THE NATURE AND STRUCTURE OF THE HEAVENS ACCORDING TO THE ANCIENTS.
Many and various have been the ideas entertained by reflecting men in
former times on the nature and construction of the heavenly vault,
wherein appeared those stars and constellations whose history we have
already traced. Is it solid? or liquid? or gaseous? Each of these and
many other suppositions have been duly formulated by the ancient
philosophers and sages, although, as we are told by modern astronomy, it
does not exist at all.
In our study of the ancient ideas about the structure of the universe,
we will commence with that early and curious system which considered the
heavenly vault to be material and solid.
The theory of a solid sky received the assent of all the most ancient
philosophers. In his commentary on Aristotle's work on the heavens,
Simplicius reveals the repugnance the ancient philosophers felt in
admitting that a star could stand alone in space, or have a free motion
of its own. It must have a support, and they therefore conceived that
the sky must be solid. However strange this idea may now appear, it
formed for many centuries the basis of all astronomical theories. Thus
Anaximenas (in the sixth century B.C.) is related by Plutarch to have
said that "the outer sky is solid and crystalline," and that the stars
are "fixed to its surface like studs," but he does not say on what this
opinion was founded, though it is probable that, like his master
Anaximander, he could not understand how the stars could move without
being supported.
Pythagoras, who lived about the same epoch, is also supposed by some to
have held the same views, and it is possible that they all borrowed
these ideas from the Persians, whose earliest astronomers are said in
the _Zend avesta_ to have believed in concentric solid skies.
Eudoxus of Cnidus, in the fifth century B.C., is said by his commentator
Aratus to have also believed in the solidity of the heavens, but his
reasons are not assigned.
Notwithstanding these previously expressed opinions, Aristotle (fourth
century, B.C.) has for a long time been generally supposed to be the
inventor of solid skies, but in fact he only gave the idea his valuable
and entire support. The sphere of the stars was his eighth heaven. The
less elevated heavens, in which he also believed, were invented to
explain as well as they might, the proper motions of the sun, moon, and
planets.
The philosopher of Stagira said that the motion of his eighth or
outermost solid sky was uniform, nor ever troubled by any perturbation.
"Within the universe there is," he says, "a fixed and immovable centre,
the earth; and without there is a bounding surface enclosing it on all
sides. The outermost part of the universe is the sky. It is filled with
heavenly bodies which we know as stars, and it has a perpetual motion,
carrying round with it these immortal bodies in its unaltering and
unending revolution."
Euclid, to whom we may assign a date of about 275 before our present
era, also considered the stars to be set in a solid sphere, having the
eye of the observer as centre; though for him this conception was simply
a deduction from exact and fundamental observations, namely, that their
revolution took place as a whole, the shape and size of the
constellation being never altered.
Cicero, in the last century before Christ, declared himself a believer
in the solidity of the sky. According to him the ether was too rarefied
to enable it to move the stars, which must therefore require to be fixed
to a sphere of their own, independent of the ether.
In the time of Seneca there seem to have been difficulties already
raised about the solidity of the heavens, for he only mentions it in the
form of a question--"Is the sky solid and of a firm and compact
substance?" (_Questions_, Book ii.)
In the fifth century the idea of the star sphere still lingered, and in
the eyes of Simplicius, the commentator of Aristotle, it was not merely
an artifice suitable for the representation of the apparent motions, but
a firm and solid reality; while Mahomet and most of the Fathers of the
Christian Church had the same conception of these concentric spheres.
It appears then from this review that the phrases "starry vault," and
especially "fixed stars," have been used in two very distinct senses.
When we meet with them in Aristotle or Ptolemy, it is obvious that they
have reference to the crystal sphere of Anaximenas, to which they were
supposed to be affixed, and to move with it; but that later the word
"fixed" carried with it the sense of immovable, and the stars were
conceived as fixed in this sense, independently of the sphere to which
they were originally thought to be attached. Thus Seneca speaks of them
as the _fixum et immobilem populum_.
