Servius, Commentary to the Aeneid 6.645

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Nec non Threicius longa cum veste sacerdos: Orpheus Calliopes musae et Oeagri fluminis filius fuit, qui primus orgia instituit, primus etiam deprehendit harmoniam, id est circulorum mundanorum sonum, quos novem esse novimus.

‘And the Thracian priest, too, in a long robe’: Orpheus was the son of the Muse Calliope and the river Oeagrus. He was the first to establish mysteries and also the first to discover harmony, that is, the sound of the earth’s spheres, which we know to be nine.

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