What it looks like
Cetus is a large equatorial constellation in the region of the sky traditionally called the Sea, drawn as a sea monster (later identified with a whale). Its brightest star, Diphda, sits at the tail; the famous variable star Mira blinks in and out of naked-eye visibility over an 11-month cycle.
Brightest stars
Cetus is anchored by Diphda and Menkar, with the famous variable Mira ranging from magnitude 2 to 10 over its cycle.
| Star | Magnitude | Distance | Meaning |
|---|---|---|---|
| Diphda (β Ceti) | +2.04 | 96 ly | The Frog |
| Menkar (α Ceti) | +2.54 | 249 ly | The Nostril |
| Mira (ο Ceti) | +3.04 (variable) | 300 ly | The Wonderful |
| Mirach-adjacent (η Ceti) | +3.45 | 118 ly | — |
| Tau Ceti | +3.50 | 12 ly | — |
Mythology and history
In Greek myth, Cetus was the sea monster sent by Poseidon to devour Andromeda, killed by Perseus with the head of Medusa.
The sea monster of Aethiopia
Cetus was the monster Poseidon sent to ravage the coast of Aethiopia after Queen Cassiopeia boasted that she was more beautiful than the Nereids. An oracle declared that only the sacrifice of Princess Andromeda could appease the god, so she was chained to a rock by the sea. Perseus, flying past on his winged sandals after killing Medusa, saw her, killed Cetus with his sickle or by showing it the Gorgon's head, and married her. The whale form of the constellation is a later, gentler interpretation of what was originally a terrifying sea beast.
How to find it
Look for a large, sprawling constellation below Pisces and Aries on autumn evenings, with Diphda as its brightest star.
- Find the Great Square of Pegasus, then look south for Diphda.
- Mira is worth checking every few weeks — it visibly brightens and fades.
- Best on autumn evenings from both hemispheres.
Key facts
- Abbreviation
- Cet
- Genitive
- Ceti
- Meaning
- The Whale
- Brightest star
- Diphda
- Main stars
- 9
- Area rank
- 4th
- Best visible
- November
- Declination
- -10°
- Meteor showers
- October Cetids, Eta Cetids, Omicron Cetids
- Neighbours
- Aries, Pisces, Aquarius, Sculptor, Fornax, Eridanus, Taurus
Deep-sky objects
M77 is a bright barred spiral galaxy in Cetus, visible in small telescopes. The constellation also contains the spiral galaxy NGC 247 and the irregular galaxy IC 1613. Mira itself is a red giant whose pulsing has been observed for over 400 years.
Frequently asked questions
What does the constellation Cetus look like?
Cetus is an equatorial constellation known as the Whale or Sea Monster, home to Mira — the first variable star ever recognised — and the nearby barred spiral galaxy M77.
What is the brightest star in Cetus?
The brightest star in Cetus is Diphda, shining at around magnitude 2.04.
When is Cetus best visible?
Cetus is best seen from both hemispheres during November.
What is the story behind Cetus?
In Greek myth, Cetus was the sea monster sent by Poseidon to devour Andromeda, killed by Perseus with the head of Medusa.
Which constellations are near Cetus?
Cetus borders Aries, Pisces, Aquarius, Sculptor, Fornax, Eridanus and Taurus.
Pisces
Aries