ASTRONOMICAL SYSTEMS.
In our former chapters we have gained some idea of the general structure
of the heavens as represented by ancient philosophers, and we no longer
require to know what was thought in the infancy of astronomy, when any
ideas promulgated were more or less random ones; but in this chapter we
hope to discuss those arrangements of the heavenly bodies which have
been promulgated by men as complete systems, and were supposed to
represent the totality of the facts.
The earliest thoroughly-established system is that of Ptolemy. It was
not indeed invented by him. The main ideas had been entertained long
before his time, but he gave it consistence and a name.
We obtain an excellent view of the general nature of this system from
Cicero. He writes:--
"The universe is composed of nine circles, or rather of nine moving
globes. The outermost sphere is that of the heavens which surrounds all
the others, and on which are fixed the stars. Beneath this revolve
seven other globes, carried round by a motion in a direction contrary to
that of the heavens. On the first circle revolves the star which men
call Saturn; on the second Jupiter shines, that beneficent and
propitious star to human eyes; then follows Mars, ruddy and awful.
Below, and occupying the middle region, revolves the Sun, the chief,
prince, and moderator of the other stars, the soul of the world, whose
immense globe spreads its light through space. After him come, like two
companions, Venus and Mercury. Lastly, the lowest globe is occupied by
the moon, which borrows its light from the star of day. Below this last
celestial circle, there is nothing but what is mortal and corruptible,
except the souls given by a beneficent Divinity to the race of men.
Above the moon all is eternal. The earth, situated in the centre of the
world, and separated from heaven on all sides, forms the ninth sphere;
it remains immovable, and all heavy bodies are drawn to it by their own
weight."
The earth, we should add, is surrounded by the sphere of air, and then
by that of fire, and by that of ether and the meteors.
With respect to the motions of these spheres. The first circle described
about the terrestrial system, namely, that of the moon, was accomplished
in 27 days, 7 hours, and 43 minutes. Next to the moon, Mercury in the
second, and Venus in the third, and the sun in the fourth circle, all
turned about the earth in the same time, 365 days, 5 hours, and 49
minutes. But these planets, in addition to the general movement, which
carried them in 24 hours round from east to west and west to east, and
the annual revolution, which made them run through the zodiacal circle,
had a third motion by which they described a circle about each point of
their orbit taken as a centre.
[Illustration: FIG. 14.--PTOLEMY'S ASTRONOMICAL SYSTEM.]
The fifth sphere, carrying Mars, accomplished its revolution in two
years. Jupiter took 11 years, 313 days, and 19 hours to complete his
orbit, and Saturn in the seventh sphere took 29 years and 169 days.
Above all the planets came the sphere of the fixed stars, or Firmament,
turning from east to west in 24 hours with inconceivable rapidity, and
endued also with a proper motion from west to east, which was measured
by Hipparchus, and which we now call the precession of the equinoxes,
and know that it has a period of 25,870 years. Above all these spheres,
a _primum mobile_ gave motion to the whole machine, making it turn from
east to west, but each planet and each fixed star made an effort against
this motion, by means of which each of them accomplished their
revolution about the earth in greater or less time, according to its
distance, or the magnitude of the orbit it had to accomplish.
One immense difficulty attended this system. The apparent motions of the
planets is not uniform, for sometimes they are seen to advance from west
to east, when their motion is called _direct_, sometimes they are seen
for several nights in succession at the same point in the heavens, when
they are called _stationary_, and sometimes they return from east to
west, and then their motion is called _retrograde_.
We know now that this apparent variation in the motion of the planets is
simply due to the annual motion of the earth in its orbit round the sun.
For example, Saturn describes its vast orbit in about thirty years, and
the earth describes in one year a much smaller one inside. Now if the
earth goes faster in the same direction as Saturn, it is plain that
Saturn will be left behind and appear to go backwards, while if the
earth is going in the same direction the velocity of Saturn will appear
to be decreased, but his direction of motion will appear unaltered.
To explain these variations, however, according to his system, Ptolemy
supposed that the planets did not move exactly in the circumference of
their respective orbits, but about an _ideal centre_, which itself moved
along this circumference. Instead therefore of describing a circle, they
described parts of a series of small circles, which would combine, as is
easy to see, into a series of uninterrupted waves, and these he called
_Epicycles_.
