Arguments of those denying the Earth's motion, and
_their confutation._
Now it will not be superfluous to weigh well the arguments of those who say
the Earth does not move; that we may be better able to satisfy the crowd of
philosophizers who assert that this constancy and stability of the Earth is
confirmed by the most convincing arguments. Aristotle does not allow that
the Earth moves circularly, on the ground that each several part of it
would be affected by this particular motion; that whereas now all the
separate parts of the Earth are borne toward the middle in straight lines,
that circular motion would be violent, and strange to nature, and not
enduring. But it has been before proved that all actual portions of the
Earth move in a circle, and that all magnetick bodies (fitly disposed) are
borne around in an orbe. They are borne, however, toward the centre of the
Earth in a {226} straight line (if the way be open) by a motion of
aggregation as though to their own origin: they move by various motions
agreeably to the conformation of the whole: a terrella is moved circularly
by its innate forces. "Besides" (says he), "all things which are borne in
an orbe, afterwards would seem to be abandoned by the first motion, and to
be borne by several motions besides the first. The Earth must also be borne
on by two sorts of motion, whether it be situate around a mid-point, or in
the middle site of the universe: and if this were so, there must needs be
at one time an advance, at another time a retrogression of the fixed stars:
This, however, does not seem to be the case, but they rise and set always
the same in the same places." But it by no means follows that a double
motion must be assigned to the Earth. But if there be but one diurnal
motion of the Earth around its poles, who does not see that the stars must
always in the same manner rise and set at the same points of the horizon,
even although there be another motion about which we are not disputing:
since the mutations in the smaller orbit cause no variation of aspect in
the fixed stars owing to their great distance, unless the axis of the Earth
have varied its position, concerning which we raise a question when
speaking of the cause of the præcession of the æquinoxes. In this argument
are many flaws. For if the Earth revolve, that we asserted must needs occur
not by reason of the first sphære, but of its innate forces. But if it were
set in motion by the first sphære, there would be no successions of days
and nights, for it would continue its course along with the _Primum
Mobile_. But that the Earth is affected by a double movement at the time
when it rotates around its own centre, because the rest of the stars move
with a double motion, does not follow. Besides, he does not well consider
the argument, nor do his interpreters understand the same. [Greek: toutou
de sumbainontos, anankaion gignesthai parodous kai tropas tôn endedemenôn
astrôn.] (Arist. _de Coelo_, ii. chap. 14.) That is, "If this be so, there
must needs be changes, and retrogressions of the fixed stars." What some
interpret as retrogressions or regressions, and changes of the fixed stars,
others explain as diversions: which terms can in no way be understood of
axial motion, unless he meant that the Earth moved by the _Primum Mobile_
is borne and turned over other poles diverse even from those which
correspond to the first sphære, which is altogether absurd. Other later
theorists suppose that the eastern ocean ought to be impelled so into
western regions by that motion, that those parts of the Earth which are dry
and free from water would be daily flooded by the eastern ocean. But the
ocean is not acted upon by that movement, since nothing opposes it; and
even the whole atmosphere is carried round: And for that reason in the
Earth's course all the things in the air are not left behind by us nor do
they seem to move toward the West: Wherefore also the clouds {227} are at
rest in the air, unless the force of the winds drive them; and objects
which are projected into the air fall again into their own place. But those
foolish folk who think that towers, temples, and buildings must necessarily
be shaken and overthrown by the Earth's motion, may fear lest men at the
Antipodes should slip off into an opposite orbe, or that ships when sailing
round the entire [249]globe should (as soon as they have dipped under the
plane of our horizon) fall into the opposite region of the sky. But those
follies are old wives' gossip, and the rubbish of certain philosophizers,
men who, when they essay to treat of the highest truths and the fabrick of
the universe, and hazard anything, can scarce understand aught _ultra
crepidam_. They would have the Earth to be the centre of a circle; and
therefore to rest motionless amid the rotation. But neither the stars nor
the wandering globes move about the Earth's centre: the high heaven also
does not move circularly round the Earth's centre; nor if the Earth were in
the centre, is it a centre itself, but a body around a centre. Nor is it
confident with reason that the heavenly bodies of the Peripateticks should
attend on a centre so decadent and perishable as that of the Earth. They
think that Nature seeks rest for the generation of things, and for
promoting their increase while growing; and that accordingly the whole
Earth is at rest. And yet all generation takes place from motion, without
which the universal nature of things would become torpid. The motion of the
Sun, the motion of the Moon, cause changes; the motion of the Earth awakens
