The inseparable twins Castor and Pollux are commemorated in the constellation Gemini, depicted here in the Atlas Coelestis of John Flamsteed (1729).
Castor, at right, carries a lyre and an arrow, Pollux a club.
The stars Castor and Pollux mark the heads of the twins.

Genitive: Geminorum

Abbreviation: Gem

Size ranking: 30th

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

Greek name: Δίδυμοι (Didymoi)

Gemini represents the mythical Greek twins Kastor (Κάστωρ) and Polydeukes (Πολυδεύκης). The Latinized forms of their names are Castor and Pollux (sometimes Polydeuces), by which they are now generally known. The Greeks referred to them jointly as the Dioskouroi (Dioscuri in Latin), literally meaning ‘sons of Zeus’. However, mythologists disputed whether both really were sons of Zeus, because of the unusual circumstances of their birth.

Their mother was Leda, Queen of Sparta, whom Zeus visited one day in the form of a swan (now represented by the constellation Cygnus). That same night she also slept with her husband, King Tyndareus. Both unions were fruitful, for Leda subsequently gave birth to four children. In the most commonly accepted version, Pollux and Helen (later to become famous as Helen of Troy) were children of Zeus, and hence immortal, while Castor and Clytemnestra were fathered by Tyndareus, and hence were mortal.

Castor and Pollux grew up the closest of friends, never quarrelling or acting without consulting each other. They were said to look alike and even to dress alike, as identical twins often do. Castor was a famed horseman and warrior who taught Heracles to fence, while Pollux was a champion boxer.

The Twins join the Argonauts

The inseparable twins joined the expedition of Jason and the Argonauts in search of the golden fleece. The boxing skills of Pollux came in use when the Argonauts landed in a region of Asia Minor ruled by Amycus, a son of Poseidon. Amycus, the world’s greatest bully, would not allow visitors to leave until they had fought him in a boxing match, which he invariably won. He stamped down to the shore where the Argo lay and challenged the crew to put up a man against him. Pollux, stirred by the man’s arrogance, accepted at once and the two pulled on leather gloves. Pollux easily avoided the rushes of his opponent, like a matador side-stepping a charging bull, and felled Amycus with a blow to the head that splintered his skull.

On the Argonauts’ homeward trip with the golden fleece Castor and Pollux were of further value to the crew. Apollonius Rhodius tells us briefly that during the voyage from the mouth of the Rhone to the Stoechades Islands (the present-day Îles d’Hyères off Toulon) the Argonauts owed their safety to Castor and Pollux. Presumably a storm was involved, but he does not elaborate on the circumstances. Ever since this episode, says Apollonius – and he assures us there were other voyages on which they were saviours – the twins have been the patron saints of sailors. Hyginus said that the twins were given the power to save shipwrecked sailors by Poseidon, the sea god, who also presented them with the white horses that they often rode.

Mariners believed that during storms at sea the twins appeared in a ship’s rigging in the form of the electrical phenomenon known as St Elmo’s fire, as described by Pliny, the Roman writer of the first century AD, in his book Natural History:

‘On a voyage stars alight on the yards and other parts of the ship… If there are two of them, they denote safety and portend a successful voyage… for this reason they are called Castor and Pollux, and people pray to them as gods for aid at sea.’

By contrast, a single glow was called Helena and was considered a sign of disaster.

Castor and Pollux clashed with another pair of twins, Idas and Lynceus, over two beautiful women. Idas and Lynceus (who were also members of the Argo’s crew) were engaged to Phoebe and Hilaira, but Castor and Pollux carried them off. Idas and Lynceus gave pursuit and the two sets of twins fought it out. Castor was run through by a sword thrust from Lynceus, whereupon Pollux killed him. Idas attacked Pollux but was repulsed by a thunderbolt from Zeus.

Another story says that the two pairs of twins made up their quarrel over the women, but came to blows over the division of some cattle they had jointly rustled. Whatever the case, Pollux grieved for his fallen brother and asked Zeus that the two should share immortality. Zeus placed them both in the sky as the constellation Gemini, where they are seen in close embrace, inseparable to the last.

