The Gods are vain;
Mortals, be warned…
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Cassiopeia was an Ethiopian Queen who dared to boast of her daughter’s beauty (as well as her own).
She compared herself, and her child, as being more enchanting than even the Nereids, the sea-children of Nereus, a sea God who served under Poseidon, God of all waters.
Nereus was offended.
Poseidon was not pleased…
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There are differing accounts at this point:
Maybe Poseidon chose to flood all of Ethiopia;
More likely, he let loose a monster –
P.S. it wasn’t the Kraken, as many of us might like to believe (blame the original 1981 movie, Clash of the Titans, starring Harry Hamlin, for that)
No, the Monster in question was called Cetus.
And while Heracles would defeat a Cetus later, this version of the sea monster had no match.
Save a wandering hero… (yeah, now we’re back to Harry Hamlin) …
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King Cepheus (Andromeda’s father) and her mother, Queen Cassiopeia, consulted an oracle.
The oracle was clear: to save the land of Ethiopia, Cetus (not to mention Nereus and Poseidon, by extension) required a sacrifice…
Andromeda.
And so, they did the unthinkable, to protect themselves and their kingdom: they chained their daughter to a rock by the ocean, to be given as an offering to Cetus…
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There is much to say about Perseus: a child gifted from Zeus to an imprisoned maiden, cast away in the ocean in a wooden box by his grandfather, along with his mother.
Perseus was given a terrible task, one that took divine intervention, including Hades’ Helm of Darkness, Hermes’ winged sandals, and an indestructible sword from his father, Zeus. At the end of his trial, not only had he slain the fierce Gorgon Medusa, but he had gained another invaluable asset:
The winged horse, Pegasus.
Born from the blood of the beheaded Medusa…
[Note: another being also came forth from Medusa’s blood, Chrysaor. Chrysaor’s main mythological claim to fame comes from his son, the three headed giant Geryon, who shows up in Heracles’ Labors].
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Pegasus has been given many tasks over time: gathering thunderbolts for Zeus, helping Perseus’ great-grandson Bellerophon defeat the Chimera, and even helping the British Army during World War II:

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For all of his service, Pegasus was eventually transformed into a constellation. For many mortals, this type of transformation is a blessing (see Orion).

For some, like Andromeda’s mother Cassiopeia, it is a curse.
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Perseus was able to rescue Andromeda (lit. ‘Queen of All Men’) with the assistance of Medusa’s head, and her bloody offspring, the winged horse Pegasus.
So, what came of Andromeda’s mother, Cassiopeia?
She was also bound for a stellar fate, though not one she might have desired…
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Poseidon was not forgiving; he chained Cassiopeia to the heavens, binding her in her throne.

In a different take, there are interpretations of Cassiopeia’s persistent celestial vanity:

Vain or chained, Cassiopeia has to spend a cosmic eternity in the heavens with the beast that was her undoing, the winged horse,
Pegasus.
And that might be the cruelest punishment of all…


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Classical Sources:
Hesiod, Theogony.
Gaius Julius Hyginus, Astronomica from The Myths of Hyginus. Online version at the Topos Text Project.
Gaius Julius Hyginus, Fabulae from The Myths of Hyginus. Online version at the Topos Text Project.
Pseudo-Apollodorus, Bibliotecha. Online version at the Perseus Digital Library.
Ovid, Metamorphoses.