Why are there so many celestial allusions in the Canterbury Tales? In this revealing work, Marijane Osborn brings insight to Chaucer's celestial references by exploring his fascination with the astrolabe ("star-catcher"), a medieval device used for calculating the position of celestial bodies. Osborn suggests that Chaucer took his symbolic structure for the famous pilgrimage story from the functions of the astrolabe, which calculates time in relation to celestial movements.While writing the Tales, Chaucer also wrote A Treatise on the Astrolabe, the oldest work on a scientific instrument in the English language. Osborn describes Chaucer's treatise so that a careful reader might learn about the astrolabe's functions.