If we would inquire a little further into the supposed nature of this
solid sphere, we find that Empedocles considered it to be a solid mass,
formed of a portion of the ether which the elementary fire has converted
into crystal, and his ideas of the connection between cold and
solidification being not very precise, he described it by names that
give the best idea of transparence, and, like Lactantius, called it
_vitreum caelum_, or said _caelum aerem glaciatum esse_, though we cannot
suppose that he made any allusion to what we now call glass, but simply
meant some body eminently transparent into which the fire had
transformed the air; while so far from having any idea of cold, as we
might imagine possible from observations of the snowy tops of mountains,
they actually believed in a warm region above the lower atmosphere. Thus
Aristotle considers that the spheres heat by their motion the air below
them, without being heated themselves, and that there is thus a
production of heat. "The motion of the sphere of fixed stars," he says,
"is the most rapid, as it moves in a circle with all the bodies attached
to it, and the spaces immediately below are strongly heated by the
motion, and the heat, thus engendered, is propagated downwards to the
earth." This however, strangely enough, does not appear to have
prevented their supposing an eternal cold to reign in the regions next
below, for Macrobius, in his commentary on Cicero, speaks of the
decrease of temperature with the height, and concludes that the extreme
zones of the heavens where Saturn moves must be eternally cold; but this
they reckoned as part of the atmosphere, beyond whose limits alone was
to be found the fiery ether.
It is to the Fathers of the Church that we owe the transmission during
the middle ages of the idea of a crystal vault. They conceived a heaven
of glass composed of eight or ten superposed layers, something like so
many skins in an onion. This idea seems to have lingered on in certain
cloisters of southern Europe even into the nineteenth century, for a
venerable Prince of the Church told Humboldt in 1815, that a large
aerolite lately fallen, which was covered with a vitrified crust, must
be a fragment of the crystalline sky. On these various spheres, one
enveloping without touching another, they supposed the several planets
to be fixed, as we shall see in a subsequent chapter.
Whether the greater minds of antiquity, such as Plato, Plutarch,
Eudoxus, Aristotle, Apollonius, believed in the reality of these
concentric spheres to carry the planets, or whether this conception was
not rather with them an imaginary one, serving only to simplify
calculation and assist the mind in the solution of the difficult problem
of their motion, is a point on which even Humboldt cannot decide. It is
certain, however, that in the middle of the sixteenth century, when the
theory involved no less than seventy-seven concentric spheres, and
later, when the adversaries of Copernicus brought them all into
prominence to defend the system of Ptolemy, the belief in the existence
of these solid spheres, circles and epicycles, which was under the
especial patronage of the Church, was very widespread.
Tycho Brahe expressly boasts of having been the first, by considerations
concerning the orbits of the comets, to have demonstrated the
impossibility of solid spheres, and to have upset this ingenious
scaffolding. He supposed the spaces of our system to be filled with air,
and that this medium, disturbed by the motion of the heavenly bodies,
opposed a resistance which gave rise to the harmonic sounds.
It should be added also that the Grecian philosophers, though little
fond of observation, but rejoicing rather in framing systems for the
explanation of phenomena of which they possessed but the faintest
glimpse, have left us some ideas about the nature of shooting stars and
aerolites that come very close to those that are now accepted. "Some
philosophers think," says Plutarch in his life of Lysander, "that
shooting stars are not detached particles of ether which are
extinguished by the atmosphere soon after being ignited, nor do they
arise from the combustion of the rarefied air in the upper regions, but
that they are rather heavenly bodies which fall, that is to say, which
escaping in some way from the general force of rotation are precipitated
in an irregular manner, sometimes on inhabited portions of the earth,
but sometimes also in the ocean, where of course they cannot be found."
Diogenes of Apollonius expresses himself still more clearly: "Amongst
the stars that are visible move others that are invisible, to which in
consequence we are unable to give any name. These latter often fall to
the earth and take fire like that star-stone which fell all on fire near
AEgos Potamos." These ideas were no doubt borrowed from some more ancient
source, as he believed that all the stars were made of something like
pumice-stone. Anaxagoras, in fact, thought that all the heavenly bodies
were fragments of rocks which the ether, by the force of its circular
motion, had detached from the earth, set fire to, and turned into
stars. Thus the Ionic school, with Diogenes of Apollonius, placed the
aerolites and the stars in one class, and assigned to all of them a
terrestrial origin, though in this sense only, that the earth, being the
central body, had furnished the matter for all those that surround it.
