What is the figure of speech in Shakespeare's Sonnet 18, "Shall I compare thee to a summer's day?"

A figure of speech is a useful tool in literature, and introduces an inferred meaning into a description other than the literal interpretation. This allows for vivid descriptions and visual images which intensify the significance contained in, in this instance, Shakespeare's Sonnet 18 which begins with the familiar words "Shall I compare thee to a Summer's Day?" Shakespeare directs a question at the object of his admiration, and not expecting an answer, he answers himself adding great emphasis to the poem. The question is, therefore, rhetorical. Of course, the answer would be a resounding affirmation and in fact Shakespeare's subject is far "more lovely and more temperate." The opening line is a much repeated line in many romantic encounters, not all of which are sincere expressions of admiration. However, Shakespeare's intention is in no doubt and he uses metaphor to make his comparison with a summer's day.

His talk of "temperate" weather and "rough winds" reveals the enduring characteristics of his subject who is not affected by change, and the metaphor is extended so that the reader cannot mistake Shakespeare's feelings of high regard for someone whose "eternal summer shall not fade." It is significant that Shakespeare talks of the month of May which passes and of summer as the seasons change. "Nature's changing course" confirms that it would not be possible to sustain a "gold complexion" but, for Shakespeare's subject, not even Death (personified) can hold him in "his shade." 

In my mind, the theme of the Sonnet is a reverence of love or feelings of emotions from the speaker to another element.  Certainly, this could be the love of the creation of art or the ability to love another.  The idea of setting this spirit of Eros to the natural setting helps to bring forth the idea that one's love is almost as natural as other phenomena experienced in the world of nature.  Connecting both of these experiences into one, making them almost seamless expressions of individual expression and nature represents a critical theme of the sonnet.  Shakespeare's ability to make the subjective universal helps to create the idea that individual expression and natural expression are one in the same.

I'm not exactly sure what you mean by my take on it, but here's what I think Shakespeare is trying to say in this poem.

In my opinion, this poem is mostly about how great he thinks his poetry is.  People usually think it's about the woman he's talking about, but I think is just as much about the poetry.  After all, what is it that gives life to her, that makes her immortal?  It is the fact that he has written this poem about her.  So it's his poetry, not something about her, that is going to make her immortal.

Other than that, I would just say that I really like the poem.  I think his way of comparing her to the summer's day and all the things he says about how she is better are quite clever.

The power of Sonnet 18 is how it uses the imagery of the natural world to help establish the feeling of love the speaker holds.  The opening line helps to establish a mood that creates a transcendent vision of love and affection.  This is continued throughout the sonnet, where the ability to unify the natural and objective order to things as well as the subjectively personal is evident.  The Sonnet accomplishes this harmonious union through the employment of imagery.  Such mental pictures can be seen in the descriptions that can be described in terms such as  "temperate" and "gold complexion."

If something is "temperate" that means that it is not too extreme.  In the case of weather, not too hot, not too cold, not too windy, and so on.  In this poem, Shakespeare is saying that a summer's day is not temperate enough -- it is too extreme -- to be compared to his love.

Specifically, he says that summer days can be too hot (the eye of heaven shines too hot).  And he says that they can be too windy (rough winds shake "the darling buds of May").

For those reasons, they are not a good comparison for how wonderful his love is.

In Sonnet XVIII, Death is personified much like the Grim Reaper who comes for the beloved, desiring to claim her in "his shade"; this shade is an allusion to the valley of the shadow of death expressed in Psalm 23:4.

Written as a Petrarchan sonnet, Sonnet XVIII is composed of an octave (8 lines) that presents an argument, while the sestet (final 6 lines) offers an answer to this argument. The speaker argues that he does not wish to compare his love to a summer's day because of sublunary corruption; that is, things of nature deteriorate: "every fair from fair sometime declines." And, as a result of this deterioration, this sublunary corruption, the poet has decided to preserve his love and the object of his love in verse. With his "eternal lines," therefore, the beauty of his love will be everlasting, so long as men "can breathe, or eyes can see." Thus, Death and his "shade" will be thwarted, and he cannot brag of having taken the  beloved, who will now live forever in verse where

...thy eternal summer shall not fade,
Nor lose possession of that fair thou owest;

The sonnet exemplifies Shakespeare's innovation of the English form of the genre. Although written in the rhyme scheme abab cdcd efef gg which is typical of the English form, sonnet 18 has a strong pause (or volta) between the first eight lines (the octave) and the last six (the sestet). This is typical of the Italian or Petrarchan sonnet. The pause in the text is signalled by the colon and by the conjunction BUT which introduces a contrast with what has been said earlier. The pause thus corresponds to the introduction of new imagery or a new topic. Other innovative sonnets are 99 which has fifteen rather than fourteen lines and 126 (six couplets).

