Describe the imagery in Shakespeare's Sonnet 73.
The first four lines compare the end of the narrator's life to the last days of autumn:
That time of year thou mayst in me behold
When yellow leaves, or none, or few, do hang
Upon those boughs which shake against the cold,
Bare ruined choirs where late the sweet birds sang.
Yellow leaves, signs of past bloom, signify decay. The tree will soon have no leaves left to hang; it will be completely bare, or absent of fruit. This absence indicates that the narrator is nearing death, which is a "cold" and "bare" state. "The sweet birds sang" is in past tense, signifying that all sense of merriment has departed.
The next four lines reduce the event of death to a day. Shakespeare uses time to chart the narrator's progress from old age, or the autumn years ("That time of year thou mayst in me behold / When yellow leaves, or none, or few, do hang") to the day on which he dies:
In me thou seest the twilight of such day
As after sunset fadeth in the west,
Which by and by black night doth take away,
Death's second self, that seals up all in rest.
"In me thou seest" parallels "thou mayst in me behold" in the first line. The narrator's dying body personifies transience, and makes the passing of time tangible to the reader/listener. "Black night" (death) "[takes] away" sunset (old age). Shakespeare characterizes "black night," or the time in which we sleep ("seals up all in rest") as "Death's second self." There is a sense, then, of death simply being a long sleep.
The next four lines direct attention to the spirit, or passion, of the narrator which is extinguished by death:
In me thou seest the glowing of such fire
That on the ashes of his youth doth lie
As the death-bed whereon it must expire,
Consumed with that which it was nourished by.
Once again, there is an emphasis on what the reader/listener can see within the dying figure. So much of his experience of death relies on her attention and sympathy. "The glowing of such fire" indicates a light that remains beyond what has expired ("the ashes of his youth"). The use of the adjective "such" can have two meanings: it can indicate the magnitude or intensity of the fire, or it can point to there being something mysterious or inexplicable about its source.
The fire "must expire," or die out, "consumed with that which it was nourished by." It is unclear what the narrator is indicating as a source of nourishment. It could be the love of the listener/reader. It could be his own sense of mortality. It could be the passion and desire he once felt. It could be all of these things.
The last two lines, as with most of Shakespeare's sonnets, help us understand the point of the poem:
This thou perceiv'st, which makes thy love more strong,
To love that well which thou must leave ere long.
In seeing both "the twilight" and "the glowing of such fire" in the narrator, his lover is better able to understand the ephemeral nature of existence, which makes love so necessary and important. It is especially meaningful to love when one understands that the object of that love will one day be gone.