the internal breath of the globe; animals themselves do not live without
motion, and the ceaseless activity of the heart and arteries. For of no
moment are the arguments for a simple straight motion toward the centre,
that this is the only kind in the Earth, and that in a simple body there is
one motion only and that a simple one. For that straight motion is only a
tendency toward their own origin, not of the parts of the Earth only, but
of those of the Sun also, of the Moon, and of the rest of the sphæres which
also move in an orbit. Joannes Costæus, who raises doubts concerning the
cause of the Earth's motion, looking for it externally and internally,
understands magnetick vigour to be internal, active, and disponent; also
that the Sun is an external promotive cause, and that the Earth is not so
vile and abject a body as it is generally considered. Accordingly there is
a diurnal movement on the part of the Earth for its own sake and for its
advantage. Those who make out that that terrestrial motion (if such there
be) takes place not only in longitude, but also in latitude, talk nonsense.
For Nature has set in the Earth determinate poles, and definite unconfused
revolutions. Thus the Moon revolves with respect to the Sun in a monthly
course; yet having her own definite poles, facing determinate parts of the
heaven. To suppose that the air moves the Earth would be {228} ridiculous.
For air is only exhalation, and is an enveloping effluvium from the Earth
itself; the winds also are only a rush of the exhalations in some part near
the Earth's surface; the height of its motion is slight, and in all regions
there are various winds unlike and contrary. Some writers, not finding in
the matter of the Earth the cause (for they say that they find nothing
except solidity and consistency), deny it to be in its form; and they only
admit as qualities of the Earth cold and dryness, which are unable to move
the Earth. The Stoicks attribute a soul to the Earth, whence they pronounce
(amid the laughter of the learned) the Earth to be an animal. This
magnetick form, whether vigour or soul, is astral. Let the learned lament
and bewail the fact that none of those old Peripateticks, nor even those
common philosophizers heretofore, nor Joannes Costæus, who mocks at such
things, were able to apprehend this grand and important natural fact. But
as to the notion that surface inequality of mountains and valleys would
prevent the Earth's diurnal revolution, there is nothing in it: for they do
not mar the Earth's roundness, being but slight excrescences compared with
the whole Earth; nor does the Earth revolve alone without its emanations.
Beyond the emanations, there is no renitency. There is no more labour
exerted in the Earth's motion than in the march of the rest of the Stars:
nor is it excelled in dignity by some stars. To say that it is frivolous to
suppose that the Earth rather seeks a view of the Sun, than the Sun of the
Earth, is a mark of great obstinacy and unwisdom. Of the theory of the
rotation we have often spoken. If anyone seek the cause of the revolution,
or of other tendency of the Earth, from the sea surrounding it, or from the
motion of the air, or from the Earth's gravity, he would be no less silly
as a theorist than those who stubbornly ground their opinions on the
sentiments of the ancients. Ptolemy's reasonings are of no weight; for when
our true principles are laid down, the truth comes to light, and it is
superfluous to refute them. Let Costæus recognize and philosophers see how
unfruitful and vain a thing it becomes then to take one's stand on the
principles and unproved opinions of certain ancients. Some raise a doubt
how it can be that, if the Earth move round its own axis, a globe of iron
or of lead dropped from the highest point of a tower falls exactly
perpendicularly to a spot of the Earth below itself. Also how it is that
cannon balls from a large culverin, fired with the same quantity and
strength of powder, in the same direction and at a like elevation through
the same air, would be cast at a like distance from a given spot both
Eastward and Westward, supposing the Earth to move Eastward. But those who
bring forward this kind of argument are being misled: not attending to the
nature of primary globes, and the combination of parts with their globes,
even though they be not adjoined by solid parts. Whereas the motion of the
Earth in the diurnal revolution does not involve the separation of her more
{229} solid circumference from the surrounding bodies; but all her effluvia
surround her, and in them heavy bodies projected in any way by force, move
on uniformly along with the Earth in general coherence. And this also takes
place in all primary bodies, the Sun, the Moon, the Earth, the parts
betaking themselves to their first origins and sources, with which they
connect themselves with the same appetence as terrene things, which we call
heavy, with the Earth. So lunar things tend to the Moon, solar things to
the Sun, within the orbes of their own effluvia. The emanations hold
together by continuity of substance, and heavy bodies are also united with
the Earth by their own gravity, and move on together in the general motion:
especially when there is no renitency of bodies in the way. And for this
cause, on account of the Earth's diurnal revolution, bodies are neither set
in motion, nor retarded; they do not overtake it, nor do they fall short
behind it when violently projected toward East or West.