Sonnet 18 also introduces a variation in terms of content compared to the first seventeen poems. The speaker expresses for the first time his ambition to make youth immortal in his lines, thus adopting a new perspective.

The speaker teasingly wonders out loud if he should compare his beloved to a summer's day. He has no intention of doing so, of course; he's gently toying with his lover. He then proceeds, much to the relief of his lover, we might think, to give reasons why such a comparison would be unfair.

For even in summertime, rough winds shake the "darling buds of May." Not only that, but a summer's day, even if extraordinarily beautiful, is also transient; its beauty isn't destined to last. Besides, the sun can often be too hot during the daytime, which makes the comparison between a summer's day and the speaker's beloved all the more inappropriate.

Summer itself has "too short a lease," so even if every day of summer were gloriously bright and sunny, summer itself would still eventually have to make way for autumn. The point that the speaker is making is that his love is nothing like this at all. It is eternal; it will live on, immortalized in the words of the poem long after his beloved has passed away.

If I understand your question correctly, then the role of the described person is completely passive.  While she is the object of the poem, she is just that -- an object.

In this poem, the described person does not do anything at all.  She is just there and she is only there in the way that Shakespeare is describing her.  While she will be famous, he says, it is not for anything that she has done.  Instead, she will be famous only because he has written about her.

So I think that the role of the described person is very passive -- sort of like the role of a model who poses for a painting, perhaps.

In this poem, poetery is given a very high status.  Its status (or at least the status of Shakespeare's own poetry) is almost that of a god because it can make someone immortal.

In the poem, Shakespeare is saying that his love is so much better than a summer day.  One of the ways in which this is true is that his love will live forever while a summer day does not last very long (hath all too short a lease).  The reason why his love will live forever is because he has written this poem about her.

There is no definitive pronoun used to describe the person addressed by the poem's speaker, and, therefore, we must conclude that this sonnet could address either a woman or a man. Because of its context within Shakespeare's oeuvre, people tend to assume that the speaker addresses an attractive young man; however, I always caution my students against making assumptions about a poem based on things happening outside of the poem. Since this speaker never specifies, I believe the sex of the person being addressed is left open for our interpretation. Certainly, it seems possible that the speaker could describe either a man or a woman as "lovely" and "temperate"; likewise, the speaker describes the addressee's "eternal summer" and the "fair[ness]" that the speaker hopes to prevent from perishing from the earth forever by writing these words. Both men and women can be attractive and worth preserving, in some way, from inevitable death.

I agree that the poem is written to a man. If you read the 17 that come before it, it becomes more clear that his intention for many of the early sonnets is to do what post #2 discusses. It is theorized that Shakespeare was hired by a young man or his family to write a series of sonnets about and/or in praise of the young man. Some experts think that the sonnets may have written to or about a young gentleman named William Herbert.

What's the meaning of the closing couplet in Shakespeare's "Shall I compare thee to a summer's day?"

In Shakespeare's Sonnet 18 ("Shall I compare thee to a summer's day"), the poet is comparing the subject of the poem with nature.

Though the summer is a beautiful time, the object of the poet's praise is even more "lovely and more temperate." Unfortunately, the winds wreak havoc with the gentle blossoms that bloomed in the spring ("May"). And in the summer, the sun can sometimes be terrible fierce ("too hot the eye of heaven shines"). He goes on to write that there is always an eventual decline in nature, with the passing of summer. So there is the question, "Shall I compare thee...?"

The pivotal point of the poem rests on the first word of the ninth line: "But..." Shakespeare summarizes an idea in the first two quatrains (four-line stanzas), but then shifts his focus in the first line of the third quatrain. In this case, he is saying that for all that happens in terms of nature and the summer, the object of his praise will NOT follow the same path: "...thy eternal summer shall not fade," even as time passes, even when death comes.

