The "cold chaste moon" may be a referent to Mary Shelley and other women whom Shelley had been involved with. Richard Holmes provides excerpts from a letter written by Emilia Viviani to Mary Shelley, and it is interesting to note that Emilia describes Mary as being "cold." Holmes begins:
Some of Emilia's letters, both to Mary and to Shelley during these weeks, have survived. They show perfectly Emilia's mixture of naivete and charming adolescent cunning in dealing with Mary, notably in a letter of the end of December: "You seem to me a little cold, on some occasions, and this makes me a little nervous of you [un po' di soggezonie]; but I realize that your Husband speaks justly, and that your apparent coldness is only the ash which covers a radiant heart [un cuore affettuoso]." (630)
Holmes, Richard. Shelley: The Pursuit. London: Weidenfeld and Nicolson, 1974.