jeudi 18 septembre 2008

Eridanus 1

Early writers seem to have regarded the Eridanus as a mythical river, flowing into the great Ocean that surrounded the lands of the known world. The first-century BC Roman poet Virgil called it ‘the king of rivers’. Eratosthenes identified it as the Nile, ‘the only river which runs from south to north’. Hyginus agreed with this identification, pointing out that the star Canopus (which marks a steering oar of the ship Argo) lay at the end of the celestial river, as the island Canopus lies at the mouth of the Nile. But Hesiod in his Theogony listed the Nile and Eridanus separately, showing that he regarded them as different rivers. Later Greek writers identified the Eridanus with the river Po in Italy.
In mythology, the Eridanus features in the story of Phaethon, son of the Sun-god Helios, who begged to be allowed to drive his father’s chariot across the sky. Reluctantly Helios agreed to the request, but warned Phaethon of the dangers he was facing. ‘Follow the track across the heavens where you will see my wheel marks’, Helios advised.

As Dawn threw open her doors in the east, Phaethon enthusiastically mounted the Sun-god’s golden chariot studded with glittering jewels, little knowing what he was letting himself in for. The four horses immediately sensed the lightness of the chariot with its different driver and they bolted upwards into the sky, off the beaten track, with the chariot bobbing around like a poorly ballasted ship behind them. Even had Phaethon known where the true path lay, he lacked the skill and the strength to control the reins.
The team galloped northwards, so that for the first time the stars of the Plough grew hot and Draco, the dragon, which until then had been sluggish with the cold, sweltered in the heat and snarled furiously. Looking down on Earth from the dizzying heights, the panic-stricken Phaethon grew pale and his knees trembled in fear. Finally, he saw the constellation of the Scorpion with its huge claws outstretched and its poisonous tail raised to strike. Young Phaethon let the reins slip from his grasp and the horses galloped out of control.
Ovid graphically describes Phaethon’s crazy ride in Book II of his Metamorphoses. The chariot plunged so low that the Earth caught fire. Enveloped in hot smoke, Phaethon was swept along by the horses, not knowing where he was. It was then, the mythologists say, that Libya became a desert, the Ethiopians acquired their dark skins and the seas dried up. To bring the catastrophic events to an end, Zeus struck Phaethon down with a thunderbolt. With his hair streaming fire, the youth plunged like a shooting star into the Eridanus. Some time later, when the Argonauts sailed up the river, they found his body still smouldering, sending up clouds of foul-smelling steam in which birds choked and died.
Eridanus is a long constellation, the sixth-largest in the sky, meandering from the foot of Orion far into the southern hemisphere, ending near Tucana, the Toucan. The constellation’s brightest star, first-magnitude Alpha Eridani, is called Achernar, from the Arabic akhir al-nahr meaning ‘the river’s end’; at declination –57.2 degrees, it does indeed mark the southern end of Eridanus.
In Ptolemy’s day, though, the river dried up 17 degrees farther north, at the star now assigned the Greek letter Theta. Eridanus was first shown flowing to the present-day Alpha Eridani on a globe of 1598 compiled by Petrus Plancius. Plancius got his information on the southern stars from observations made by the navigator Pieter Dirkszoon Keyser during the first Dutch voyage to the East Indies (the ‘Eerste Schipvaart’) in 1595–97. However, it is not clear whether the idea of extending Eridanus was due to Plancius, Keyser, or even some earlier navigators who had previously seen this star.
Bedouin Arabs visualized Achernar and Fomalhaut (in Piscis Austrinus) as a pair of ostriches.

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