[Illustration]
Let E F G be the Earth's globe, A its centre, L E the ascending effluvia:
Just as the orbe of the effluvia progresses with the Earth, so also does
the unmoved part of the circle at the straight line L E progress along with
the general revolution. At L and E, a heavy body, M, falls perpendicularly
toward E, taking the shortest way to the centre, nor is that right movement
of weight, or of aggregation compounded with a circular movement, but is a
simple right motion, never leaving the line L E. But when thrown with an
equal force from E toward F, and from E toward G, it completes an equal
distance on either side, even though the daily rotation of the Earth is in
process: just as twenty paces of a man mark an equal space whether toward
East or West: so the Earth's diurnal motion {230} is by no means refuted by
the illustrious Tycho Brahe, through arguments such as these.
[Illustration] The tendency toward its origin (which, in the case of the
Earth, is called by Philosophers weight) causes no resistance to the
diurnal revolution, nor does it direct the Earth, nor does it retain the
parts of the Earth in place, for in regard to the Earth's solidity they are
imponderous, nor do they incline further, but are at rest in the mass. If
there be a flaw in the mass, such as a deep cavity (say 1000 fathoms), a
homogenic portion of the Earth, or compacted terrestrial matter, descends
through that space (whether filled with water or air) toward an origin more
assured than air or water, seeking a solid globe. But the centre of the
Earth, as also the Earth as a whole, is imponderous; the separated parts
tend toward their own origin, but that tendency we call weight; the parts
united are at rest; and even if they were ponderable, they would introduce
no hindrance to the diurnal revolution. For if around the axis A B, there
be a weight at C, it is balanced from E; if at F, from G; if at H, from I.
So internally at L, they are balanced from M: the whole globe, then, having
a natural axis, is balanced in æquilibrio, and is easily set in motion by
the slighted cause, but especially because the Earth in her own place is
nowise heavy nor lacking in balance. Therefore weight neither hinders the
diurnal revolution, nor influences either the direction or continuance in
position. Wherefore it is manifest that no sufficiently strong reason has
yet been found out by Philosophers against the motion of the Earth.
* * * * *
{231} CHAP. VI.
On the cause of the definite time, of an entire
_rotation of the Earth._
Diurnal motion is due to causes which have now to be sought, arising from
magnetick vigour and from the confederated bodies; that is to say, why the
diurnal rotation of the Earth is completed in the space of twenty-four
hours. For no curious art, whether of Clepsydras or of sand-clocks, or
those contrivances of little toothed wheels which are set in motion by
weights, or by the force of a bent steel band, can discover any degree of
difference in the time. But as soon as the diurnal rotation has been gone
through, it at once begins over again. But we would take as the day the
absolute turning of a meridian of the Earth, from sun to sun. This is
somewhat greater than one whole revolution of it; in this way the yearly
course is completed in 365 and nearly ¼ turnings with respect to the sun.