Another objection, which even this arrangement did not overcome, was the
variation of the size of the planets. To overcome this Hipparchus gave
to the sphere of each planet a considerable thickness, and saw that the
planet did not turn centrally round the earth, but round a centre of
motion placed outside the earth. Its revolution took place in such a
manner, that at one time it reached the inner boundary, at another time
the outer boundary of its spherical heaven.
But this reply was not satisfactory, for the differences in the apparent
sizes proved by the laws of optics such a prodigious difference between
their distances from the earth at the times of conjunction and
opposition, that it would be extremely difficult to imagine spheres
thick enough to allow of it.
It was a gigantic and formidable piece of machinery to which it was
necessary to be continually adding fresh pieces to make observation
accord with theory. In the thirteenth century, in the times of the
King-Astronomer, Alphonso X. of Castile, there were already seventy-five
circles, one within the other. It is said that one day he exclaimed, in
a full assemblage of bishops, that if the Deity had done him the honour
to ask his advice before creating the world, he could have told Him how
to make it a little better, or at all events more simply. He meant to
express how unworthy this complication was of the dignity of nature.
[Illustration: FIG. 15.--THE EPICYCLES OF PTOLEMY.]
Fracastor, in his _Homocentrics_, says that nothing is more monstrous or
absurd than all the excentrics and epicycles of Ptolemy, and proposes
to explain the difference of velocity in the planets at different parts
of their orbits by the medium offering greater or less resistance, and
their alteration in apparent size by the effect of refraction.
The essential element of this system was that it took appearances for
realities, and was founded on the assumption that the earth is fixed in
the centre of the universe, and of course therefore neglected all the
appearances produced by its motion, or had to explain them by some
peculiarity in the other planets.
Although it was corrected from time to time to make it accord better
with observation, it was the same essentially that was taught officially
everywhere. It reigned supreme in Egypt, Greece, Italy, and Arabia, and
in the great school of Alexandria, which consolidated it and enriched it
by its own observations.
But though the same in essence, the details, and especially the means of
overcoming the difficulties raised by increased observations, have much
varied, and it will be interesting and instructive to record some of the
chief of them.
One of the most important influences in modifying the astronomical
systems taught to the world has been that of the Fathers of the
Christian Church. When, after five centuries of patient toil, of hopes,
ambitions, and discussions, the Christian Church took possession of the
thrones and consciences of men, they founded their physical edifice on
the ancient system, which they adapted to their special wants. With them
Aristotle and Ptolemy reigned supreme. They decreed that the earth
constituted the universe, that the heavens were made for it, that God,
the angels, and the saints inhabited an eternal abode of joy situated
above the azure sphere of the fixed stars, and they embodied this
gratifying illusion in all their illuminated manuscripts, their
calendars, and their church windows.
The doctors of the Church all acknowledged a plurality of heavens, but
they differed as to the number. St. Hilary of Poitiers would not fix it,
and the same doubt held St. Basil back; but the rest, for the most part
borrowing their ideas from paganism, said there were six or seven, or up
to ten. They considered these heavens to be so many hemispheres
supported on the earth, and gave to each a different name. In the system
of Bede, which had many adherents, they were the Air, Ether, Fiery
Space, Firmament, Heaven of the Angels, and Heaven of the Trinity.
The two chief varieties in the systems of the middle ages may be
represented as follows:--
Those who wished to have everything as complete as possible combined the
system of Ptolemy with that of the Fathers of the Church, and placed in
the centre of the earth the infernal regions which they surrounded by a
circle. Another circle marked the earth itself, and after that the
surrounding ocean, marked as water, then the circle of air, and lastly
that of fire. Enveloping these, and following one after the other, were
the seven circles of the seven planets; the eighth represented the
sphere of the fixed stars on the firmament, then came the ninth heaven,
then a tenth, the _coelum cristallinum_, and lastly an eleventh and
outermost, which was the empyreal heaven, where dwelt the cherubim and
seraphim, and above all the spheres was a throne on which sat the
Father, as Jupiter Olympus.
