Ordinarily, when one uses the phrase "to shoulder", it means that they are taking on a burden of some kind, either literal or figurative. Therefore, it could suggest that the hill must bear the weight of the moon or, perhaps more interestingly, the responsibility for the passing of time. Given Fotheringham Cato's enduring interest in environmental and conservation issues, it could imply that nature has been overburdened by some force, possibly caused by human actions.