From this sure and regular motion of the Earth, the number and time of 365
days, 5 hours, 55 minutes, in solar tropical years is always certain and
definite, except that there are some slight differences due to other
causes. The Earth therefore revolves not fortuitously, or by chance, or
precipitately; but with a rather high intelligence, equably, and with a
wondrous regularity, in no other way than all the rest of the movable
stars, which have definite periods belonging to their motions. For the Sun
himself being the agent and incitor of the universe in motion, other
wandering globes set within the range of his forces, when acted on and
stirred, also regulate each its own proper courses by its own forces; and
they are turned about in periods corresponding to the extent of their
greater rotation, and the differences of their effused forces, and their
intelligence for higher good. And for that cause Saturn, having a wider
orbit, is borne round it in a longer time, Jupiter a shorter, and Mars
still less; while Venus takes nine months, Mercury 80 days, on the
hypotheses of Copernicus; the Moon going round the Earth with respect to
the Sun in 29 days, 12 hours, 44 minutes. We have asserted that the Earth
moves circularly about its centre, completing a day by an entire revolution
with respect to the Sun. The Moon revolves in a monthly course around the
Earth, and, repeating a conjunction with the Sun after a former synodic
conjunction, constitutes the month or Lunar day. The Moon's mean
concentrick orbit, according to numerous observations of Copernicus and
later astronomers, is found to be distant 29 and about 5/6 diameters of the
Earth from the Earth's centre. The Moon's revolution with respect to the
Sun takes place in 29½ days and 44 minutes of time. We reckon the motion
with respect to the sun, not the periodic motion, {232} just as a day is
one entire revolution of the Earth with respect to the Sun, not one
periodick revolution; because the Sun is the cause of lunar as of
terrestrial motion: also, because (on the hypotheses of later observers)
the synodical month is truly periodic, on account of the Earth's motion in
a great orbit. The proportion of diameters to circumferences is the same.
And the concentrick orbit of the Moon contains twice over 29 and ½ great
circles of the Earth & a little more. The Moon & the Earth, then, agree
together in a double proportion of motion; & the Earth moves in the space
of twenty-four hours, in its diurnal motion; because the Moon has a motion
proportional to the Earth, but the Earth a motion agreeing with the lunar
motion in a nearly double proportion. There is some difference in details,
because the distances of the stars in details have not been examined
sufficiently exactly, nor are mathematicians as yet agreed about them. The
Earth therefore revolves in a space of 24 hours, as the Moon in her monthly
course, by a magnetick confederation of both stars, the globes being
forwarded in their movement by the Sun, according to the proportion of
their orbits, as Aristotle allows, _de Coelo_, bk. ii., chap. 10. "It
happens" (he says) "that the motions are performed through a proportion
existing between them severally, namely, at the same intervals in which
some are swifter, others slower," But it is more agreeable to the relation
between the Moon and the Earth, that that harmony of motion should be due
to the fact that they are bodies rather near together, and very like each
other in nature and substance, and that the Moon has more evident effects
upon the Earth than the rest of the stars, the Sun excepted; also because
the Moon alone of all the planets conducts her revolutions, directly
(however diverse even), with reference to the Earth's centre, and is
especially akin to the Earth, and bound to it as with chains. This, then,
is the true symmetry and harmony between the motions of the Earth and the
Moon; not that old oft-besung harmony of coelestial motions, which assumes
that the nearer any sphære is to the _Primum Mobile_ and that fictitious
and pretended rapidest Prime Motion, the less does it offer resistance
thereto, and the slower it is borne by its own motion from west to east:
but that the more remote it is, the greater is its velocity, and the more
freely does it complete its own movement; and therefore that the Moon
(being at the greatest distance from the _Primum Mobile_) revolves the most
swiftly. Those vain tales have been conceded in order that the _Primum
Mobile_ may be accepted, and be thought to have certain effects in
retarding the motions of the lower heavens; as though the motion of the
stars arose from retardation, and were not inherent and natural; and as
though a furious force were perpetually driving the rest of the heaven
(except only the _Primum Mobile_) with frenzied incitations. Much more
likely is it that the stars are borne around symmetrically by their own
forces, with a certain mutual concert and harmony.
* * * * *
{233} CHAP. VII.