Plutarch speaks thus of this curious combination:--"Anaxagoras teaches
that the ambient ether is of an igneous nature, and by the force of its
gyratory motion it tears off blocks of stone, renders them incandescent,
and transforms them into stars." It appears that he explained also by an
analogous effect of the circular motion the descent of the Nemaean Lion,
which, according to an old tradition, fell out of the moon upon the
Peloponnesus. According to Boeckh, this ancient myth of the Nemaean
Lion had an astronomical origin, and was symbolically connected in
chronology with the cycle of intercalation of the lunar year, with the
worship of the moon in Nemaea, and the games by which it was
accompanied.
[Illustration: PLATE VI.--THE NEMAEAN LION.]
Anaxagoras explains the apparent motion of the celestial sphere from
east to west by the hypothesis of a general revolution, the interruption
of which, as we have just seen, caused the fall of meteoric stones. This
hypothesis is the point of departure of the theory of vortices, which
more than two thousand years later, by the labours of Descartes,
Huyghens, and Hooke, took so prominent a place among the theories of
the world.
It may be worth adding with regard to the famous aerolite of AEgos
Potamus, alluded to above, that when the heavens were no longer believed
to be solid, the faith in the celestial origin of this, as of other
aerolites, was for a long time destroyed. Thus Bailly the astronomer,
alluding to it, says, "if the fact be true, this stone must have been
thrown out by a volcano." Indeed it is only within the last century that
it has been finally accepted for fact that stones do fall from the sky.
Laplace thought it probable that they came from the moon; but it has now
been demonstrated that aerolites, meteors, and shooting stars belong all
to one class of heavenly bodies, that they are fragments scattered
through space, and circulate like the planets round the sun. When the
earth in its motion crosses this heavenly host, those which come near
enough to touch its atmosphere leave a luminous train behind them by
their heating by friction with the air: these are the _shooting stars_.
Sometimes they come so close as to appear larger than the moon, then
they are _meteors;_ and sometimes too the attraction of the earth makes
them fall to it, and these become _aerolites_.
But to return to our ancient astronomers:--
They believed the heavens to be in motion, not only because they saw the
motion with their eyes, but because they believed them to be animated,
and regarded motion as the essence of life. They judged of the rapidity
of the stars' motion by a very ingenious means. They perceived that it
was greater than that of a horse, a bird, an arrow, or even of the
voice, and Cleomenas endeavoured to estimate it in the following way. He
remarks that when the king of Persia made war upon Greece he placed men
at certain intervals, so as to lie in hearing of each other, and thus
passed on the news from Athens to Susa. Now this news took two days and
nights to pass over this distance. The voice therefore only accomplished
a fraction of the distance that the stars had accomplished twice in the
same time.
The heavens, as we have seen, were not supposed to consist of a single
sphere, but of several concentric ones, the arrangement and names of
which we must now inquire into.
The early Chaldeans established three. The first was the empyreal
heaven, which was the most remote. This, which they called also the
solid firmament, was made of fire, but of fire of so rare and
penetrating a nature, that it easily passed through the other heavens,
and became universally diffused, and in this way reached the earth. The
second was the ethereal heaven, containing the stars, which were simply
formed of the more compact and denser parts of this substance; and the
third heaven was that of the planets. The Persians, however, gave a
separate heaven to the sun, and another to the moon.
The system which has enjoyed the longest and most widely-spread reign
is that which places above, or rather round, the solid firmament a
heaven of water--(the nature of which is not accurately defined), and
round this a _primum mobile_, prime mover, or originator of all the
motions, and round all this the empyreal heaven, or abode of the
blessed. In the most anciently printed scientific encyclopaedia known,
the _Magarita philosophica_, edited in the fifteenth century, that is,
two centuries before the adoption of the true system of the world, we
have the curious figure represented on the next page, in which we find
no less than eleven different heavens. We here see on the exterior the
solid empyreal heaven, which is stated in the body of the work to be the
abode of the blessed and to be immovable, while the next heaven gives
motion to all within, and is followed by the aqueous heaven, then the
crystal firmament, and lastly by the several heavens of the planets,
sun, and moon. The revolution of these spheres was not supposed to take
place, like the motion of the earth in modern astronomy, round an
imaginary axis, but round one which had a material existence, which was
provided with pivots moving in fixed sockets. Thus Vitruvius, architect
to Augustus, teaches it expressly in these words:--
"The heaven turns continually round the earth and sea upon an axis,
where two extremities are like two pivots that sustain it: for there are
two places in which the Governor of Nature has fashioned and set these
pivots as two centres; one is above the earth among the northern stars;
the other is at the opposite end beneath the earth to the south; and
around these pivots, as round two centres, he has placed little naves,
like those of a wheel upon which the heaven turns continually."
