What metaphor is in the poem i compare thee to a summers day?

In the poem "I Compare Thee to a Summer's Day" by William Shakespeare, the poet uses a metaphor to compare his beloved to a summer day. The metaphor is that the beloved is like a summer day in the sense that she is beautiful, fair, and pleasant. The poet says that his beloved is more lovely and temperate than a summer day, and that her beauty is eternal and will not fade like a summer day.