On the primary magnetick nature of the Earth,
whereby its poles are parted from the poles
_of the Ecliptick._
Primarily having shown the manner and causes of the diurnal revolution of
the Earth, which is partly brought about from the vigour of the magnetick
virtue, partly effected by the præ-eminence and light of the Sun; there now
follows an account of the distance of its poles from the poles of the
Ecliptick--a supremely necessary fact. For if the poles of the universe or
of the Earth remained fast at the poles of the Zodiack, then the Æquator of
the Earth would lie exactly beneath the line of the Ecliptick, and there
would be no variation in the seasons of the year, no Winter, no Summer, nor
Spring, nor Autumn: but one and the same invariable aspect of things would
continue. The direction of the axis of the Earth has receded therefore from
the pole of the Zodiack (for lasting good) just so far as is sufficient for
the generation and variety of things. Accordingly the declination of the
tropicks and the inclination of the Earth's pole remain perpetually in the
twenty-fourth degree; though now only 23 degrees 28 minutes are counted;
or, as others make out, 29 minutes: But once it was 23 degrees 52 minutes,
which are the extreme limits of the declinations hitherto observed. And
that has been prudently ordained by nature, and is arranged by the primary
excellence of the Earth. For if those poles (of the Earth and the
Ecliptick) were to be parted by a much greater distance, then when the Sun
approached the tropick, all things in the other deserted part of the globe,
in some higher latitude, would be desolate and (by reason of the too
prolonged absence of the Sun) brought to destruction. As it is, however,
all is so proportioned that the whole terrestrial globe has its own varying
seasons in succession, and alternations of condition, appropriate and
needful: either from the more direct and vertical radiation of light, or
from its increased tarriance above the horizon.
Around these poles of the Ecliptick the direction of the poles of the Earth
is borne: and by this motion the præcession of the æquinoxes is apparent to
us.
* * * * *
{234} CHAP. VIII.
On the Præcession of the Æquinoxes, from the magnetick
motion of the poles of the Earth, in the Arctick
_and Antarctick circle of the Zodiack._
Primitive mathematicians, since they did not pay attention to the
inequælities of the years, made no distinction between the æquinoctial, or
solstitial revolving year, and that which is taken from some one of the
fixed stars. Even the Olympick years, which they used to reckon from the
rising of the dogstar, they thought to be the same as those counted from
the solstice. Hipparchus of Rhodes was the first to call attention to the
fact that these differ from each other, and discovered that the year was
longer when measured by the fixed stars than by the æquinox or solstice:
whence he supposed that there was in the fixed stars also some motion in a
common sequence; but very slow, and not at once perceptible. After him
Menelaus, a Roman geometer, then Ptolemy, and long afterward Mahometes
Aractensis, and several more, in all their literary memoirs, perceived that
the fixed stars and the whole firmament proceeded in an orderly sequence,
regarding as they did the heaven, not the earth, and not understanding the
magnetical inclinations. But we shall demomstrate that it proceeds rather
from a certain rotatory motion of the Earth's axis, than that that eighth
sphære (so called) the firmament, or non-moving empyrean, revolves studded
with innumerable globes and stars, whose distances from the Earth have
never been proved by anyone, nor can be proved (the whole universe gliding,
as it were). And surely it should seem much more likely that the
appearances in the heavens should be clearly accounted for by a certain
inflection and inclination of the comparatively small body of the Earth,
than by the setting in motion of the whole system of the universe;
especially if this motion is to be regarded as ordained solely for the
Earth's advantage: While for the fixed stars, or for the planets, it is of
no use at all. For this motion the rising and settings of stars in every
Horizon, as well as their culminations at the height of the heavens, are
shifted so much that the stars which once were vertical are now some
degrees distant from the zenith. For nature has taken care, through the
Earth's soul or magnetick vigour, that, just as it was needful in
tempering, receiving, and warding off the sun's rays and light, by suitable
seasons, that the points toward which the Earth's pole is directed should
be 23 degrees and more {235} from the poles of the Ecliptick[250]: so now
for moderating and for receiving the luminous rays of the fixed stars in
due turn and succession, the Earth's poles should revolve at the same
distance from the Ecliptick at the Ecliptick's arctick circle; or rather
that they should creep at a gentle pace, that the actions of the stars
should not always remain at the same parallel circles, but should have a
rather slow mutation. For the influences of the stars are not so forceful
as that a swifter course should be desired. Slowly, then, is the Earth's
axis inflected; and the stars' rays, falling upon the face of the Earth,
shift only in so long a time as a diameter of the arctick or polar circle
is extended: whence the star at the extremity of the tail of the Cynosure,
which once was 12 degrees 24 minutes (namely, in the time of Hipparchus)
distant from the pole of the universe, or from that point which the pole of
the Earth used to face, is now only 2 degrees and 52 minutes distant from
the same point; whence from its nearness it is called by the moderns
_Polaris._ Some time it will be only ½ degree away from the pole: afterward
it will begin to recede from the pole until it will be 48 degrees distant;
and this, according to the Prutenical tables, will be in Anno Domini 15000.