Another identification – Apollo and Heracles

Aratus referred to the constellation only as the twins (Δίδυμοι, i.e. Didymoi), without identifying who they were, but a century later Eratosthenes named them as the Dioskouroi, in reference to Castor and Pollux. An alternative view, reported by Hyginus, says that the constellation represents Apollo and Heracles (i.e. Hercules), both sons of Zeus but not twins. Ptolemy called the constellation the Twins (Δίδυμοι) in the Almagest, but in a later, more obscure treatise about astrology, called Tetrabiblos, he referred to Castor as ‘the star of Apollo’ and Pollux as ‘the star of Heracles’, supporting the identifications given by Hyginus.

Several star charts personify the twins as Apollo and Heracles. On the illustration from John Flamsteed’s atlas shown above, for example, one twin is depicted holding a lyre and arrow, attributes of Apollo, while the other carries a club, as did Heracles. Bode’s Uranographia depicts them in the same way. On his chart Bode labelled Castor and Pollux with the alternative names Apollo and Abrachaleus, the latter seemingly a strangely corrupted form of Hercules that had entered star catalogues from Arabic sources.

Stars of Gemini

The two brightest stars in the constellation, marking the heads of the twins, are named Castor and Pollux. Astronomers have found that Castor is actually a complex system of six stars linked by gravity, although to the eye they appear as one. Pollux is an orange giant star. Although Castor is labelled Alpha Geminorum, it is nearly half a magnitude fainter than Pollux, which is Beta Geminorum. This fact has led to speculation that Pollux was once fainter than Castor and has since brightened, or Castor has faded; but the reality is that Johann Bayer, who allotted the Greek letters in 1603, labelled the three brightest stars of Gemini α, β, and γ in order of declination, not brightness.

Unlike the twins that they represent, the stars Castor and Pollux are not related since they lie at different distances from us, 51 and 34 light years respectively. Eta Geminorum is called Propus, from the Greek πρόπουϛ (propous) meaning ‘forward foot’, a name that first appears with Eratosthenes; it arises from the star’s position in the leading (left) foot of the advance twin, Castor. Tycho in his star catalogue published in Astronomiae Instauratae Progymnasmata (1602) wrongly gave the name Propus to the star now known as 1 Geminorum, a mistake that was followed by Bayer (who labelled it H Geminorum on his atlas) and Bode in his Uranographia.

Chinese associations

In Chinese astronomy, the largest part of present-day Gemini was taken up by Jing, meaning Well (sometimes called the Eastern Well), which consisted of eight stars in the legs of the Twins: Lambda, Zeta, 36, Epsilon, Xi, Gamma, Nu, and Mu Geminorum. Together they form a shape resembling that of the Chinese character for ‘well’. The 22nd Chinese lunar mansion was named Jing after this constellation; it is the widest of the 28 mansions, extending for 33°, greater even than any of the 12 western houses of the zodiac, which were all 30° wide. The modern star Eta Geminorum next to the Well was known as Yue, a battle axe, used for decapitating the corrupt and immoral.

Castor and Pollux were not part of Jing. Instead, with nearby Rho Geminorum they formed Beihe, the Northern River (the Southern River, Nanhe, was to be found in Canis Minor, incorporating Procyon and two other stars). Beihe and Nanhe lay north and south of the ecliptic respectively, so the pair were also interpreted as gates or sentries. At either end of Beihe were Jishui and Jixin, each marked by a single star, representing a supply of water for winemaking or brewing and a pile of firewood for cooking. Sun and Kistemaker identify the relevant stars as Omicron and Phi Geminorum, although Kappa looks a better fit for the latter.

Five stars from Theta to Kappa or Phi Geminorum were Wuzhuhou, five feudal lords or princes who acted as the Emperor’s advisors and teachers. Delta Geminorum was one of a triangle of stars on the ecliptic forming Tianzun, a wine cup or water jar with three feet.

Shuiwei, ‘water level’, a curved line of four stars, was usually seen as extending from Canis Minor into Cancer, but some older versions show it as the stars 68 to 85 Geminorum, in an example of how Chinese constellations have literally changed their spots with time.

© Ian Ridpath. All rights reserved