Footnote 19
Part I, §17.  We may place under the Rete any plates we please.  If only the Mother be under it, without any plate, we may suppose the Mother marked as in fig. 2.  The plate or disc (tympanum) which was usually dropped in under the Rete is that shewn in fig. 5, Plate III, and which Chaucer now describes.  Any number of these, marked differently for different latitudes, could be provided for the Astrolabe.  The greatest declination of the sun measures the obliguity of the ecliptic, the true value of which is slightly variable, but was about 23° 31' in Chaucer's time, and about 23° 40' in the time of Ptolemy, who certainly assigns to it too large a value.  The value of it must be known before the three circles can be drawn.  The method of finding their relative magnitudes is very simple.  Let ABCD (fig 3, Plate IV) be the tropic of Capricorn, BO the South line, OC the West line.  Make the angle EOB equal to the obliquity (say 23½°), and join EA, meeting BO in F.  Then OF is the radius of the Equatorial circle, and if GH be drawn parallel to EF, OH is the radius of the Tropic of Cancer.  In the phrase angulus primi motus, angulus must be taken to mean angular motion.  The `first moving' (primus motus) has its name of `moving' (motus) from its denoting motion due to the primum mobile or `first moveable'.  This primum mobile (usually considered as the ninth sphere) causes the rotation of the eighth sphere, or sphoera stellarum fixarum.  See the fig. in MS. Univ. Camb. li. 3. 3 (copied in fig. 10, Plate IV).  Some authors make 12 heavens, viz. those of the 7 planets, the firmamentum (stellarum fixarum), the nonum cælum, decimum cælum, decimum cælum, primum mobile, and cælum empyræum.