Thus _Lucida Lyræ_ (which to us southern Britons now almost culminates)
will some time approach to the pole of the world, to about the fifth
degree. So all the stars shift their rays of light at the surface of the
Earth, through this wonderful magnetical inflection of the Earth's axis.
Hence come new varieties of the seasons of the year, and lands become more
fruitful or more barren; hence the characters and manners of nations are
changed; kingdoms and laws are altered, in accordance with the virtue of
the fixed stars as they culminate, and the strength thence received or lost
in accordance with the singular and specifick nature of each; or on account
of new configurations with the planets in other places of the Zodiack; on
account also of risings and settings, and of new concurrences at the
meridian. The Præcession of the æquinoxes arising from the aequable motion
of the Earth's pole in the arctick circle of the Zodiack is here
demonstrated. Let A B C D be the Ecliptick line; I E G the arctic circle of
the Zodiack. Then if the Earth's pole look to E, the æquinoxes are at D, C.
Let this be at the time of Metho, when the horns of Aries were in the
æquinoctial colure. Now if the Earth's pole have advanced to I; then the
æquinoxes will be at K, L; and the stars in the ecliptick C will seem to
have progressed, in the order of the signs, along the whole arc K C: L will
be moved on by the præcession, against the order of the signs, along the
arc D L. But this would occur in the contrary order, if the point G were to
face the poles of the earth, and the motion were from E to G: for then the
æquinoxes would be M N, and the fixed stars would anticipate the same at C
and D, counter to the order of the signs.
[Illustration]
* * * * *
{236} CHAP. IX.
On the anomaly of the Præcession of the Equinoxes,
_and of the obliquity of the Zodiack._
At one time the shifting of the æquinoxes is quicker, at another slower,
being not always equal: because the poles of the earth travel unequally in
the arctick and antarctick circle of the Zodiack; and decline on both sides
from the middle path: whence the obliquity of the Zodiack to the Æquator
seems to change. And as this has become known by means of long
observations, so also has it been perceived, that the true æquinoctial
points have been elongated from the mean æquinoctial points, on this side
and on that, by 70 minutes (when the prostaphæresis is greatest): but that
the solstices either approach the equator unequally 12 minutes nearer, or
recede as far behind; so that the nearest approach is 23 degrees 28
minutes, and the greatest elongation 23 degrees 52 minutes. Astronomers
have given various explanations to account for this inequality of the
præcession and also of the obliquity of the tropicks. Thebit, with the view
of {237} laying down a rule for such considerable inequalities in the
motion of the stars, explained that the eighth sphære does not move with a
continuous motion from west to east; but is shaken with a certain motion of
trepidation, by which the first points of Aries and Libra in the eighth
heaven describe certain small circles with diameters equal to about nine
degrees, around the first points of Aries and Libra in the ninth sphære.
But since many things absurd and impossible as to motion follow from this
motion of trepidation, that theory of motion is therefore long since
obsolete. Others therefore are compelled to attribute the motion to the
eighth sphære, and to erect above it a ninth heaven also, yea, and to pile
up yet a tenth and an eleventh: In the case of mathematicians, indeed, the
fault may be condoned; for it is permissible for them, in the case of
difficult motions, to lay down some rule and law of equality by any
hypotheses. But by no means can such enormous and monstrous celestial
structures be accepted by philosophers. And yet here one may see how hard
to please are those who do not allow any motion to one very small body, the
Earth; and notwithstanding they drive and rotate the heavens, which are
huge and immense above all conception and imagination: I declare that they
feign the heavens to be three (the most monstrous of all things in Nature)
in order that some obscure motions forsooth[251] may be accounted for.
