John Somer was one of the leading English astronomers of the late fourteenth century. Geoffrey Chaucer likely consulted Somer's Kalendarium to relate dates, times, and movements of the stars and planets to events in his tales. In her introduction to this scholarly edition, Linne Mooney discusses not only Somer's importance but also Chaucer's use of the Kalendarium in composing his texts from The Parliament of Fowls through The Canterbury Tales. She examines the thirty-three complete and nine fragmentary copies of the work known today and explains Somer's innovative and influential eclipse tables, adopted by some scribes in later copies of the Kalendarium of Nicholas of Lynn, a contemporary of Somer's. Somer's Kalendarium itself is presented in the original Latin text with English translation on facing pages. Mooney also provides full textual apparatus for the eleven complete manuscripts closest to the base text.