Source Verification: Andromeda

Every claim below is either what a named ancient source attests, in its own line or section, or a point where the sources themselves disagree.

What the Sources Attest

Why was Andromeda sacrificed to the serpent?

The ancient sources describe a sea monster rather than a serpent, and Andromeda was offered to it to end a punishment brought on by her mother. Apollodorus says Queen Cassiopeia claimed to be more beautiful than the Nereids. Poseidon, sharing the sea nymphs' anger, sent a flood and a monster against Ethiopia. The oracle of Ammon then declared that the land would be saved only if Andromeda were given to the monster, and the Ethiopians forced her father Cepheus to chain her to a rock. Hyginus's Fabulae tells it differently: there Cassiopeia boasted that her daughter was more beautiful than the Nereids.

Source: Apollodorus, Library 2.4.3

Did Perseus marry Andromeda?

Yes. Apollodorus says Perseus fell in love when he saw her bound to the rock and swore an agreement with Cepheus to kill the monster in exchange for her hand. Ovid describes the wedding feast that followed, where Perseus took her without a dowry. Her former betrothed Phineus, her uncle, attacked the feast and was turned to stone with the Gorgon's head. Andromeda went with Perseus to Greece and bore him six sons and a daughter, among them Perses, Alcaeus, Sthenelus and Electryon.

Source: Ovid, Metamorphoses 4.706-764

Is Andromeda a demigod?

No. The ancient sources make her a mortal princess, with no god as her parent. Her father was Cepheus, king of the Ethiopians. Herodotus calls him a son of Belus, and Apollodorus notes that Euripides made Cepheus and Phineus sons of Belus, whom Poseidon fathered on Libya. That makes her a distant descendant of Poseidon, the same god who sent the monster against her. Her husband Perseus, son of Zeus and Danae, was the demigod of the pair. Her own honor came later, when she was said to have been placed among the stars.

Source: Herodotus, Histories 7.61

What is the Andromeda Rock?

It is a rock off the coast of Jaffa, ancient Joppa, pointed out as the place where Andromeda was chained. The main mythographers, Apollodorus and Ovid, set her story in Ethiopia, but the Joppa location is itself ancient. Strabo's Geography records writers who placed her exposure at Joppa. Pliny the Elder's Natural History says a rock in front of the town still showed the marks of the chains that bound her. Pausanias adds that people near Joppa showed a spring of blood-red water where Perseus washed off the monster's blood.

Source: Pausanias, Description of Greece 4.35.9

Where Accounts Differ

whose beauty Cassiopeia boasted of
Ethiopia or Joppa
who plotted at the wedding

Whose beauty was boasted of differs by source. Apollodorus has Cassiopeia claim that she herself surpassed the Nereids, and Hyginus's Astronomica says Euripides and Sophocles told it that way. Hyginus's Fabulae instead has Cassiopeia claim that her daughter Andromeda's beauty surpassed theirs.
The place of the rescue varies. Apollodorus and Ovid set it in Ethiopia, the kingdom of Cepheus. Strabo reports, and rejects as mythic fiction, the version that moved the story to Joppa on the Phoenician coast, and Pausanias records a local spring at Joppa tied to the monster's blood.
The plot at the wedding is told in two ways. Apollodorus and Ovid make Phineus, Cepheus's brother and Andromeda's first betrothed, the plotter, with Cepheus on Perseus's side in Ovid. Hyginus's Fabulae names the rival suitor Agenor and has Cepheus join the plot against Perseus.

Sources

Primary sources

Apollodorus, Library 2.4.3
Apollodorus, Library 2.4.4
Apollodorus, Library 2.4.5
Apollodorus, Library 2.1.4
Ovid, Metamorphoses 5.1-235
Herodotus, Histories 7.61
Herodotus, Histories 7.150
Pausanias, Description of Greece 4.35.9
Hyginus, Fabulae 64
Strabo, Geography
Ovid, Metamorphoses 4.604-705
Ovid, Metamorphoses 4.706-764