The others who wished for more simplicity, represented the earth in the
centre of the universe, with a circle to indicate the ocean, the second
sphere was that of the moon; the third was that of the sun; on the
fourth were placed the four planets, Jupiter, Mars, Venus and Mercury;
there was a fifth for the space outside the planets, and the last
outside one was the firmament; altogether seven spheres instead of
eleven. As a specimen of the style of representation of the astronomical
systems of the middle ages, we may take the figure on the following
page:--
Here we see the earth placed immovable in the centre of the universe,
and represented by a disc traversed by the Mediterranean, and surrounded
by the ocean. Round this are circumscribed the celestial spheres. That
of the moon first, then that of Mercury, in which several
constellations, as the Lyre, Cassiopeia, the Crown, and others, are
roughly indicated, then comes the sphere of Venus with Sagittarius and
the Swan. After this comes the _celestis_ _paradisus_, and the legend
that, "the paradise to which Paul was raised is in this third locality;
some of these must reach to us, since in them repose the souls of the
prophets." In the other circles are yet other constellations: for
example Pegasus, Andromeda, the Dog, Argo, the He-goat, Aquarius, the
Fishes, and Canopus, figured by a star of the first magnitude. To the
north is seen near the constellation of the Swan a large star with seven
rays, meant to represent the brightest of those which compose the Great
Bear. The stars of Cassiopeia are not only misplaced, but roughly
represented. The Lyre is curiously drawn. The positions of the
constellations just named are all wrong in this figure, just as we find
those of towns in maps of the earth. The cartographers of the middle
ages, with incredible ignorance, misplaced in general every locality.
They did the same for the constellations in the celestial hemispheres.
In the heaven of Jupiter, and in that of Saturn we read the
words--Seraphim, Dominationes, Potestates, Archangeli, Virtutes
coelorum, Principatus, Throni, Cherubim, all derived from their
theology. A veritable muddle! The angels placed with the heroes of
mythology, the immortal virgins with Venus and Andromeda, and the Saints
with the Great Bear, the Hydra, and the Scorpion!
[Illustration: FIG. 16.--HEAVENS OF THE MIDDLE AGES.]
Another such richly illuminated manuscript in the library at Ghent,
entitled Liber Floridus, contains a drawing similar to this under the
title _Astrologia secundum Bedum_. Only, instead of the earth, there is
a serpent in the centre with the name Great Bear, and the twins are
represented by a man and woman, Andromeda in a chasuble, and Venus as a
nun!
Several similar ones might be quoted, varying more or less from this;
one, executed in a geographical manuscript of the fifteenth century, has
the tenth sphere, being that of the fixed stars, then the crystalline
heaven, and then the immovable heaven, "which," it says, "according to
sacred and certain theology, is the dwelling-place of the blessed, where
may we live for ever and ever, Amen;" "this is also called the empyreal
heaven." Near each planet the author marks the time of its revolution,
but not at all correctly.
[Illustration: PLATE VII.--HEAVENS OF THE FATHERS.]
The constructors of these systems were not in the least doubt as to
their reality, for they actually measured the distance between one
sphere and another, though in every case their numbers were far from the
truth as we now know it. We may cite as an example an Italian system
whose spheres were as follows:--Terra, Aqua, Aria, Fuoco, Luna,
Mercurio, Venus, Sol, Marte, Giove, Saturno, Stelle fixe, Sfera nona,
Cielo empyreo. Attached to the design is the following table of
dimensions which we may copy:--
Miles.
From the centre of the Earth to the surface 3,245
" " " " inner side of the
heaven of the Moon 107,936
Diameter of Moon 1,896
From the centre of the Earth to Mercury 209,198
Diameter of Mercury 230
From the centre of the Earth to Venus 579,320
Diameter of Venus 2,884
From the centre of the Earth to the Sun 3,892,866
Diameter of the Sun 35,700
From the centre of the Earth to Mars 4,268,629
Diameter of Mars 7,572
From the centre of the Earth to Jupiter 8,323,520
Diameter of Jupiter 29,641
From the centre of the Earth to outside of Saturn's
heaven 52,544,702
Diameter of Saturn 29,202
From the centre of the Earth to the fixed stars 73,387,747
The author states that he cannot pursue his calculations further, and
condescends to acknowledge that it is very difficult to know accurately
what is the thickness of the ninth and of the crystalline heavens!
