21. The Riet of thyn Astrolabie with thy zodiak, shapen in maner of a net or of a loppe-webbe after the olde descripcioun, which thow mayst tornen up and doun as thy-self lyketh, conteneth certein nombre of sterres fixes, with hir longitudes and latitudes determinat; yif so be that the makere have nat erred. The names 5 of the sterres ben writen in the margin of the riet ther as they sitte; of whiche sterres the smale poynt is cleped the Centre. And understond also that alle sterres sittinge with-in the zodiak of thyn Astrolabie ben cleped 'sterres of the north,' for they arysen by northe the est lyne. And alle the remenant fixed, out of the 10 zodiak, ben cleped 'sterres of the south;' but I sey nat that they arysen alle by southe the est lyne; witnesse on Aldeberan and Algomeysa. Generally understond this rewle, that thilke sterres that ben cleped sterres of the north arysen rather than the degree of hir longitude, and alle the sterres of the south arysen after the 15 degree of hir longitude; this is to seyn, sterres fixed in thyn Astrolabie. The mesure of this longitude of sterres is taken in the lyne ecliptik of hevene, under which lyne, whan that the sonne and the mone ben lyne-right or elles in the superfice of this lyne, than is the eclips of the sonne or of the mone; as I shal declare, 20 and eek the cause why. But sothly the Ecliptik Lyne of thy zodiak is the outtereste bordure of thy zodiak, ther the degrees ben marked.