28. 'And observe, that this first moving (_primus motus_) is so called from the first movable (_primum mobile_) of the eighth sphere, which moving or motion is from East to West,' &c. There is an _apparent_ confusion in this, because the _primum mobile_ was the _ninth_ sphere (see Plate V, fig. 10); but it may be called the movable of the eighth, as _giving motion to it_. An attempt was made to explain the movements of the heavenly bodies by imagining the earth to be in the centre, surrounded by a series of concentric spheres, or rather shells, like the coats of an onion. Of these the seven innermost, all revolving with different velocities, each carried with it a planet. Beyond these was an eighth sphere, which was at first supposed to be divided into two parts, the inner part being the _firmamentum_, and the outer part the _primum mobile_; hence the _primum mobile_ might have been called 'the first moving of the eighth sphere,' as accounting for the more important part of the motion of the said sphere. It is simpler, however, to make these distinct, in which case the eighth sphere is _firmamentum_ or _sphæra stellarum fixarum_, which was supposed to have a very slow motion from West to East round the poles of the _zodiac_ to account for the precession of the equinoxes, whilst the ninth sphere, or _primum mobile_, whirled round from East to West once in 24 hours, carrying all the inner spheres with it, by which means the ancients accounted for the diurnal revolution. This ninth sphere had for its poles the north and south poles of the heavens, and its 'girdle' (or great circle equidistant from the poles) was the equator itself. Hence the equator is here called the 'girdle of the first moving.' As the planetary spheres revolved _in an opposite direction_, thus accounting for the _forward_ motion of the sun and planets in the ecliptic or near it, the _primum mobile_ was considered to revolve in a _backward_ or _unnatural_ direction, and hence Chaucer's apostrophe to it (Man of Lawes Tale, B 295):-- 'O firste moevyng cruel firmament, With thy diurnal sweigh that crowdest ay And hurlest all from Est til Occident, That naturelly wolde holde another way.'