vendredi 19 septembre 2008

Canis Major 1

Canis Major is dominated by the star Sirius, popularly called the Dog Star, the most brilliant star in the entire sky; almost certainly the constellation originated with this star alone. Aratus referred to Canis Major as the guard-dog of Orion, following on the heels of its master and standing on its hind legs with Sirius carried in its jaws. Manilius called it “the dog with the blazing face”. Canis Major seems to cross the sky in pursuit of the hare, represented by the constellation Lepus under Orion’s feet.

Mythologists such as Eratosthenes and Hyginus said that the constellation represented Laelaps, a dog so swift that no prey could outrun it. This dog had a long list of owners, one of them being Procris, daughter of King Erechtheus of Athens and wife of Cephalus, but accounts differ about how she came by it. In one version the dog was given to her by Artemis, goddess of hunting; but a more likely account says that it is the dog given by Zeus to Europa, whose son Minos, King of Crete, passed it on to Procris. The dog was presented to her along with a javelin that could never miss; this turned out to be an unlucky gift, for her husband Cephalus accidentally killed her with it while out hunting.
Cephalus inherited the dog, and took it with him to Thebes (not Thebes in Egypt but a town in Boeotia, north of Athens) where a vicious fox was ravaging the countryside. The fox was so swift of foot that it was destined never to be caught – yet Laelaps the hound was destined to catch whatever it pursued. Off they went, almost faster than the eye could follow, the inescapable dog in pursuit of the uncatchable fox. At one moment the dog would seem to have its prey within grasp, but could only close its jaws on thin air as the fox raced ahead of it again. There could be no resolution of such a paradox, so Zeus turned them both to stone, and the dog he placed in the sky as Canis Major, without the fox.
The name of the star Sirius comes from the Greek word seirius meaning ‘searing’ or ‘scorching’, highly appropriate for something so brilliant. In Greek times its rising at dawn just before the Sun marked the start of the hottest part of the summer, a time that hence became known as the Dog Days. ‘It barks forth flame and doubles the burning heat of the Sun’, said Manilius, expressing a belief held by the Greeks and Romans that the star had a heating effect. The ancient Greek writer Hesiod wrote of ‘heads and limbs drained dry by Sirius’, and Virgil in the Georgics said that ‘the torrid Dog Star cracks the fields’.
Germanicus Caesar outlined clearly the effects that the rising of Sirius with the Sun was supposed to have. Healthy crops it strengthens, but those with shrivelled leaves or feeble roots it kills. ‘There is no star the farmer likes more or hates more’, according to Germanicus.
‘Hardly is it inferior to the Sun, save that its abode is far away’, wrote Manilius, anticipating the modern view that stars are bodies like the Sun only vastly more distant. Yet, in contradiction of the supposed heating effects of Sirius, Manilius continued: ‘The beams it launches from its sky-blue face are cold’. That description of the colour of Sirius is in contrast to Ptolemy’s surprising reference to it as reddish, which has caused all manner of arguments.
In fact, Manilius was nearly correct, for Sirius is a blue-white star, even larger and brighter than the Sun. It lies 8.6 light years away, making it one of the Sun’s closest neighbours. It has a white dwarf companion star, visible only through telescopes, that orbits it every 50 years.

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