Evaluate "Sonnet 18" as a Shakespearean sonnet?

In addition, the quatrains of the Shakespearean sonnet each express a single, differing thought.  For instance, in the first quatrain, Shakespeare places the young man beyond comparison:  "Shall I compare thee..../Thou art more lovely and more temperate:" 

Then, in the second quatrain, the poet draws a series of implicit analogies regarding the young man's temperament, likening the heat of the sun to the man's self-will, and "nature's changing course" to the coolness of the young man toward the narrator: "By chance, or nature's changing course untrimmed:"

Again, in the third quatrain the poet places the young man outside of a natural context as he declares, "But thy eternal summer shall not fade...." because of the eternal lines of poetry.

Finally, the couplet makes the suggestion that the "eternal lines" will preserve the youth, although the temperament metioned in the second quatrain ("too hot the eye" )challenges the idea that the young man will be satisfied with the reliance of the poet's "lines."