[Illustration: FIG. 13.]
Similarly curious ideas we shall find to have prevailed with respect to
the meaning of everything that they observed in the heavens: thus what a
number of opinions have been hazarded on the nature of the "Milky Way"
alone! some of which we may learn from Plutarch. The Milky Way, he says,
is a nebulous circle, which constantly appears in the sky, and which
owes its name to its white appearance. Certain Pythagoreans assert that
when Phaeton lit up the universe, one star, which escaped from its
proper place, set light to the whole space it passed over in its
circular course, and so formed the Milky Way. Others thought that this
circle was where the sun had been moving at the beginning of the world.
According to others it is but an optical phenomenon produced by the
reflection of the sun's rays from the vault of the sky as from a mirror,
and comparable with the effects seen in the rainbow and illuminated
clouds. Metrodorus says it is the mark of the sun's passage which moves
along this circle. Parmenidas pretends that the milky colour arises from
a mixture of dense and rare air. Anaxagoras thinks it an effect of the
earth's shadow projected on this part of the heavens, when the sun is
below. Democritus says that it is the lustre of several little stars
which are very near together, and which reciprocally illuminate each
other. Aristotle believes it to be a vast mass of arid vapours, which
takes fire from a glowing tress, above the region of the ether, and far
below that of the planets. Posidonius says that the circle is a
compound of fire less dense than that of the stars, but more luminous.
All such opinions, except that of Democritus, are of little value,
because founded on nothing; perhaps the worst is that of Theophrastus,
who said it was the junction between the two hemispheres, which together
formed the vault of heaven: and that it was so badly made that it let
through some of the light that he supposed to exist everywhere behind
the solid sky.
We now know that the Milky Way, like many of the nebulae, is an immense
agglomeration of suns. The Milky Way is itself a nebula, a mass of
sidereal systems, with our own among them, since our sun is a single
star in this vast archipelago of eighteen million orbs. The Greeks
called it the Galaxy. The Chinese and Arabians call it the River of
Heaven. It is the Path of Souls among the North American Indians, and
the Road of S. Jacques de Compostelle among French peasants.
In tracing the history of ideas concerning the structure of the heavens
among the Greek philosophers, we meet with other modifications which it
will be interesting to recount. Thus Eudoxus, who paid greater attention
than others to the variations of the motions of the planets, gave more
than one sphere to each of them to represent these observed changes.
Each planet, according to him, has a separate part of the heaven to
itself, which is composed of several concentric spheres, whose
movements, modifying each other, produce that of the planet. He gave
three spheres to the sun: one which turned from east to west in
twenty-four hours, to represent the diurnal rotation; a second, which
turned about the pole of the ecliptic in 365-1/4 days, and produced its
annual movement; and a third was added to account for a certain supposed
motion, by which the sun was drawn out of the ecliptic, and turned about
an axis, making such an angle with that of the ecliptic, as represented
the supposed aberration. The moon also had three spheres to produce its
motions in longitude and latitude, and its diurnal motion. Each of the
other planets had four, the extra one being added to account for their
stations and retrogressions. It should be added that these concentric
spheres were supposed to fit each other, so that the different planets
were only separated by the thicknesses of these crystal zones.
Polemarch, the disciple of Eudoxus, who went to Athens with his pupil
Calippus for the express purpose of consulting Aristotle on these
subjects, was not satisfied with the exactness with which these spheres
represented the planetary motions, and made changes in the direction of
still greater complication. Instead of the twenty-six spheres which
represented Eudoxus' system, Calippus established thirty-three, and by
adding also intermediary spheres to prevent the motion of one planet
interfering with that of the adjacent ones, the number was increased to
fifty-six.
