This, in combination with the words "nature's changing course", creates an oxymoron: the unchanging change of nature, or the fact that the only thing that does not change is change. This line in the poem creates a shift from the mutability of the first eight lines, into the eternity of the last six.
This, combined with the words "nature's shifting trajectory", generates an antithesis: the unchanging alteration of nature, or the fact that the sole constant is change. This line in the poem marks a shift from the fluidity of the preceding lines to the enduring quality of the latter six.