libro i

. cap. 8. Primi motus celi duo sunt, quorum vnus est qui mouet totum semper ab Oriente in Occidentem vno modo super orbes, &c. Item aliter vero motus est qui mouet orbem stellarum currencium contra motum primum, videlicet, ab Occidente in Orientum super alios duos polos[72].' That is, the two chief motions are that of the _primum mobile_, which carries everything round from East to West, and that of the fixed stars, which is a slow motion from West to East round the axis of the zodiac, to account for precession. This exactly explains the well-known passage in the Frankeleines Tale (C. T., F 1280):--

'And by his eighte spere in his werking, He knew ful wel how fer Alnath was shove Fro the heed of thilke fixe Aries above That in the ninthe spere considered is.'

Here the eight spheres are the eight inner spheres which revolve round the axis of the zodiac in an easterly direction, whilst the ninth sphere, or _primum mobile_, contained both the theoretical or _fixed_ first point of Aries from which measurements were made, and also the _signs_ of the zodiac as distinct from the _constellations_. But Alnath, being an actual star, viz, [alpha] Arietis[73], was in the _eighth_ sphere; and the distance between its position and that of the first point of Aries at any time afforded a measure of the amount of precession. Mr. Brae rightly remarks that Tyrwhitt's readings in this passage are correct (except that _eighte speres_ should be _eightespere_), and those of Mr. Wright and Dr. Morris (from the Harleian MS.) are incorrect.

It may be as well to add that a later refinement was to insert a crystalline sphere, to account for the precession; so that the order stood thus: seven spheres of planets; the eighth, of fixed stars; the ninth, or crystalline; the tenth, or _primum mobile_; and, beyond these, an empyræan or theological heaven, so to speak, due to no astronomical wants, but used to express the place of residence of celestial beings[74]. Hence the passage in Milton, P. L. iii. 481:--

'They pass the planets seven, and pass the fix'd, And that crystalline sphere whose balance weighs The trepidation talk'd, and that first mov'd.'

i.e. They pass the seven planetary spheres; then the sphere of fixed stars; then the crystalline or transparent one, whose swaying motion or libration measures the amount of the precession and nutation so often talked of; and then, the sphere of the _primum mobile_ itself. But Milton clearly himself believed in the Copernican system; see Paradise Lost, viii. 121-140, where the _primum mobile_ is described in the lines--

'that swift Nocturnal and diurnal rhomb supposed, Invisible else above all stars, the wheel Of day and night.'

§ 18. 8. _compowned by 2 & 2._ This means that in the _best_ astrolabes, _every_ almicantarath for every degree of latitude was marked; as may be seen in Metius. In others, including the one given by Chaucer to his son, they were marked only for every other degree. See