Perhaps, however, these reckonings are better than those of the
Egyptians, who came to the conclusion that Saturn was only distant 492
miles, the sun only 369, and the moon 246.
These numerous variations and adaptations of the Ptolemaic system, prove
what a firm hold it had taken, and how it reigned supreme over all
minds. Nor are we merely left to gather this. They consciously looked to
Ptolemy as their great light, if we may judge from an emblematic drawing
taken from an authoritative astronomical work, the _Margarita
Philosophica_, which we give on the opposite page.
[Illustration: FIG. 17.]
In all the systems derived from Ptolemy, the order of the planets
remained the same, and Mercury and Venus were placed nearer to the earth
than the sun is. According to many authors, however, Plato made a
variation in this respect, by putting them outside the sun, on the
ground that they never were seen to pass across its surface. He had
obviously never heard of the "Transit of Venus." This arrangement was
adopted by Theon, in his commentary on the _Almagesta_ of Ptolemy, and
afterwards by Geber, who alone among the Arabians departed from the
strict Ptolemaic system.
[Illustration: FIG. 18.--EGYPTIAN SYSTEM.]
The Egyptians improved upon this idea, and made the first step towards
the true system, by representing these two planets, Mercury and Venus,
as revolving round the sun instead of the earth. All the rest of their
system was the same as that of Ptolemy, for the sun itself, and the
other planets and the fixed stars all revolved round the earth in the
centre. This system of course accounted accurately for the motions of
the two inferior planets, whose nearness to the sun may have suggested
their connection with it. This system was in vogue at the same time as
Ptolemy's, and numbers Vitruvius amongst its supporters.
[Illustration: FIG. 19.--CAPELLA'S SYSTEM.]
In the fifth century of our era Martian Capella taught a variation on
the Egyptian system, in which he made Mercury and Venus revolve in the
same orbit round the sun. In the treatise entitled _Quod Tellus non sit
Centrum Omnibus Planetis_, he explains that when Mercury is on this side
of the orbit it is nearer to us than Venus, and farther off from us
than that planet when it is on the other side. This hypothesis was also
adopted in the middle ages.
We have here indicated the time of the revolution of the various
planets, and notice that the firmament is said to move round from west
to east in 7,000 years; the second heaven in 49,000, while the _primum
mobile_ outside moved in the contrary direction in twenty-four hours.
These Egyptian systems survived in some places the true one, as they
were thought to overcome the chief difficulties of the Ptolemaic without
interfering with the stability of the earth, and they were known as the
_common system_, _i.e._ containing the elements of both.
Such were the astronomical systems in vogue before the time of
Copernicus--all of them based upon the principle of the earth being the
immovable centre of the universe. We must now turn to trace the history
of the introduction of that system which has completely thrown over all
these former ones, and which every one knows now to be the true one--the
Copernican.
No revolution is accomplished, whether in science or politics, without
having been long in preparation. The theory of the motion of the earth
had been conceived, discussed, and even taught many ages before the
birth of Copernicus. And the best proof of this is the acknowledgment of
Copernicus himself in his great work _De Revolutionibus Orbium
Caelestium_, in which he laid down the principles of his system. We will
quote the passage in which it is contained.
"I have been at the trouble," he writes, "to read over all the works of
philosophers that I could procure, to see if I could find in them any
different opinion to that which is now taught in the schools respecting
the motions of the celestial spheres. And I saw first in Cicero that
Maetas had put forth the opinion that the earth moves. (Maetam sensisse
terram moveri.) Afterwards I found in Plutarch that others had
entertained the same idea."
Here Copernicus quotes the original as far as it relates to the system
of Philolaus, to the effect "that the earth turns round the region of
fire (ethereal region), and runs through the zodiac like the sun and the
moon." The principal Pythagoreans, such as Archytas of Tarentum,
Heraclides of Pontium, taught also the same doctrine, saying that "the
earth is not immovable in the centre of the universe, but revolves in a
circle, and is far from occupying the chief place among the celestial
bodies."
Pythagoras learnt this doctrine, it is said, from the Egyptians, who in
their hieroglyphics represented the symbol of the sun by the stercoral
beetle, because this insect forms a ball with the excrement of the oxen,
and lying down on its back, turns it round and round with its legs.
