The main stylistic element of this great sonnet is the way in which Shakespeare builds this poem around a central comparison between stars and his lover's eyes. Note how Shakespeare introduces this comparison at the beginning of the poem, alluding to the way in which "astronomers" through studying the stars could predict the future:
Not from the stars do I my judgment pluck;
And yet methinks I have astronomy...
The poem begins by the speaker stating that he is not able to predict the future through study of the stars or through any other such method, and yet he still feels that he is able to practice "astronomy," but on a different source:
But from thine eyes my knowledge I derive,
And, constant stars, in them I read such art
As truth and beauty shall together thrive,
If from thyself to store thou wouldst convert;
The stars, then, with others build their lives around and base their predictions upon, do not serve for the speaker. For him, it is the eyes of his beloved, who is addressed in the sonnet, that he uses to "derive" his "knowledge." From these eyes he is able to see a "thriving" of truth and beauty. The prediction that the speaker is able to make based on these eyes forms the ending of the poem:
Thy end is truth's and beauty's doom and date.
His beloved personifies truth and beauty to such an extent, that his death will mark the end of these qualities. Thus this sonnet is built around the central comparison of the stars which are compared to the eyes of the speaker's beloved.