Lines 7-9 Summary

Line 7

The Child is father of the Man; 

• Famous line alert! Here we move away from talking specifically about the rainbow. This line is an example of a paradox—a contradictory statement. It's definitely a paradox that a child could father a man, right? You'd think it was the other way around. 
• Yet, in the context of this poem, the statement makes sense. The speaker has shown us how important it is that something that thrilled him when he was young continues to thrill him when he grows old. He is saying here that his childhood formed who he is as an adult—his self, as a child, fathered, or gave birth to, his adult self. It seems the speaker treasures the fact that he still has a childlike capacity for wonder.
• Also note the capitalization of the words "Child" and "Man" in this line. This is a way to draw attention to the general truth of the line. It is meant to have a wider meaning than just in the speaker's life. A rainbow brings out the child in all of us.

Lines 8-9

And I could wish my days to be
Bound each to each by natural piety.

• The speaker now expresses that he hopes nature will tie his days together forever, as we can imagine a child's days would be tied together by playing outside.
• What do we mean by tied together? Well, they would all have the same thing in common. Think about when you went to the same park to play, every day of summer vacation. That experience tied your summer together. Well, here the speaker wants all of his days to feature this same feeling of wonder for the natural world. 
• We suspect that the speaker doesn't mean literal days here, but rather his time on earth—his life. 
• The glue, or rope, between these days is "natural piety." There are a few different ways to interpret this phrase. Piety normally has a religious connotation.
• Someone who follows the laws of their religion and is very devoted to God would be called pious. So we might interpret "natural piety" as a religion that is natural, or not forced. 
• But there's not really much else about religion in this poem, so that interpretation seems a little off. What if "natural" referred not to something being genuine and sincere, but to the object of the piety? We think the speaker wants his days to be tied together by reverence and piety toward the natural world, rather than toward religion. 
• These two lines sort of put the rest of the poem in context. The rainbow, which thrills the speaker throughout his life, is an example of a form of natural piety, his sense of joy and wonder at the natural world. That sense is what he hopes to experience for the rest of his days, his time on earth.