In his biography--Shelley: the Pursuit--Richard Holmes claims that the moon became for Shelley an emblem for Mary (268). Although the emblem is viewed negatively here, during their courtship the moon was a positive signifier. Consider the following passage from Shelley's Letters:
Mary love--we must be united. I will not part from you again after Saturday night. We must devise some scheme. I must return. York thoughts alone can waken mine to energy. My mind without yours is dead & cold as the dark midnight river when the moon is down. It seems as if you alone could shield me from impurity and vice. If I were absent from you long I should shudder with horror at myself. My understanding becomes undisciplined without you. (qtd. in Holmes 268)
Holmes, Richard. Shelley: The Pursuit. London: Weidenfeld and Nicolson, 1974.