"the further one goes, the deeper one sinks. It's because the earth keeps turning around, around, around, around"
The Old Man provides an answer for why it gets darker earlier in Part One. He and the Old Woman lead lives of repetition, of routines that never advance but cycle continuously around themselves, and his image of the earth's revolutions captures this revolving stasis. He also alludes to the death such repetition comes before. Their lives are devoid of novelty and, while their routines, such as storytelling and fantasy conversations, aim to stave off boredom and make life bearable. However, the repetitions actually grind them into a routine that approaches death, "the deeper one sinks," as into the ground, just as the earth's revolutions, in the Old Man's mind, increase the darkness.