Cetus Constellation

Cetus represents the sea monster of Greek myth — sometimes described as a whale, sometimes as a far more fearsome creature — sent by Poseidon to ravage the coast of Aethiopia and devour Princess Andromeda, before the hero Perseus swooped in to save her. It's one of the largest constellations in the entire sky, sprawling across the boundary between the northern and southern sky near the watery constellations of Aquarius and Pisces. Despite its size, Cetus is fairly faint, but it hosts one of the most famous variable stars ever discovered: Mira, a star that visibly brightens and fades over the course of nearly a year.

Cetus at a Glance

• AbbreviationCet
• Genitive FormCeti
• MeaningThe Whale (or Sea Monster)
• Constellation FamilyPerseus family
• Area1,231 sq. degrees (4th largest of 88)
• Right Ascension0h 26m – 3h 24m
• Declination+10.5° to -24.9°
• Brightest StarDiphda (magnitude 2.02)
• Best Evening ViewingNovember, visible 70°N to 90°S

Mythology and History

The myth of Cetus is tightly bound to its neighboring constellations: Queen Cassiopeia boasted that her daughter Andromeda was more beautiful than the Nereids, sea nymphs attending Poseidon, and the insulted god sent the monster Cetus to punish the kingdom. An oracle declared that only the sacrifice of Andromeda herself would appease the beast, so she was chained to a rock on the coast to await her fate. Perseus, flying home from slaying the Gorgon Medusa, spotted her in time and turned Cetus to stone using Medusa's severed head, then freed and married Andromeda. Cetus was one of the 48 constellations catalogued by Ptolemy in the 2nd century AD, and it remains one of the largest constellations in the sky by area, ranking fourth of the 88 modern constellations.

Notable Stars in Cetus

Diphda, also known as Deneb Kaitos, is Cetus's brightest star at magnitude 2.02, an orange giant roughly 96 light-years from Earth. Far more famous, however, is Mira (Omicron Ceti), a red giant star that pulsates dramatically over a roughly 332-day cycle, swinging from around magnitude 2, easily visible to the naked eye, down to magnitude 10, invisible without a telescope. Its discovery in 1596 made it the first variable star (other than a nova or supernova) recognized in Western astronomy, and it remains the prototype for an entire class of pulsating stars known as Mira variables.

Deep-Sky Objects

Cetus's most notable deep-sky object is Messier 77, a bright barred spiral galaxy roughly 47 million light-years away with an unusually active core, powered by a supermassive black hole feeding on surrounding gas and dust — making it one of the closest examples of what astronomers call a Seyfert galaxy.

• Messier 77 — a bright barred spiral galaxy roughly 47 million light-years away, notable for its active galactic nucleus powered by a supermassive black hole

Neighboring Constellations and How to Find It

Cetus sits in a relatively empty stretch of sky south of Aries and Pisces, best found by first locating the bright star Diphda and then tracing the constellation's long, faint body westward. It reaches its highest point around November and, straddling the celestial equator, is visible from most inhabited latitudes. Cetus borders Aries, Pisces, Aquarius, Sculptor, Fornax, Eridanus, and Taurus.

Cetus is also associated with the October Cetids, Eta Cetids, and Omicron Cetids meteor showers, visible to observers who track where these annual displays radiate from.