Yep, you heard us. "Piedra de sol" is made up of 584 hendecasyllabic lines (that's the fancy pants way of saying that each line has eleven syllables) in the original Spanish (English translations vary). This meter comes from ancient Greek and Latin poetry, and was used in medieval and modern European poetry. In other words, it's super traditional. Think of it as the iambic pentameter of Romance languages.
Maybe Paz is trying to show the European tradition that is so important in Mexico, and that kind of took over the Aztec and other native cultures. He's putting ancient, indigenous content into a European context. The effect is kind of hypnotic, and also reminds the reader of the mix of cultures present in Mexico in the 20th century, when Paz wrote the poem.
We know, it can be hard to learn a meter without hearing and seeing it in action, so check out the first line of the poem in its original Spanish:
In Spanish the "de" and the first syllable of "agua" are smooshed together to create one super-syllable, so the final count is eleven syllables, with five stressed ones.
There are a few line breaks, which split the lines before they reach eleven syllables, but Paz keeps the meter going by indenting the next line. We count these broken lines as one line to give props to the form. The strict hendecasyllable also creates a lot of enjambment, which is when the line breaks in the middle of a clause. But that's no matter here, as the entire poem is one circular, ever-continuing sentence. Sure, the enjambment can be a bit jarring to read, but it forces the reader to continue—you can't just cut off a line and stop and come back later. The lines keep coming, because they're all connected, like the days and seasons of the year.
As a last note on form, check out those stanzas. Notice anything? They're of varying length, though the longest ones come at the end of the poem. You might say that Paz is mimicking the varying lengths of days through the year, with the seasons (after all, he does mention the seasons in order). Or you could argue that the poem builds to a kind of crescendo, with each stanza stretching a bit longer than the last as you get deeper into the poem. Whatever the case, this, along with the enjambment, are ways that the poem's structure gives us a clue as to what its meaning is. It's not just content, but also form, that can tell us what's going on.