Footnote 20
Part I, §18.  See fig. 5, Plate III.  This is made upon the alt-azimuth system, and the plates are marked according to the latitude.  The circles, called in Latin circuli progressionum, in Arabic Almucantarat, are circles of altitude, the largest imperfect one representing the horizon (horizon obliquus), and the central dot being the zenith, or pole of the horizon.  In my figure, they are `compounded by' 5 and 5, but Chaucer's shewed every second degree, i.e. it possessed 45 such circles.  For the method of drawing them, see Stöffler, leaf 5, back.