Cassiopeia was born into the royal house of Aethiopia, the kingdom that lay at the farthest southern edge of the Greek world, a land of burning sun and dark-skinned peoples that the Greeks linked with both wealth and the distant unknown. She married Cepheus, the son of Belus, the Phoenician king whose descendants ruled Egypt and founded the city of Joppa on the coast of what is now Israel. By Cepheus she bore a daughter named Andromeda, a princess of legendary beauty, and possibly other children whose names the myths did not preserve. The family ruled from a palace in Aethiopia, and their wealth was said to be immense, drawn from the gold and incense of the southern lands.

The disaster began with a boast. Cassiopeia, looking at her daughter or perhaps at her own reflection, declared that the beauty of the house of Cepheus surpassed that of the Nereids. The Nereids were not ordinary nymphs. They were the fifty daughters of Nereus, the ancient sea god called the Old Man of the Sea because of his wisdom and his age, and they were the companions of Poseidon, the earth-shaker who ruled the waves from his palace beneath the Aegean. To claim that a mortal woman, even a princess, outshone the divine attendants of the god of the sea was an act of hubris so extreme that it demanded divine punishment. The Nereids themselves went to Poseidon and demanded vengeance. The god of the sea, never slow to anger when his honour was slighted, struck the coast of Aethiopia with a flood and sent a sea monster named Cetus to ravage the land. The creature rose from the depths like a moving mountain, its jaws large enough to swallow ships whole, and it laid waste to the harbours and fields of Cepheus' kingdom.

Desperate, Cepheus consulted the oracle of Ammon, the Egyptian god whom the Greeks identified with Zeus, whose sanctuary lay in the Libyan desert. The oracle gave a chilling answer: the land would be spared only if Cepheus sacrificed his daughter Andromeda to the monster. The king, torn between love for his child and duty to his people, yielded to the will of his subjects. Andromeda was stripped of her royal garments, chained naked to a rock on the shore near Joppa, and left for Cetus. The princess waited in the spray, her chains cutting into her wrists, while the monster circled in the water below. Some ancient writers placed the scene at Joppa in Phoenicia, where a rock was shown to visitors for centuries with the marks of Andromeda's chains still visible upon it. Others placed it on the coast of Aethiopia itself. Either way, the image was the same: a young woman bound to stone, offered up to appease a god's anger at her mother's pride.

Salvation came from the sky. Perseus, the son of Zeus and the princess Danae, was returning from his quest to slay the Gorgon Medusa, the creature whose face turned all who looked upon it to stone. He flew over the coast on winged sandals given to him by Hermes, the messenger god, or rode the winged horse Pegasus that had sprung from the blood of the slain Gorgon. Looking down, he saw Andromeda chained to the rock and the monster rising from the sea. He fell in love at first sight. He descended, struck a bargain with Cepheus and Cassiopeia - he would save their daughter in exchange for her hand in marriage - and then turned to face Cetus. Some say he killed the beast with his sword, driving his curved blade again and again into its barnacle-covered hide until purple blood and sea foam mingled in the water. Others say he simply uncovered the Gorgon's head and the monster turned to stone in an instant, becoming the very rock that tourists would later visit at Joppa. Either way, Andromeda was free. Perseus cut her chains, wrapped her in his cloak, and claimed her as his bride.

The wedding was not peaceful. Andromeda had been betrothed before her sacrifice to her uncle Phineus, the brother of Cepheus, and Phineus arrived at the feast with an armed band to reclaim her. Perseus, outnumbered, did what he had done to the Gorgon: he raised Medusa's severed head. Phineus and his allies froze where they stood, turned to stone in the middle of the palace, their faces forever fixed in expressions of rage and disbelief. Cassiopeia watched her brother-in-law become a statue and said nothing. She had already said too much.

But Poseidon was not finished with her. The god who had sent the flood and the monster decreed that Cassiopeia should not escape punishment even in death. When she and Cepheus died, Athena, the goddess of wisdom, placed them among the stars as constellations, along with Andromeda, Perseus, and the monster Cetus itself. Cassiopeia was set near the north celestial pole, close enough to her husband and daughter that the family is still visible together on winter nights, but she was fixed to her throne in such a way that as the sky turns, she spends half of every revolution hanging upside down. The five bright stars that form the familiar W or M shape of her constellation are the arms and legs of the queen clinging to her throne so that she does not fall into the void. For half the year she sits upright in dignity. For the other half she dangles head-down, her royal robes falling about her face, the most famous humiliation in the heavens.

The constellation became one of the forty-eight listed by Ptolemy in the Almagest and remains one of the eighty-eight recognised by modern astronomy. It contains some of the most remarkable objects in the sky: the supernova remnant Cassiopeia A, the strongest radio source outside the solar system; Tycho's Supernova, observed in 1572; and the Andromeda Galaxy itself, which lies just beyond her daughter's constellation. Sailors in the ancient Mediterranean used her W-shape to find the Pole Star. Navigators in the age of exploration steered by her. Every winter night, when the W rises high above the northern horizon, the queen of Aethiopia is still being punished for a boast she made three thousand years ago.