Cassiopeia, the wife of King Cepheus,
ruler of Ethiopia, was beautiful, arrogant and vain, and it was these latter
two characteristics which were to lead to her downfall. Her continual boast
that both she and her daughter
Andromeda were more beautiful than
the sea nymphs, daughters of Nereus, the sea god, brought the wrath of Poseidon,
ruling god of the sea, upon the kingdom of Ethiopia.
(Differing accounts state that either Cetus,
the sea monster, on Poseidon's instructions, was to destroy the land, or that the
whole country was to be flooded.)
Trying to avert such a fate, Cepheus and Cassiopeia consulted a wise oracle,
who told them that the only way to appease the sea gods was to sacrifice their
daughter. Accordingly, Andromeda was chained to a rock at the sea's edge,
and left there to helplessly await her fate at the mercy of the sea monster.
But Perseus arrived in time and saved Andromeda.
Poseidon considered that Cassiopeia should not escape punishment entirely,
and placed her in the heavens in such a position that she circles the celestial
pole in an upside-down position for half the time.