Chaucer's treatment of astronomical lore in his poetry differs much from his use of it in his prose writings. In poetical allusions to heavenly phenomena, much attention to detail and a pedantic regard for accuracy would be inappropriate. References to astronomy in Chaucer's poetry are, as a rule rather brief, specific but not technical, often purely conventional but always truly poetic. There are, indeed, occasional passages in Chaucer's poetry showing so detailed a knowledge of observational[50] astronomy that they would seem astonishing and, to many people, out of place, in modern poetry. They were not so in Chaucer's time, when the exigencies of practical life demanded of the ordinary man a knowledge of astronomy far surpassing that possessed by most of our contemporaries. Harry Bailly in the _Introduction to the Man of Lawes Tale_ determines the day of the month and hour of the day by making calculations from the observed position of the sun in the sky, and from the length of shadows, although, says Chaucer, "he were not depe expert in lore."[51] Such references to technical details of astronomy as we find in this passage are, however, not common in Chaucer's poetry; in his _Treatise on the Astrolabe_, on the other hand, a professedly scientific work designed to instruct his young son Louis in those elements of astronomy and astrology that were necessary for learning the use of the astrolabe, we have sufficient evidence that he was thoroughly familiar with the technical details of the astronomical science of his day. In Chaucer's poetry the astronomical references employed are almost wholly of two kinds: references showing the time of day or season of the year at which the events narrated are supposed to take place; and figurative allusions for purposes of illustration or comparison. Figurative uses of astronomy in Chaucer vary from simple similes as in the _Prologue to the Canterbury Tales_, where the friar's eyes are compared to twinkling stars[52] to extended allegories like the _Compleynt of Mars_ in which the myth of Venus and Mars is related by describing the motions of the planets Venus and Mars for a certain period during which Venus overtakes Mars, they are in conjunction[53] for a short time, and then Venus because of her greater apparent velocity leaves Mars behind. One of the most magnificent astronomical figures employed by Chaucer is in the _Hous of Fame_. Chaucer looks up into the heavens and sees a great golden eagle near the sun, a sight so splendid that men could never have beheld its equal 'unless the heaven had won another sun:' "Hit was of golde, and shone so bright, That never saw men such a sighte, But-if the heven hadde y-wonne Al newe of golde another sonne; So shoon the egles fethres brighte, And somwhat dounward gan hit lighte."[54]