Timaeus of Locris was more precise than the other Pythagoreans in calling
"the five planets the organs of time, on account of their revolutions,"
adding that we must conclude that the earth is not immovable in one
place, but that it turns, on the contrary, about itself, and travels
also through space.
Plutarch records that Plato, who had always taught that the sun turned
round the earth, had changed his opinion towards the end of his life,
regretting that he had not placed the sun in the centre of the universe,
which was the only place, he then thought, that was suitable for that
star.
Three centuries before Jesus Christ, Aristarchus of Samos is said by
Aristotle to have composed a special work to defend the motion of the
earth against the contrary opinions of philosophers. In this work, which
is now lost, he laid down in the most positive manner that "the sun
remains immovable, and that the Earth moves round it in a circular
curve, of which that star is the centre." It would be impossible to
state this in clearer terms; and what makes his meaning more clear, if
possible, is that he was persecuted for it, being accused of irreligion
and of troubling the repose of Vesta--"because," says Plutarch, "in
order to explain the phenomena, he taught that the heavens were
immovable, and that the earth accomplished a motion of translation in an
oblique line, at the same time that it turned round its own axis." This
is exactly the opinion that Copernicus took up, after an interval of
eighteen centuries--and he too was accused of irreligion.
In passing from the Greeks to the Romans, and from them to the middle
ages, the doctrine of Aristarchus underwent a curious modification,
assimilating it to the system of Tycho Brahe, which we shall hereafter
consider, rather than to that of Copernicus. This consisted in making
the planets move round the sun, while the sun itself revolved round the
earth, and carried them with him, and the heavens revolved round all.
Vitruvius and Macrobius both taught this doctrine. Although Cicero and
Seneca, with Aristotle and the Stoics, taught the immobility of the
earth in the centre of the universe, the question seemed undecided, to
Seneca at least, who writes:--"It would be well to examine whether it is
the universe that turns about the immovable earth, or the earth that
moves, while the universe remains at rest. Indeed some men have taught
that the earth is carried along, unknown to ourselves, that it is not
the motion of the heavens that produces the rising and setting of the
stars, but that it is we who rise and set relatively to them. It is a
matter worthy of contemplation, to know in what state we are--whether we
are assigned an immovable or rapidly-moving home--whether God makes all
things revolve round us, or we round them."
The double motion of the earth, then, is an idea revived from the
Grecian philosophers. The theory was known indeed to Ptolemy, who
devotes a whole chapter in his celebrated _Almagesta_ to combat it. From
his point of view it seemed very absurd, and he did not hesitate to call
it so; and it was in reality only when fresh discoveries had altered the
method of examining the question that the absurdities disappeared, and
were transferred to the other side. Not until it was discovered that the
earth was no larger and no heavier than the other planets could the idea
of its revolution and translation have appeared anything else than
absurd. We are apt to laugh at the errors of former great men, while we
forget the scantiness of the knowledge they then possessed. So it will
be instructive to draw attention to Ptolemy's arguments, that we may see
where it is that new knowledge and ideas have led us, as they would
doubtless have led him, had he possessed them, to a different
conclusion.
His argument depends essentially on the observed effects of weight.
"Light bodies," he says, "are carried towards the circumference, they
appear to us to go _up_; because we so speak of the space that is over
our heads, as far as the surface which appears to surround us. Heavy
bodies tend, on the contrary, towards the middle, as towards a centre,
and they appear to us to fall _down_, because we so speak of whatever is
under our feet, in the direction of the centre of the earth. These
bodies are piled up round the centre by the opposed forces of their
impetus and friction. We can easily see that the whole mass of the
earth, being so large compared with the bodies that fall upon it, can
receive them without their weight or their velocity communicating to it
any perceptible oscillation. Now if the earth had a motion in common
with all the other heavy bodies, it would not be long, on account of its
weight, in leaving the animals and other bodies behind it, and without
support, and it would soon itself fall out of heaven. Such would be the
consequences of its motion, which are most ridiculous even to imagine."
