Cetus is one of the clearest cases in Greek myth of a monster that grew out of real animals. The Greeks did not have one sea monster called Cetus so much as a category: a kētos was any enormous creature of the sea, the kind of thing sailors glimpsed as a rolling back or found rotting on a beach. Poets and painters gave it a shape, part serpent, part dog, part fish, and it became the standard villain whenever a god wanted to punish a seaside kingdom. The most famous of these monsters is the one that came for Andromeda.

What I enjoy about Cetus is that the story is still visible in the night sky and in the name biologists gave to whales. Few monsters leave a legacy that lasting.

The story of Andromeda and Perseus

Queen Cassiopeia of Ethiopia boasted that she, or in some versions her daughter Andromeda, was more beautiful than the Nereids, the sea nymphs. It was a dangerous thing to say about Poseidon’s family.

The Nereids complained to Poseidon, who sent a flood and a sea monster to ravage the coast of King Cepheus’s kingdom. The Ethiopia of Greek myth was not the modern country but a vague land at the edge of the known world, and later writers placed the story on the coast of the eastern Mediterranean, at Joppa, today’s Jaffa in Israel. An oracle told Cepheus that the only way to end the attacks was to sacrifice his daughter. Andromeda was chained to a rock on the shore to wait for the monster.

Perseus was flying home on winged sandals after beheading the Gorgon Medusa. He saw Andromeda, fell in love, and struck a deal with her parents: he would kill the monster if he could marry her. The accounts differ on how. In Ovid’s Metamorphoses, he swoops down and stabs the beast repeatedly with his curved sword until it dies. In other retellings, especially later ones, he simply holds up Medusa’s severed head and the monster turns to stone. The stone version is the one most people know today, partly thanks to films, and it neatly explains the rocks along the shore at Jaffa.

Heracles, Hesione and the monster of Troy

Andromeda’s was not the only kētos in Greek myth. An earlier hero, Heracles, fought one at Troy, and the story follows almost the same plan.

King Laomedon of Troy had hired Poseidon and Apollo to build the city walls, then refused to pay them. Apollo sent a plague and Poseidon sent a sea monster. The oracle again demanded a royal daughter, and Laomedon’s daughter Hesione was chained to the shore. Heracles arrived and agreed to kill the monster in return for the king’s divine horses. In some versions he let the monster swallow him and cut his way out from inside after three days. Laomedon then cheated Heracles too, and the hero later came back and sacked Troy. A Corinthian vase from the 6th century BC, now in the Museum of Fine Arts in Boston, shows the scene, with Hesione throwing stones at a monstrous head emerging from a cliff.

What did a kētos look like?

Greek artists settled on a fairly consistent design: a long serpent-like body, a fish tail, flippers or fins, and a big head with a long snout full of teeth, often with a crest or frill.

In vase paintings and mosaics the kētos looks a bit like a cross between a dragon, a dog and a dolphin. Nereids are often shown riding on their backs, so they were not always enemies. The image was so well established that early Christian artists borrowed it to show the “great fish” that swallowed Jonah. On Roman-era sarcophagi, Jonah’s fish is not a fish at all but a classic kētos with curling tail and snout. That shows how the word and the image travelled together for centuries.

Our word for whales comes from the same root. The Greek kētos became the Latin cetus, which could mean whale, large fish or sea monster. When naturalists began to classify animals scientifically, they gave the name Cetacea to the group that includes the whales, dolphins and porpoises. Scientists who study whales are still called cetologists.

The real animals behind the monster

There is good evidence that the kētos legend was shaped by real bones and real whales.

The Roman naturalist Pliny the Elder records that the bones of the monster Andromeda had faced were displayed at Joppa and were brought to Rome by Marcus Scaurus during his time as aedile in 58 BC. Pliny says the skeleton was about 40 feet long, with ribs taller than an Indian elephant and a spine a foot and a half thick. That is a very good description of a large whale skeleton, probably a stranded animal from the eastern Mediterranean, where sperm whales and fin whales still occur. A whale skeleton on a beach, with a skull longer than a person, would have been all the proof anyone needed that the story was true.

The historian Adrienne Mayor has argued that the head on the Boston Hesione vase looks less like a whale and more like the fossil skull of an extinct mammal, the kind of fossil that erodes out of Mediterranean coastal cliffs. If she is right, the “monster of Troy” may have been based on a fossil the painter had seen or heard about. Either way, the Greeks were working from real things: whale bones, fossil skulls and the occasional glimpse of a sperm whale or giant shark at sea.

The constellation Cetus

The Greeks put the whole Andromeda story in the sky, and Cetus is part of it.

Cetus is one of the 48 constellations listed by the astronomer Ptolemy in the 2nd century AD, and it is the fourth-largest of the 88 constellations recognised today. It sits in the part of the sky astronomers call “the Sea”, near Pisces, Aquarius and Eridanus, and close to Andromeda, Perseus, Cassiopeia and Cepheus, so the whole cast of the myth is gathered in one region. Cetus contains two famous stars. Mira, whose name means “wonderful”, was the first star recognised as changing in brightness on a regular cycle, brightening and fading over roughly eleven months. Tau Ceti, about 12 light-years away, is one of the nearest stars like our Sun and has been a target in the search for other planets.

Older star atlases drew Cetus in the same style as the vase painters: a beast with a long snout, forelegs and a coiled fish tail, looking nothing like a modern whale. Johann Bayer’s Uranometria of 1603 and later atlases by Hevelius and Flamsteed kept that monstrous shape, so for centuries anyone learning the night sky was learning a sea monster rather than a cetacean. The constellation is best seen from the Northern Hemisphere in autumn and early winter evenings, low in the southern sky. It is not bright, apart from Mira at its peak and the star Diphda, but on a dark night away from city lights the long body of the monster is easy to trace once you find Pisces above it. From a boat at sea on a clear night, with no lights for miles, the old constellations make a lot more sense.

Why Cetus still matters

Cetus is a good reminder that sea monsters were not pure invention. They were attempts to explain big, real animals that people rarely saw alive.

Until the last century or two, most people only ever met a whale as a corpse on a beach or a skeleton in a temple. It is easy to see how a 40-foot skeleton with a huge skull and rows of teeth turned into a monster that ate princesses. Today the same animals are protected, studied and watched from boats around the world, and the name of the old monster has become the name of the group scientists work to protect. If you are curious how other legends map onto real animals, read our guide to sea monsters, the story of Scylla and Charybdis, and the kraken, which turned out to be the giant squid.

I also like how Cetus shows the Greek habit of placing a moral inside a sea story. Cassiopeia’s vanity and Laomedon’s cheating both bring the monster, and in each case an innocent daughter pays for a parent’s fault until a hero steps in. The sea in these stories is not evil. It is the instrument of a god who has been wronged. For a culture that depended on the sea for food, trade and travel, keeping the gods of the sea satisfied was not a small matter, and the kētos was the cautionary tale that kept that idea alive.