The Petrarchan or Italian sonnet is divided into two parts. The first part has eight lines and is called an octet. The second part has six lines and is called a sextet. The division between these two parts is called the turn, or volta, which sounds fancy until you realize that it's just the Italian word for "turn." This kind of sonnet is named after the Italian Renaissance poet Petrarch, who made the form popular throughout Europe.
This poem's "turn" at line 9 is about as dramatic an example of change as you can find. The turn is marked in the poetic equivalent of a huge neon sign – the word "But." This little three-letter word brings the entire vision of Judgment Day to a screeching halt. The poem is hurtling along with this vision, and then "but" shows up like a turtle waddling across the road, forcing the poem to slam on the brakes. In the second part of the poem, the speaker completely changes his mind about the whole project.
"At the round earth's imagined corners" has a typical rhyme scheme for a Petrarchan sonnet: ABBA ABBA CDCD EE. Then again, not all the rhymes are perfect, a case in point being "arise" and "infinities" in lines 2 and 3. But you try to make a rhyme out of "infinities"!
The poem's meter is iambic pentameter: an unstressed beat followed by a stressed beat. "'Tis late to ask a-bun-dance of thy grace." But, the poem has exceptions. Many exceptions. One of the most obvious can be found at the beginning: "At the round earth's […]." The poem has two unstressed beats followed by two stressed beats. The complicated rhythm makes this poem fun to listen to and read aloud.