Size ranking: 4th
Origin: One of the 48 Greek constellations listed by Ptolemy in the Almagest
Greek name: Κῆτος (Ketos)
When Cassiopeia, wife of King Cepheus of Ethiopia, boasted that she was more beautiful than the sea nymphs called the Nereids she set in motion one of the most celebrated stories in mythology, whose characters are commemorated in the sky. In retribution for the insult to the Nereids, the sea god Poseidon sent a fearsome monster to ravage the coast of Cepheus’s territory. That monster, a dragon of the sea, is represented by the constellation Cetus.
To rid himself of the monster, Cepheus was instructed by the Oracle of Ammon to offer up his daughter Andromeda as a sacrifice to the monster. Andromeda was chained to the cliffs at Joppa (the modern Tel-Aviv) to await her terrible fate.
Cetus was visualized by the Greeks as a hybrid creature, with enormous gaping jaws and the forefeet of a land animal, attached to a scaly body with huge coils like a sea serpent. Hence Cetus is drawn on star maps as a most unlikely looking creature, more comical than frightening, nothing like a whale although it is sometimes identified as one.
The bizarre-looking sea monster Cetus emerging from the ocean of the
southern skies, illustrated in the Atlas Coelestis of John Flamsteed (1729).
Andromeda trembled as the B-movie monster made towards her, cleaving through the waves like a huge ship. Fortunately, at this moment the hero Perseus happened by and sized up the situation. Swooping down like an eagle onto the creature’s back, Perseus drove his diamond-hard sword deep into its right shoulder. Agonized and enraged, the wounded monster reared up on its coils and twisted around, its cruel jaws snapping at its attacker. Again and again Perseus plunged his sword into the beast – through its ribs, its barnacle-encrusted back and at the root of its tail. Spouting blood, the monster finally collapsed into the sea and lay there like a waterlogged hulk. Its corpse was hauled on shore by the appreciative locals who skinned it and put its bones on display.
Amazing Mira and the stars of Cetus
Cetus (Κῆτος in Greek) is the fourth-largest constellation, as befits such a monster, but none of its stars is particularly bright. The brightest of them is second-magnitude Beta Ceti, officially named Diphda by the IAU but also known as Deneb Kaitos from the Arabic meaning sea monster’s tail. Ptolemy in the Almagest described this star as lying on the end of the southern tail fin; the northern fin was marked by the star we now know as Iota Ceti. Alpha Ceti is called Menkar from the Arabic meaning ‘nostrils’, a misnomer since this star lies on the beast’s jaw (in Ptolemy’s description, the star on the nostrils was actually the one to the north we know as Lambda Ceti).
The most celebrated star in the constellation is Mira, a Latin name meaning ‘the amazing one’, given on account of its variability in brightness. At times it can easily be seen with the naked eye, but for most of the time it is so faint that it cannot be seen without binoculars or a telescope. Mira is a red giant star whose brightness variations are caused by changes in size. The star was first recorded in 1596 by the Dutch astronomer David Fabricius, but the cyclic nature of the changes was not recognized until 1638. The name Mira was given to the star by the Polish astronomer Johannes Hevelius in 1662, when it was the only variable star known.
When the Chinese constellation system was first established some 2,000 years ago, this area of sky was rising in the early evening in the autumn, so it became associated with agriculture and harvesting, particularly the storage of cereals. Two granaries can be found in Cetus: Tianjun, a circular one, formed by thirteen stars in what we visualize as the head and neck of the sea monster, including Alpha, Gamma, Delta, and Xi Ceti; and Tiancang, a square one, formed by six stars in the monster’s body (Iota, Eta, Theta, Zeta, Tau, and Upsilon). Despite their names, the shapes formed by the stars of Tianjun and Tiancang are not circular or square. A third granary, Tianlin, was just over the border in present-day Taurus.
South of Tianjun was Chuhao or Chugao, a group of six stars near the border with Eridanus that included Epsilon and Rho Ceti; this constellation represented a supply of animal feed or, in another interpretation, medicinal herbs. A loop of seven stars near Eta Ceti formed Tianhun, a manure pit or pigsty on a farm, although the stars’ identities are uncertain. The single star Beta Ceti was Tusikong, a controller of land and minister of works.
Some maps show Bakui, a net for catching birds, in the region of 2, 6, and 7 Ceti. However, older maps place this constellation farther south, in Sculptor and Phoenix, which suggests that Chinese astronomers moved it northwards over time as precession took this part of the sky below their horizon.
© Ian Ridpath. All rights reserved