Cassiopeia was the wife of Cepheus, King of Ethipoia. Both she and her daughter Andromeda were very beautiful. Being vain she boasted that her daugheter was more beautiful than the Nereids, or sea nymphs . When they heard this they complained to Posedion, God of th Sea. The Sea God planned to destory their country. They consulted an oracle who told them the only way to appease the gods would be to chain Andromeda, their daughter, to a rock. There she was left to be devoured by Cetus, a terrible sea-monster.

Luckily Perseus, son of Zeus, was traveling back from a mission. He has just killed the Gorgon, Medusa (who had snakes as hair and was so terrible that one look at her would turn anyone to stone). He was carrying the head in a bag and wearing winged sandals when he saw the maiden. Perseus fell in love with her beauty a once and longed to marry her. He slew the sea-monster and set her free. Perseus and Andromeda were then married despite the fact that she had been previously been promised to Phineus. A fight took place at the wedding and ended with Perseus turning his rival into stone with Medusa's head.

Posedion thinking she had not fully learned her lesson placed her up in the heavens. There she sits chained to a chair for all eternity. Half of the year she circles the poles upside down, a humiliating position.