Reading the essays in Owen Gingerich’s The Eye of Heaven* I ran across an off-hand mention of “Chaucer’s Treatise on the Astrolabe” that I just had to investigate. It turns out that Geoffrey Chaucer (Yes, the Canterbury Tales Geoffrey Chaucer) was the author of an incomplete essay on how to use the astrolabe, addressed to his son (or godson) Lewis.
Although Chaucer never finished the manuscript, the parts he did write explain how to use the astrolabe to locate fixed stars and planets in the night sky, and to determine the time of day and the moment of dawn, dusk, noon, etc. base on the motions of the sun. Together with the various astrological references in Chaucer’s other work, the Treatise on the Astrolabe demonstrates his interest in astronomy.
You can read the text of the Treatise in the Internet Medieval Sourcebook at Fordham, and there’s a wonderful website from St. John’s College, Cambridge (home of the manuscript) that not only has images of the Treatise but also instructions on how to build your own working astrolabe.
*It’s terribly off topic, but Gingerich’s The Book Nobody Read is one of the best history of science detective story–travelogues in existence.