There is extant a small work, ascribed to Aristotle, entitled "Letter of
Aristotle to Alexander on the system of the world," which gives so clear
an account of the ideas entertained in his epoch that we shall venture
to give a somewhat long extract from it. The work, it should be said, is
not by all considered genuine, but is ascribed by some to Nicolas of
Damas, by others to Anaximenas of Lampsacus, a contemporary of
Alexander's, and by others to the Stoic Posidonius. It is certain,
however, that Aristotle paid some attention to astronomy, for he records
the rare phenomena of an eclipse of Mars by the moon, and the
occultation of one of the Gemini by the planet Jupiter, and the work may
well be genuine. It contains the following:--
"There is a fixed and immovable centre to the universe. This is occupied
by the earth, the fruitful mother, the common focus of every kind of
living thing. Immediately surrounding it on all sides is the air. Above
this in the highest region is the dwelling-place of the gods, which is
called the heavens. The heavens and the universe being spherical and in
continual motion, there must be two points on opposite sides, as in a
globe which turns about an axis, and these points must be immovable, and
have the sphere between them, since the universe turns about them. They
are called the poles. If a line be drawn from one of these points to the
other it will be the diameter of the universe, having the earth in the
centre and the two poles at the extremities; of these two poles the
northern one is always visible above our horizon, and is called the
Arctic pole; the other, to the south, is always invisible to us--it is
called the Antarctic pole.
"The substance of the heavens and of the stars is called ether; not that
it is composed of flame, as pretended by some who have not considered
its nature, which is very different from that of fire, but it is so
called because it has an eternal circular motion, being a divine and
incorruptible element, altogether different from the other four.
"Of the stars contained in the heavens some are fixed, and turn with the
heavens, constantly maintaining their relative positions. In their
middle portion is the circle called the _zoophore_, which stretches
obliquely from one tropic to the other, and is divided into twelve
parts, which are the twelve signs (of the zodiac). The others are
wandering stars, and move neither with the same velocity as the fixed
stars, nor with a uniform velocity among themselves, but all in
different circles, and with velocities depending on the distances of
these circles from the earth.
"Although all the fixed stars move on the same surface of the heavens,
their number cannot be determined. Of the movable stars there are seven,
which circulate in as many concentric circles, so arranged that the
lower circle is smaller than the higher, and that the seven so placed
one within the other are all within the spheres of the fixed stars.
"On the nearer, that is inner, side of this ethereal, immovable,
unalterable, impassible nature is placed our movable, corruptible, and
mortal nature. Of this there are several kinds, the first of which is
fire, a subtle inflammable essence, which is kindled by the great
pressure and rapid motion of the ether. It is in this region of air,
when any disturbance takes place in it, that we see kindled
shooting-stars, streaks of light, and shining motes, and it is there
that comets are lighted and extinguished.
"Below the fire comes the air, by nature cold and dark, but which is
warmed and enflamed, and becomes luminous by its motion. It is in the
region of the air, which is passive and changeable in any manner, that
the clouds condense, and rain, snow, frost, and hail are formed and fall
to the earth. It is the abode of stormy winds, of whirlwinds, thunder,
lightning, and many other phenomena.
"The cause of the heaven's motion is God. He is not in the centre, where
the earth is a region of agitation and trouble, but he is above the
outermost circumference, which is the purest of all regions, a place
which we call rightly _ouranos_, because it is the highest part of the
universe, and _olympos_, that is, perfectly bright, because it is
altogether separated from everything like the shadow and disordered
movements which occur in the lower regions."
We notice in this extract a curious etymology of the word ether, namely,
as signifying perpetual motion ([Greek: aei teein]), though it is more
probable that its true, as its more generally accepted derivation is
from [Greek: aithein], to burn or shine, a meaning doubtless alluded to
in a remarkable passage of Hippocrates, [Greek: Peri Sarkon]. "It
appears to me," he says, "that what we call the principle of heat is
immortal, that it knows all, sees all, hears all, perceives all, both in
the past and in the future. At the time when all was in confusion, the
greater part of this principle rose to the circumference of the
universe; it is this that the ancients have called _ether_."
The first Greek that can be called an astronomer was Thales, born at
Miletus 641 B.C., who introduced into Greece the elements of astronomy.
