Genitive: Cassiopeiae

Abbreviation: Cas

Size ranking: 25th

Origin: One of the 48 Greek constellations listed by Ptolemy in the Almagest

Greek name: Κασσιέπεια (Kassiepeia)

Cassiopeia was the vain and boastful wife of King Cepheus of Ethiopia, who stands next to her in the sky. They are the only husband-and-wife couple among the constellations. Classical authors spell her name Cassiepeia, from the original Greek Κασσιέπεια, but Cassiopeia is the form used by astronomers.

While combing her long locks one day, Cassiopeia dared to claim that she was more beautiful than the sea nymphs called the Nereids. Such hubris by a mortal could not go unpunished so the Nereids went in search of retribution. There were 50 Nereids, all daughters of Nereus, the so-called Old Man of the Sea, and one of them, Amphitrite, was married to Poseidon, the sea god. Amphitrite and her sisters appealed to Poseidon to punish Cassiopeia for her vanity.

Bowing to their request, the sea god sent a monster to ravage the coast of King Cepheus’s country. This monster is commemorated in the constellation Cetus. To appease the monster, Cepheus and Cassiopeia chained their daughter Andromeda to a rock as a sacrifice, but Andromeda was saved from the monster’s jaws by the hero Perseus in one of the most famous rescue stories in history.

In the sky, Cassiopeia is depicted sitting on her throne. Each night she circles the celestial pole, sometimes upright, sometimes hanging upside down in apparent danger of falling out. The mythologists interpreted the indignity of this celestial fairground ride as part of her punishment from the gods, who made her a figure of fun. Aratus wrote that she plunged headlong into the sea like a diver (some translate it as ‘tumbler’), her feet waving in the air, because as seen from Greek latitudes she would have received a ducking at the lowest point on each circuit. Her long-suffering husband Cepheus alongside her endured the same fate.

Germanicus Caesar described Cassiopeia thus: ‘Her face contorted in agony, she stretches out her hands as if bewailing abandoned Andromeda, unjustly atoning for the sin of her mother,’ and this is how she is drawn in early manuscripts illustrating the works of Aratus and Hyginus. In some cases, her wrists are shown bound to the throne in which she sits. However, from Dürer’s chart of 1515 onwards she was portrayed not with her arms outstretched but holding aloft a palm frond in one hand. With her other hand she is either holding a robe or fussing with her hair, as in the illustration here. In al-Ṣūfī's Book of the Fixed Stars (c. AD 964), an Arabic version of the Almagest, the constellation was called Dhāt al-Kursīy, the woman with the throne.

Stars of Cassiopeia

The five brightest stars of Cassiopeia are arranged in a distinctive W-shape which writers such as Aratus likened to a key or a folding door. Alpha Cassiopeiae, magnitude 2.2, is called Schedar, from the Arabic al-sadr meaning ‘the breast’, where Ptolemy said it lay. Beta Cassiopeiae, magnitude 2.3, is known as Caph from the Arabic al-kaff al-khaḍīb meaning ‘stained hand’, because the stars of Cassiopeia were thought by the Arabs to represent a hand tattooed with henna. Delta Cassiopeiae, magnitude 2.7, is named Ruchbah, from the Arabic for ‘knee’, rukbat, again following Ptolemy's description of its location in the constellation figure. The central star of the W, Gamma Cassiopeiae, is an erratic variable, given to occasional outbursts in brightness; it has no official name. 

Tycho’s Star

In November 1572 the familiar W-shape of Cassiopeia was disturbed by the intrusion of a bright interloper, now called Tycho’s Star after the Danish astronomer Tycho Brahe who first spotted it. Tycho published a treatise about the ‘new’ star the following year, De nova et nullius ævi memoria prius visa stella, which included the chart of the star’s position reproduced below.