Against the idea of the earth's diurnal rotation he argued as
follows:--"There are some who pretend that nothing prevents us from
supposing that the heaven remains immovable, and the earth turns round
upon its axis from west to east, accomplishing the rotation each day. It
is true that, as far as the stars are concerned, there is nothing
against our supposing this, if guided only by appearances, and for
greater simplicity; but those who do so forget how thoroughly ridiculous
it is when we consider what happens near us and in the air. For even if
we admit, which is not the case, that the lighter bodies have no motion,
or only move as bodies of a contrary nature, although we see that aerial
bodies move with greater velocity than terrestrial--if we admit that
very dense and heavy bodies have a rapid and constant motion of their
own, whereas in reality they obey but with difficulty the impulses
communicated to them--we should then be obliged to assert that the
earth, by its rotation, has a more rapid motion than any of the bodies
that are round it, as it makes so large a circuit in so short a time. In
this case the bodies which are not supported by it would appear to have
a motion contrary to it, and no cloud or any flying bird could ever
appear to go to the east, since the earth would always move faster than
it in that direction."
The _Almagesta_ was for a long time the gospel of astronomers; to
believe in the motion of the earth was to them more than an innovation,
it was simply folly. Copernicus himself well expresses the state of
opinion in which he found the question, and the process of his own
change, in the following words:--"And I too, taking occasion by these
testimonies, commenced to cogitate on the motion of the earth, and
although that opinion appeared absurd, I thought that as others before
me had invented an assemblage of circles to explain the motion of the
stars, I might also try if, by supposing the earth to move, I could not
find a better account of the motions of the heavenly bodies than that
with which we are at present contented. After long researches, I am at
last convinced that if we assign to the circulation of the earth the
motions of the other planets, calculation and observation will agree
better together. And I have no doubt that mathematicians will be of my
opinion, if they will take the trouble to consider carefully and not
superficially the demonstrations I shall give in this work." Although
the opinions of Copernicus had been held before, it is very just that
his should be the name by which they are known; for during the time that
elapsed before he wrote, the adherents of such views became fewer and
fewer, until at last the very remembrance of them was almost forgotten,
and it required research to know who had held them and taught them. It
took him thirty years' work to establish them on a firm basis. We shall
make no excuse for quoting further from his book, that we may know
exactly the circumstances, as far as he tells us, of his giving this
system to the world.
"I hesitated for a long time whether I should publish my commentaries on
the motions of the heavenly bodies, or whether it would not be better to
follow the example of certain Pythagoreans, who left no writings, but
communicated the mysteries of their philosophy orally from man to man
among their adepts and friends, as is proved by the letter of Lysidas to
Hipparchus. They did not do this, as some suppose, from a spirit of
jealousy, but in order that weighty questions, studied with great care
by illustrious men, might not be disparaged by the idle, who do not care
to undertake serious study, unless it be lucrative, or by shallow-minded
men, who, though devoting themselves to science, are of so indolent a
spirit that they only intrude among philosophers, like drones among
bees.
"When I hesitated and held back, my friends pressed me on. The first was
Nicolas Schonberg, Cardinal of Capua, a man of great learning. The other
was my best friend, Tideman Gysius, Bishop of Culm, who was as well
versed in the Holy Scriptures as in the sciences. The latter pressed me
so much that he decided me at last to give to the public the work I had
kept for more than twenty-seven years. Many illustrious men urged me, in
the interest of mathematics, to overcome my repugnance and to let the
fruit of my labours see the light. They assured me that the more my
theory of the motion of the earth appeared absurd, the more it would be
admired when the publication of my work had dissipated doubts by the
clearest demonstrations. Yielding to these entreaties, and buoying
myself with the same hope, I consented to the printing of my work."
He tried to guard himself against the attacks of dogmatists by saying,
"If any evil-advised person should quote against me any texts of
Scripture, I deprecate such a rash attempt. Mathematical truths can only
be judged by mathematicians."
Notwithstanding this, however, his work, after his death, was condemned
by the Index in 1616, under Paul V.
On examining the ancient systems, Copernicus was struck by the want of
harmony in the arrangements proposed, and by the arbitrary manner in
which new principles were introduced and old ones neglected, comparing
the system to a collection of legs and arms not united to any trunk, and
it was the simplicity and harmony which the one idea of the motion of
the earth introduced into the whole system that convinced him most
thoroughly of its truth.
He knew well that new views and truths would appear as paradoxes, and be
rejected by men who were wedded to old doctrines, and on this account he
took such pains to show that these views had been held before, and thus
to disarm them of their apparent novelty.
