§ 17. [6]. Ptholome. The St. John's MS. has ptolomeys almagest. 'Almagest, a name given by the Arabs to the μεγάλη σύνταξις, or great collection, the celebrated work of Ptolemy, the astronomer of Alexandria [floruit A.D. 140-160]. It was translated into Arabic about the year A.D. 827, under the patronage of the Caliph Al Mamun, by the Jew Alhazen ben Joseph, and the Christian Sergius. The word is the Arabic article al prefixed to the Greek megistus, "greatest," a name probably derived from the title of the work itself, or, as we may judge from the superlative adjective, partly from the estimation in which it was held.'—English Cyclopædia; Arts and Sciences, i. 223. The Almagest 'was in thirteen books. Ptolemy wrote also four books of judicial astrology. He was an Egyptian astrologist, and flourished under Marcus Antoninus. He is mentioned in the Sompnour's Tale [D 2289], and the Wif of Bathes Prologue, ll. 182, 324.'—Warton, Hist. E. P. ii. 356, ed. 1871. The word almagest occurs in the Milleres Tale, near the beginning (A 3208), and twice in the Wif of Bathes Prologue (D 183, 325).
Chaucer says the obliquity of the ecliptic, according to Ptolemy, was 23° 50′. The exact value, according to Ptolemy, was 23° 51′ 20″; Almagest, lib. i. c. 13. But Chaucer did not care about the odd degree, and gives it nearly enough. See note to ii. 25. 19.
[8]. tropos, a turning; Chaucer gives it the sense of agaynward, i.e. in a returning direction.
[14]. The equinoctial was supposed to revolve, because it was the 'girdle' of the primum mobile, and turned with it. See note below to l. 28.
[14, 15]. 'As I have shewed thee in the solid sphere.' This is interesting, as shewing that Chaucer had already given his son some lessons on the motions of the heavenly bodies, before writing this treatise.
[27]. angulus. We should rather have expected the word spera or sphera; cf. 'the sper solide' above, l. 15.
[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,