The couplet is used to draw the sonnet to its conclusion, but to present a summary of Shakespeare's thought: as long as there are people left to read this sonnet, the beauty and life of this person will be immortalized.

Whereas the beginning eight lines speak literally of a day in summer, the third quatrain becomes more figurative, metaphorical. The beauty of the poet's subject will not fade (literally, it will) or be less "fair;" death will not be a threat (though literally, it will), when this person looks towards dying and eternity. In a literal sense, the ravages of time will leave their mark, and death will come.

However the rhyming couplet works because Shakespeare (or the speaker) is saying that in the sonnet, time will not pass, and the object of the poem will be immortal, as if he or she were frozen in time at that very moment.

I am not sure to what extent we can apply the world of logic and reason to the world of literature, emotion and love. Certainly the last couplet of this famous sonnet of Shakespeare probably don't make much logical sense, but I guess to understand and fully appreciate poetry you have to suspend your logical faculties and try and embrace the imagination. Let us just remind ourselves what the last couplet says:

So long as men can breathe, or eyes can see,
So long lives this, and this gives life to thee.

These last two lines are actually an example of a convention that is known as an eternalising conceit, which suggests that the sonnet itself immortalises or gives eternity to the human love explored in this poem. The whole sonnet is a celebration of how the "eternal summer" of the youth addressed in the sonnet will not fade like everything else. This is because, as the last couplet states, so long as there are humans around, this sonnet will remain. And as long as that happens, the beauty of the youth will be "given life" and resurrected in Shakespeare's poetry. So, perhaps not much logical sense, but a beautiful ending to a beautiful sonnet.

How does the structure of Sonnet 18 influence its content?

The volta (turn in thought) at line nine signals a change in mood between the lament in the first eight lines and the optimism expressed in the final six lines. 

In the first eight lines, the speaker recognizes that the beauty of his beloved is superior to that of a summer day since Nature is unpredictable, unreliable, destructive, and to exist within it, one is ultimately mortal. 

After the volta, the speaker is cheered by the thought that through this poem, as long as there are people to read it, his beloved's beauty will be immortalized, not subject to the mortality found in the natural world. 

The sonnet's structure poses a problem, the inconvenient truth of our humanity, and the solution, a re-framing of this truth through the lens of artistic triumph, which is eternal. 

The three quatrain plus couplet form of Shakespeare’s sonnets also allow the speaker to use a dominant metaphor or image for each quatrain as he leads to his resolution in the concluding couplet. Each quatrain then contains a certain logic of its own.  In Sonnet 18 the first quatrain argues the beloved is more gentle than the season of summer, which can be harsh and brief; the second quatrain becomes more specific, using the metaphor of the “eye of heaven,” which is the sun, to argue that summer can reduce beauty. This quatrain concludes with a colon, signifying a shift in thought in what follows, which we see with the “But” that introduces the third quatrain. Here the speaker extends his images from the season of summer (quatrain 1), to the sun (quatrain 2), to time and eternity—the beloved’s beauty will never end.  Why is all of this so? The couplet provides the answer, which is that his beloved’s beauty and life exist in the words the speaker gives them; the beloved exists outside time altogether because of the poem which “gives life” to him or her.

As you'll find in the analysis for this sonnet (see link below), the way Shakespeare would pose a rhetorical question in his sonnets, then only allow for one answer (his own), was important in getting the point of his sonnet across to his reader, and most especially, to the intended recipient of the sonnet.

Shakespeare used the traditional sonnet form for all but three of his sonnets, which is 14 lines, broken into three quatrains (stanzas with four lines) and one concluding couplet (two lines with end rhymes). The rhetorical question is typically posed within the first quatrain, then expounded upon through the other two quatrains, and finally, a conclusion/answer is offered in the couplet.

This form - 14-line sonnet in iambic pentameter - was very conducive to the meaning of the sonnet. It works particularly well with Sonnet 18, as he is positing the fact that his beloved, when compared to the beauty of nature, is far more lovely, more calm, etc. Shakespeare is also making the point that his beloved can be immortal, despite the usual deteriorating effects of aging and nature. He presents this in the quatrains, leading up to his final point in the couplet, which reads:

"So long as men can breathe, or eyes can see,
So long lives this, and this gives life to thee."

He is saying at the end that as long as his sonnet exists, and as long as people are still living and can read it, his beloved will be immortal through the lines of his sonnet.