This thesis has examined the moon and sun imagery of Yeats's Last Poems (1936-1939), contrasting the diminution and transmutations of astronomical imagery with its rich symbolic development from 1889 to 1936. Using lyric poetry of The Collected Works of W. B. Yeats and selected prose works of William Butler Yeats, the study has traced the multiple antinomial forces embodied in moon and sun and the change in imagery that marked Yeat's poetic growth. The thesis has discussed vestiges of overt moon and sun imagery in Last Poems, Yeats's reasons for the changes that characterize his career, and the substitutes that replaced moon and sun.