His opinions were these: that the stars were of the same substance as
the earth, but that they were on fire; that the moon borrowed its light
from the sun, and caused the eclipses of the latter, while it was itself
eclipsed when it entered the earth's shadow; that the earth was round,
and divisible into five zones, by means of five circles, _i.e._ the
Arctic and Antarctic, the two tropics, and the equator; that the latter
circle is cut obliquely by the ecliptic, and perpendicularly by the
meridian. Up to his time no division of the sphere had been made beyond
the description of the constellations. These opinions do not appear to
have been rapidly spread, since Herodotus, one of the finest intellects
of Greece, who lived two centuries later, was still so ill-instructed as
to say, in speaking of an eclipse, "The sun abandoned its place, and
night took the place of day."
Anaxagoras, of whom we have spoken before, asserted that the sun was a
mass of fire larger than the Peloponnesus. Plutarch says he regarded it
as a burning stone, and Diogenes Laertius looked upon it as hot iron.
For this bold idea he was persecuted. They considered it a crime that he
taught the causes of the eclipses of the moon, and pretended that the
sun is larger than it looks. He first taught the existence of one God,
and he was taxed with impiety and treason against his country. When he
was condemned to death, "Nature," he said, "has long ago condemned me to
the same; and as to my children, when I gave them birth I had no doubt
but they would have to die some day." His disciple Pericles, however,
defended him so eloquently that his life was spared, and he was sent
into exile.
Pythagoras, who belonged to the school of Thales, and who travelled in
Phoenicia, Chaldea, Judaea, and Egypt, to learn their ideas, ventured,
in spite of the warnings of the priests, to submit to the rites of
initiation at Heliopolis, and thence returned to Samos, but meeting with
poor reception there, he went to Italy to teach. From him arose the
_Italian School_, and his disciples took the name of philosophers
(lovers of wisdom) instead of that of sages. We shall learn more about
him in the chapter on the Harmony of the Spheres.
His first disciple, Empedocles, famous for the curiosity which led him
to his death in the crater of AEtna, as the story goes, thought that the
true sun, the fire that is in the centre of the universe, illuminated
the other hemisphere, and that what we see is only the reflected image
of that, which is invisible to us, and all of whose movements it
follows.
His disciple, Philolaus, also taught that the sun was a mass of glass,
which sent us by reflection all the light that it scattered through the
universe. We must not, however, forget that these opinions are recorded
by historians who probably did not understand them, and who took in the
letter what was only intended for a comparison or figure.
If we are to believe Plutarch, Xenophanes, who flourished about 360
B.C., was very wild in his opinions. He thought the stars were lighted
every night and extinguished every morning; that the sun is a fiery
cloud; that eclipses take place by the sun being extinguished and
afterwards rekindled; that the moon is inhabited, but is eighteen times
larger than the earth; that there are several suns and several moons for
giving light to different countries. This can only be matched by those
who said the sun went every night through a hole in the earth round
again to the east; or that it went above ground, and if we did not see
it going back it was because it accomplished the journey in the night.
Parmenidas was the disciple of Xenophanes. He divided the earth, like
Thales, into zones; and he added that it was suspended in the centre of
the universe, and that it did not fall because there was no reason why
it should move in one direction rather than another. This argument is
perfectly philosophical, and illustrates a principle employed since the
time of Archimedes, and of which Leibnitz made so much use.
Such are some of the general ideas which were held by the Greeks and
others on the nature of the heavens, omitting that of Ptolemy, of which
we shall give a fuller account hereafter. We see that they were all
affected by the dominant idea of the superiority of the earth over the
rest of the universe, and were spoiled for want of the grand conception
of the immensity of space. The universe was for them a closed space,
outside of which there was _nothing_; and they busied themselves with
metaphysical questions as to the possibility of space being infinite. In
the meantime their conceptions of the distances separating us from other
visible parts of the universe were excessively cramped. Hesiod, for
instance, thinks to give a grand idea of the size of the universe by
saying that Vulcan's anvil took seven days to fall from heaven to earth,
when in reality, as now calculated, it would take no less than
seventy-two years for the light, even travelling at a far greater rate,
to reach us from one of the nearest of the fixed stars.