Sonnet 18 - the analysis

ANALYSIS
temperate (1): i.e., evenly-tempered; not overcome by passion.
the eye of heaven (5): i.e., the sun.
every fair from fair sometime declines (7): i.e., the beauty (fair) of everything beautiful (fair) will fade (declines).
Compare to Sonnet 116: "rosy lips and cheeks/Within his bending sickle's compass come."
nature's changing course (8): i.e., the natural changes age brings.
that fair thou ow'st (10): i.e., that beauty you possess.
in eternal lines...growest (12): The poet is using a grafting metaphor in this line. Grafting is a technique used to join parts from two plants with cords so that they grow as one. Thus the beloved becomes immortal, grafted to time with the poet's cords (his "eternal lines"). For commentary on whether this sonnet is really "one long exercise in self-glorification", please see below.
Sonnet 18 is the best known and most well-loved of all 154 sonnets. It is also one of the most straightforward in language and intent. The stability of love and its power to immortalize the poetry and the subject of that poetry is the theme.
The poet starts the praise of his dear friend without ostentation, but he slowly builds the image of his friend into that of a perfect being. His friend is first compared to summer in the octave, but, at the start of the third quatrain (9), he is summer, and thus, he has metamorphosed into the standard by which true beauty can and should be judged.

The poet's only answer to such profound joy and beauty is to ensure that his friend be forever in human memory, saved from the oblivion that accompanies death. He achieves this through his verse, believing that, as history writes itself, his friend will become one with time. The final couplet reaffirms the poet's hope that as long as there is breath in mankind, his poetry too will live on, and ensure the immortality of his muse.
Interestingly, not everyone is willing to accept the role of Sonnet 18 as the ultimate English love poem. As James Boyd-White puts it:
What kind of love does 'this' in fact give to 'thee'? We know nothing of the beloved’s form or height or hair or eyes or bearing, nothing of her character or mind, nothing of her at all, really. This 'love poem' is actually written not in praise of the beloved, as it seems, but in praise of itself. Death shall not brag, says the poet; the poet shall brag. This famous sonnet is on this view one long exercise in self-glorification, not a love poem at all; surely not suitable for earnest recitation at a wedding or anniversary party, or in a Valentine. (142)
Note that James Boyd-White refers to the beloved as "her", but it is almost universally accepted by scholars that the poet's love interest is a young man in sonnets 1-126.
Sonnets 18-25 are often discussed as a group, as they all focus on the poet's affection for his friend.
For more on how the sonnets are grouped, please see the general introduction to Shakespeare's sonnets.
For more on the theme of fading beauty, please see Sonnet 116.
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References
Boyd-White, James. The Desire for Meaning in Law and Literature. Current Legal Problems. Volume 53. Ed. M. Freeman. Oxford: Oxford UP, 2000.
Shakespeare, William. Shakespeare's Sonnets. Ed. Tucker Brooke. London: Oxford UP: 1936.
Shakespeare, William. The Works of Shakespeare. Ed. John Dover Wilson. Cambridge: Cambridge UP, 1969.
Smith, Hallett. The Tension of the Lyre. San Marino: Huntington Library, 1981.
How to Cite this Article
Mabillard, Amanda. An Analysis of Shakespeare's Sonnet 18. Shakespeare Online. 2000. (day/month/year you accessed the information) < http://www.shakespeare-online.com/sonnets/18detail.html >.