vendredi 12 septembre 2008

Delphinus 4

Delphinus, "The Dolphin", is an ancient constellation located just west of Pegasus.
Some references name a certain "Arion" as the inspiration for the constellation.
There were two Arions in antiquity. One was a (mythic?) poet who may have lived in the eighth century BC. This Arion, travelling from Sicily to Corinth, was thrown overboard by the ship's crew, eager for the valuables he was carrying. A dolphin is said to have rescued the poet. But this dolphin probably isn't the constellation's origin.
The second Arion was a son of Poseidon and Demeter, and was in fact a horse (like his half-brother Pegasus). Instead of hooves, he had feet on his right side. And, unlike most horses, he could talk. But this Arion also has nothing to do with the constellation.
It is most likely however that the constellation is associated with Poseidon. It was probably his way of thanking one of his messengers for a job well done.
As God of the Sea, Poseidon had fifty sea-nymphs at his court. These were all born of Nereus and known therefore as the Nereids.
While Poseidon had many casual love affairs, when he set out to find a wife he was concerned that she be accustomed to life in the sea. His first choice was Thetis, one of the fifty Nereids. But he learned that any son born of Thetis would grow to become greater than his father. Clearly Poseidon couldn't accept that prophecy.
As a side note, Thetis married Peleus, a mortal, and they had a famous son named Achilles. Thetis dipped Achilles in the river Styx to make him invulnerable to his enemies. As most people now knows, since his mother grasped him by the heels, they were the only part of Achilles which were vulnerable. Wouldn't you know, the day would come when he'd get a poisoned arrow in his heel and die from it.
Poseidon's next choice in marriage was a sister of Thetis, called Amphitrite. But when Poseidon pressed Amphitrite to marry him, she was quite disgusted by the thought and fled to the far-off Atlas Mountains. Poseidon sent a number of messengers to persuade her to return, as his wife, to his underwater realm.
The messenger who succeeded in this task was the dolphin Delphinus. Amphitrite was so beguiled by Delphinus' pleadings she relented and returned to Poseidon and became the Queen of the Sea. They had many children.
Delphinus was later put in the heavens as a constellation by a grateful Poseidon.

The asterism is rather curious, for its four main stars form a rectangle called "Job's Coffin". This is probably a hang-over from the time Delphinus was interpreted as a whale, as in Chapter 41 of Job where God challenged Job: "Canst thou draw out leviathan with an hook?" However there is no reference to Job being swallowed by a whale, as happened with Jonah, so the name Job's Coffin remains a bit of a mystery.
The constellation's Bayer stars are not complete, and are mostly in the fourth and fifth magnitude range.

Double stars:
Delphinus has several fine binaries, a Mira-type variable, and a very remote globular cluster.
Beta Delphini is a very close visual binary with orbit of 26.7 years. Epoch 2000 values: 4.0, 4.9; PA 343º, separation 0.5".
Gamma1 and gamma2 Del form a fine binary with (perhaps) subtle colour change (observers argue over this; some find them both yellow, others that the companion is greenish or bluish): 4.5, 5.5; PA 268º, 9.6"
Struve 2725 is a wonderful sight in the same field as gamma Del (to the SW): 7.3, 8.0; PA 9º, separation 5.7".

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