vendredi 19 septembre 2008

Ursa Major 2

Undoubtedly the most familiar star pattern in the entire sky is the seven stars that make up the shape popularly termed the Plough or Big Dipper, part of the constellation Ursa Major, the Great Bear. The seven stars form the rump and tail of the bear, while the rest of the animal is comprised of fainter stars. It is the third-largest constellation.
In mythology, the Great Bear is identified with two separate characters: Callisto, a paramour of Zeus; and Adrasteia, one of the ash-tree nymphs who nursed the infant Zeus. To complicate matters, there are several different versions of each story, particularly the one involving Callisto.
Callisto is usually said to have been the daughter of Lycaon, king of Arcadia in the central Peloponnese. (An alternative story says that she is not Lycaon’s daughter but the daughter of Lycaon’s son Ceteus. In this version, Ceteus is identified with the constellation Hercules, kneeling and holding up his hands in supplication to the gods at his daughter’s transformation into a bear.)
Callisto joined the retinue of Artemis, goddess of hunting. She dressed in the same way as Artemis, tying her hair with a white ribbon and pinning together her tunic with a brooch, and she soon became the favourite hunting partner of Artemis, to whom she swore a vow of chastity. One afternoon, as Callisto laid down her bow and rested in a shady forest grove, Zeus caught sight of her and was entranced. What happened next is described fully by Ovid in Book II of his Metamorphoses. Cunningly assuming the appearance of Artemis, Zeus entered the grove to be greeted warmly by the unsuspecting Callisto. He lay beside her and embraced her. Before the startled girl could react, Zeus revealed his true self and, despite Callisto’s struggles, had his way with her. Zeus returned to Olympus, leaving the shame-filled Callisto scarcely able to face Artemis and the other nymphs.
On a hot afternoon some months later, the hunting party came to a cool river and decided to bathe. Artemis stripped off and led them in, but Callisto hung back. As she reluctantly undressed, her advancing pregnancy was finally revealed. Artemis, scandalized, banished Callisto from her sight.
Worse was to come when Callisto gave birth to a son, Arcas. Hera, the wife of Zeus, had not been slow to realize her husband’s infidelity and was now determined to take revenge on her rival. Hurling insults, Hera grabbed Callisto by her hair and pulled her to the ground. As Callisto lay spreadeagled, dark hairs began to sprout from her arms and legs, her hands and feet turned into claws and her beautiful mouth which Zeus had kissed turned into gaping jaws that uttered growls.
For 15 years Callisto roamed the woods in the shape of a bear, but still with a human mind. Once a huntress herself, she was now pursued by hunters. One day she came face to face with her son Arcas. Callisto recognized Arcas and tried to approach him, but he backed off in fear. He would have speared the bear, not knowing it was really his mother, had not Zeus intervened by sending a whirlwind that carried them up into heaven, where Zeus transformed Callisto into the constellation Ursa Major and Arcas into Boötes.
Hera was now even more enraged to find her rival glorified among the stars, so she consulted her foster parents Tethys and Oceanus, gods of the sea, and persuaded them never to let the bear bathe in the northern waters. Hence, as seen from mid-northern latitudes, the bear never sets below the horizon.
That this is the most familiar version of the myth is due to Ovid’s pre-eminence as a storyteller, but there are other versions, some older than Ovid. Eratosthenes, for instance, says that Callisto was changed into a bear not by Hera but by Artemis as a punishment for breaking her vow of chastity. Later, Callisto the bear and her son Arcas were captured in the woods by shepherds who took them as a gift to King Lycaon. Callisto and Arcas sought refuge in the temple of Zeus, unaware that Arcadian law laid down the death penalty for trespassers. (Yet another variant says that Arcas chased the bear into the temple while hunting – see Boötes.) To save them, Zeus snatched them up and placed them in the sky.
The Greek mythographer Apollodorus says that Callisto was turned into a bear by Zeus to disguise her from his wife Hera. But Hera saw through the ruse and pointed out the bear to Artemis who shot her down, thinking that she was a wild animal. Zeus sorrowfully placed the image of the bear in the sky.
Aratus makes a completely different identification of Ursa Major. He says that the bear represents one of the nymphs who raised Zeus in the cave of Dicte on Crete. That cave, incidentally, is a real place where local people still proudly point out the supposed place of Zeus’s birth. Rhea, his mother, had smuggled Zeus to Crete to escape Cronus, his father. Cronus had swallowed all his previous children at birth for fear that one day they would overthrow him – as Zeus eventually did. Apollodorus names the nurses of Zeus as Adrasteia and Ida, although other sources give different names. Ida is represented by the neighbouring constellation of Ursa Minor, the Little Bear.
These nymphs looked after Zeus for a year, while armed Cretan warriors called the Curetes guarded the cave, clashing their spears against their shields to drown the baby’s cries from the ears of Cronus. Adrasteia laid the infant Zeus in a cradle of gold and made for him a golden ball that left a fiery trail like a meteor when thrown into the air. Zeus drank the milk of the she-goat Amaltheia with his foster-brother Pan. Zeus later placed Amaltheia in the sky as the star Capella, while Adrasteia became the Great Bear – although why Zeus turned her into a bear is not explained.
Aratus named the constellation Helice, meaning ‘twister’, apparently from its circling of the pole, and said that the ancient Greeks steered their ships by reference to it, whereas the Phoenicians used the Little Bear (Ursa Minor). Aratus said that the bears were also called wagons or wains, from the fact that they wheel around the pole. The adjacent constellation Boötes is visualized as either the herdsman of the bear or the wagon driver. But Germanicus Caesar said that he bears were also called ploughs because, as he wrote, ‘the shape of a plough is the closest to the real shape formed by their stars’. According to Hyginus the Romans referred to the Great Bear as Septentrio, meaning ‘seven plough oxen’, although he added the information that in ancient times only two of the stars were considered oxen, the other five forming a wagon. On a star map of 1524 the German astronomer Peter Apian showed Ursa Major as a team of three horses pulling a four-wheeled cart, which he called Plaustrum.
One puzzle, never explained by any mythologist, is why the celestial bears have long tails, which real bears do not. Thomas Hood, an English astronomical writer of the late 16th century, offered the tongue-in-cheek suggestion that the tails had become stretched when Zeus pulled the bears up into heaven. ‘Other reason know I none’, he added apologetically.

