Shakespeare's Sonnet 18 and 73 are thematically opposite of each other. Sonnet 18 praises the beloved as more wonderful than a summer's day. Sonnet 73 praises love that grows more strong under the influence of old and withered age. Another difference is that Sonnet 18 ends with the confidence that the sonnet will make the beloved live as long as eyes can see, while Sonnet 73 ends with the confidence that love will grow stronger because of an anticipated shortened life for the speaker.
The two sonnets are similar in that the poet calls upon nature to develop the conceits that express the themes. In Sonnet 18, the opening conceit, from which other nature metaphors are drawn, is that of comparing a beloved to a summer's day: "Shall I compare thee to a summer's day?" In Sonnet 73, the opening conceit, from which other nature metaphors are likewise drawn, is that of comparing the speaker to the time of year, that being the autumn of the year: "That time of year thou mayst in me behold / When yellow leaves,..do hang / Upon those boughs... ."
Concerning comparisons of Sonnet 18 and Sonnet 73 by William Shakespeare, both sonnets deal with love and the longings associated with having a meaningful relationship with someone.
In addition, in both Sonnets, Shakespeare makes reference to Death:
Nor shall Death brag thou wander'st in his shade, (Sonnet 18)
Death's second self, that seals up all in rest. (Sonnet 73)
Furthermore, both Sonnet 18 and Sonnet 73 refer to seasons. Sonnet 18 speaks of summer as well as the month of May. Sonnet 73 speaks of the changing leaves of the autumn season.
Regarding contrasts, a significant one is the tone or mood of the respective sonnets. Sonnet 18 has a more positive lilt to it. This sonnet is Shakespeare’s celebration of female beauty in all its aspects as concerns the man’s person of desire. In this sonnet, Shakespeare talks of this woman’s beauty never fading. Even in Death, the remembrance of her beauty will carry on.
In Sonnet 73, the tone is more resigned to the ravages of growing old and the man not being of his former resilience and vigour. Shakespeare speaks of the man fading into an advanced age and alludes to the fact that he is fading as autumn leaves fade and fall from the branch, before the cold winter hits with full force.
In this sonnet, the man understands that his lover now sees him as such, a waning sun so-to-speak, evidenced by the line:
As after sunset fadeth in the west,
In Sonnet 73, Shakespeare notes that the woman still loves the man greatly, even though she knows they will soon take leave of each other because he will die. In Sonnet 18, Shakespeare talks of vibrant beauty in the present concerning the woman he loves, and a continued celebration of her beauty through memories, despite a Death that will one day come to her.