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While I personally feel the planet Uranus and Aquarius (as well as Saturn and Aquarius) are a good match, the Graeco-Roman mythology concerning the god Uranus and his castration are about as unhelpful as you can get.

If we trace the history of mythology for the sign, Aquarius was Gula "the great one" to the Babylonians, the source of the winter rains that fed the Tigris and Euphrates. Aquarius was possibly the god Hapi of the Egyptians, the source of the Nile. The water supposedly flowing from the constellation's vase is the Aquarid meteor shower. The parallel wavy lines in the Aquarius glyph resemble the Babylonian cuniform and Egyptian heiroglyph for water.

The Hellenists had an unfortunate tendency to dumb-down early meanings of the signs and constellations, hence Manilius's description of Aquarius as the abducted boy Ganymede.