What are the changes that happen to the summer sun according to "Shall I Compare Thee to a Summer's Day" (Sonnet 18)?

In Sonnet XVIII, Shakespeare writes that the summer sun can be too short, too hot, and sometimes it only shines dimly.

In this sonnet of Shakespeare's, which is written in the Petrarchan form, the first four lines interrelate in order to form an argument that the last few lines will answer. In these first lines, the poet states that comparing his beloved to a summer's day lends his love only a temporal nature. So, in his argument against the summer sun, the poet states that

1. Summer does not last long enough since it "hath all too short a date."


2. Often the sun is unbearably hot when it shines upon the earth: "sometimes too hot the eye of heaven shines."


3. The summer sun often loses its brilliance because of clouds and such: "and often is his gold complexion dimm'd . . ."

For these reasons, the poet decides to preserve his beloved's beauty, not in a comparison to the sun, but in the written verse: "when in eternal lines to time thou growest." As long as people live and read, the beloved's beauty will be preserved in the poet's verse.