The turn in line 8 is basically the after affect of October fading or the end of the season taking place. Before this element, the poem talks about how everything is coming to a end. The narrator states "As bleak November's iron skies descend"(2) also "we see our fruitful times approaching end"(4). However, after the turn everything came into action. A good example would be from lines 8 and 10 stating "Each Golden Age that flared has soon expired" and "Persepolis has crumbled back to dust'. This poem is overall about how time is passing fast and how we make a lot of history which will soon be our ancient history and how we should embrace our times as the days will become shorter.
Persepolis has crumbled back to dust.
Embrace what time remains; it will not last.
Be mindful of the present moment as it shall make history