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This reference is a little tricky, but based on the context, it's pretty clear that the "silver sphere" is an allusion to the planet Venus. Venus is visible as a bright object in the heavens, and it really stands out at dusk and at dawn (just like this silver sphere).
Line 22: In the line above this, the speaker says that the bird's song is "keen as the arrows" of the silver sphere. That tips us off that this is a simile, a comparison between the beautiful bright solitary planet Venus and the invisible bird.