Two stars in Ursa Major called Dubhe and Merak are popularly termed the Pointers because a line drawn through them points to the north celestial pole. Dubhe’s name comes from the Arabic al-dubb, ‘the bear’, while Merak comes from the Arabic word al-maraqq meaning ‘the flank’ or ‘groin’. At the tip of the bear’s tail lies Eta Ursae Majoris, known both as Alkaid, from the Arabic al-qa’id meaning ‘the leader’, or as Benetnasch, from the Arabic banat na’sh meaning ‘daughters of the bier’ – for the Arabs regarded this figure not as a bear but as a bier or coffin. They saw the tail of the bear as a line of mourners (the ‘daughters’) leading the coffin.
Second in line along the tail is the wide double star Zeta Ursae Majoris. The two members of the double, visible separately with keen eyesight, are called Mizar and Alcor. They were depicted as a horse and its rider on the 1524 star chart of Peter Apian, apparently following a popular German tradition. The name Mizar is a corruption of the Arabic al-maraqq, the same origin as the name Merak. Its companion, Alcor, gets its name from a corruption of the Arabic al-jaun, meaning ‘the black horse or bull’. This is the same origin as the name Alioth which is applied to the next star along the tail, Epsilon Ursae Majoris. The name the Arabs used for Alcor was al-suha, which Paul Kunitzsch translates as meaning either the ‘forgotten’ or ‘neglected’ one.
Delta Ursae Majoris is named Megrez, from the Arabic meaning ‘root of the tail’. Gamma Ursae Majoris is called Phad or Phecda, from the Arabic word meaning ‘the thigh’.
In addition to the famous seven stars of Ursa Major there are three pairs of stars that mark the feet of the bear. The Arabs imagined these as forming the tracks of a leaping gazelle. The pair Nu and Xi Ursae Majoris are called Alula Borealis and Alula Australis. The word Alula comes from an Arabic phrase meaning ‘first leap’; the distinctions ‘northern’ (Borealis) and ‘southern’ (Australis) are added in Latin. The second leap is represented by Lambda and Mu Ursae Majoris, known as Tania Borealis and Tania Australis, while the third leap is represented by Iota and Kappa Ursae Majoris, although Iota alone bears the name Talitha, from the Arabic meaning ‘third’.

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