Understanding Chaucer's instruction on using an astrolabe may be a little confusing, but Silfren, who posted the link, offers a suggestion: build your own. Here's how: http://cse.ssl.berkeley.edu/AtHomeAstronomy/activity_07.html Or, for a history of navigational instruments including the sextant, try this site: http://www.mat.uc.pt/~helios/Mestre/Novemb00/H61iflan.htm
Silfren adds other sources:
While hunting info about the astrolabe, I came across http://users.rcn.com/detroyes/astrolabe/intro.html which is a useful adjunct to the Chaucer piece. It has a list of different manuscripts and errors in them and some discussion of the astronomy.
http://www.math.nus.edu.sg/aslaksen/teaching/hm/projects/astronomy_and_a... is another fun one; it explains the astronomy then talks about astronomical and astrological allusions in the Canterbury Tales.