of Poseidon.

The king chained Andromeda to a sea cliff. Fortunately, at this same moment, Perseus, the nephew of the king of Argos, was traveling along the coast. Perseus noticed the beautiful woman and fell in love with her. Learning of Andromeda's story, he offered to rescue her if her parents agreed to let him marry their daughter. With the help of some magical sandals that allowed Perseus to fly, and a magical sword given to him by the god Hermes, Perseus killed the monster and married Andromeda.