Endnote 20
Part I, §17, l. 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 d!vided 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 the 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.'
That is -- `O thou primum mobile, thou cruel firmament, that with
thy diurnal revolution (or revolution once in 24 hours round the axis of
the equator) continually forcest along and whirlest all the celestial bodies
from East to West, which naturally would wish to follow the course
of the sun in the zodiac from West to East.' This is well illustrated
by a sidenote in the Ellesmere MS. to the passage in question, to this
effect: -- `Vnde Ptholomeus, 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.' (E20b)
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, a Arietis (E20c),
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. (E20d)
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.'