Ptolemy, who compares with his own the observations of Timocharis and
Hipparchus, one of whom flourished 260 years, the other 460 years before
him, thought that there was this motion of the eighth sphære, and of the
whole firmament; and proved by help of numerous phenomena that it took
place over the poles of the Zodiack, and, supposing its motion to be so far
æquable, that the non-planetary stars in the space of 100 years completed
just one degree beneath the _Primum Mobile_. After him 750 years
Albategnius discovered that one degree was completed in a space of 66
years, so that a whole period would be 23,760 years. Alphonsus made out
that this motion was still slower, completing one degree and 28 minutes
only in 200 years; and that thus the course of the fixed stars went on,
though unequally. At length Copernicus, by means of the observations of
Timocharis, Aristarchus of Samos, Hipparchus, Menelaus, Ptolemy, Mahometes
Aractensis, Alphonsus, and of his own, detected the anomalies of the motion
of the Earth's axis: though I doubt not that other anomalies also will come
to light some ages hence. So difficult is it to observe motion so slow,
unless extending over a period of many centuries; on which account we still
fail to understand the intent of Nature, what she is driving after through
such inequality of motion. Let A be the pole of the Ecliptick, B C the
Ecliptick, D the Æquator; when the pole of the Earth near the arctick
circle of the Zodiack faces the point M, then there is an anomaly of the
præcession of the æquinox at F; {238} but when it faces N, there is an
anomaly of the præcession at E. But when it faces I directly, then the
maximum obliquity G is observed at the solstitial colure; but when it faces
L, there is the minimum obliquity H at the solstitial colure.
[Illustration]
_Copernicus' contorted circlet in the Arctick circle of the Zodiack._
Let F B G be the half of the Arctick circle described round the pole of the
Zodiack: A B C the solstitial colure: A the pole of the Zodiack; D E the
anomaly of longitude 140 minutes at either side on both ends: B C the
anomaly of obliquity 24 minutes: B the greater obliquity of 23 degrees 52
minutes: D the mean obliquity of 23 degrees 40 minutes: C the minimum
obliquity of 23 degrees 28 minutes.
{239} [Illustration]
[Illustration]
{240} The period of motion of the præcession of the æquinoxes is 25,816
Ægyptian years; the period of the obliquity of the Zodiack is 3434 years,
and a little more. The period of the anomaly of the præcession of the
æquinoxes is 1717 years, and a little more. If the whole time of the motion
AI were divided into eight equal parts: in the first eighth the pole is
borne somewhat swiftly from A to B; in the second eighth, more slowly from
B to C; in the third, with the same slowness from C to D; in the fourth,
more swiftly again from D to E; in the fifth, with the same swiftness from
E to F; again more slowly from F to G; and with the same slowness from G to
H; in the last eighth, somewhat swiftly again from H to I. And this is the
contorted circlet of Copernicus, fused with the mean motion into the curved
line which is the path of the true motion. And thus the pole attains the
period of the anomaly of the præcession of the æquinoxes twice; and that of
the declination or obliquity once only. It is thus that by later
astronomers, but especially by Copernicus (the Restorer of Astronomy)[252],
the anomalies of the motion of the Earth's axis are described, so far as
the observations of the ancients down to our own times admit; but there are
still needed more and exact observations for anyone to establish aught
certain about the anomaly of the motion of the præcessions, and at the same
time that also of the obliquity of the Zodiack. For ever since the time at
which, by means of various observations, this anomaly was first observed,
we have only arrived at half a period of the obliquity. So that all the
more all these matters about the unequal motion both of the præcession and
of the obliquity are uncertain and not well known: wherefore neither can we
ourselves assign any natural causes for it, and establish it for certain.
Wherefore also do we to our reasonings and experiments magnetical here set
an end and period.[253]
[Illustration]
* * * * *
{241}