In Sonnet XVIII Shakespeare uses the concept of sublunary, or earthly, corruption to demonstrate that summer weather is unpredictable.
In his Petrarchan sonnet, Shakespeare invokes the sense of harmony of the classical form and order demanded by this particular sonnet form. But, to expect such order in the universe is not possible because of sublunary corruption: Summer is but one season and then changes; sometimes it is too hot, or "rough winds" may disturb the beauty of nature. At any rate, this lovely season can end in destruction: "And every fair from fair sometime declines."
Since the elements of nature are transitory, verse is then appropriated as the form for the perpetuation of the speaker's love. Indeed, beauty will last forever in verse. In this way, the beloved's "eternal summer" will not wither or fade, and the beloved will remain as fair as she is at the time of the composition of this sonnet. The final couplet summarizes this eternalness:
So long as men can breath, or eyes can see,
So long lives this, and this gives life to thee.