Sentence:  By the 'hissing snake' Spenser presumably means the scorpion sent by Diana that killed Orion. Like Orion it, too, was <WSD>stellified</WSD>. But since, as Scorpio, it rises in the east as Orion's sign sets in the west, the two were regarded as being kept forever apart, Orion perpetually avoiding in the heavens his vanquisher on earth. 
Meaning of the word marked with <WSD> tags:  To transform from an earthly body into a celestial body; to place in the sky as such