[Illustration: FIG. 20.--THE COPERNICAN SYSTEM.]
Copernicus dealt only with the six planets then known and the sun and
moon. As to the stars, he had no idea that they were suns like our own,
at immense and various distances from us. The knowledge of the magnitude
of the sidereal universe was reserved for our own century, when it was
discovered by the method of parallaxes. We will give Copernicus's own
sketch of the planetary system:--
"In the highest place is the sphere of the fixed stars, an immovable
sphere, which surrounds the whole of the universe. Among the movable
planets the first is Saturn, which requires thirty years to make its
revolution. After it Jupiter accomplishes its journey in twelve years;
Mars follows, requiring two years. In the fourth line come the earth and
the moon which in the course of one year return to their original
position. The fifth place is occupied by Venus, which requires nine
months for its journey. Mercury occupies the sixth place, whose orbit is
accomplished in eighty days. In the midst of all is the sun. What man is
there, who in this majestic temple could choose another and better place
for that brilliant lamp which illuminates all the planets with their
satellites? It is not without reason that the sun is called the lantern
of the world, the soul and thought of the universe. In placing it in the
centre of the planets, as on a regal throne, we give it the government
of the great family of celestial bodies."
The hypothesis of the motion of the earth in its orbit appeared simply
to Copernicus as a good basis for the exact determination of the ratios
of the distances of the several planets about the sun. But he did not
give up the excentrics and epicycles for the explanation of the
irregular motions of the planets, and certain imaginary variations in
the precession of the equinoxes and the obliquity of the ecliptic.
According to him the earth was endowed with three different motions, the
first about its axis, the second along the ecliptic, and a third, which
he called the declination, moving it backwards along the signs of the
zodiac from east to west. This last motion was invented to explain the
phenomena of the seasons. He thought, like many other ancient
philosophers, that a body could not turn about another without being
fixed in some way to it--by a crystal sphere, or something--and in this
case that the same surface would each day be presented to the sun, and
so it requires a third rotation, by which its axis may remain constantly
parallel to itself. Galileo, however, afterwards demonstrated the
independence of the two motions in question, and proved that the third
was unnecessary.
Copernicus was born in the Polish village of Thorn, in 1473, and died in
1543, at Warmia, of which he was canon, and where he built an
observatory. The voyages of his youth, his labours, adversities, and old
age at last broke him down, and in the winter of 1542 he took to his
bed, and was incapable of further work. His work, which was just
finished printing at Nuremberg, was brought to him by his friends before
he died. He soon after completely failed in strength, and passed away
tranquilly on the 23rd of May, 1543.
[Illustration: PLATE VIII.--DEATH OF COPERNICUS.]
The Copernican system required, however, establishing in the minds of
astronomers generally before it took the place it now holds, and this
work was done by Galileo--a name as celebrated as that of Copernicus
himself, if not more so. This perhaps is due not only to his
demonstration of the motion of the earth, but to his introduction of
experimental philosophy, and his observational method in astronomy.
The next advance was made by Kepler, who overthrew at one blow all the
excentrics and epicycles of the ancients, when by his laborious
calculations he proved the ellipticity of the orbit of Mars.
The Grecian hypotheses were the logical consequences of two propositions
which were universally admitted as axioms in the early and middle ages.
First, that the motions of the heavenly bodies were uniform; second,
that their orbits were perfect circles. Nothing appeared more natural
than this belief, though false. So then when Kepler, in 1609, recognised
the fact, by incontestable geometrical measurements, that Mars described
an oval orbit round the sun, in which its velocity varied periodically,
he could not believe either his observation or his calculation, and he
puzzled his brain to discover what secret principle it was that forced
the planet to approach and depart from the sun by turns. Fortunately for
him, in this inquietude he came across a treatise by Gilbert, _De
Magnate_, which had been published in London nine years before. In this
remarkable work Gilbert proved by experiment that the earth acts on
magnetized needles and on bars of iron placed near its surface just as a
magnet does--and by a conjectural extension of this fact, which was a
vague presentiment of the truth, he supposed that the earth itself might
be retained in its constant orbit round the sun by a magnetic
attraction. This idea was a ray of light to Kepler. It led him to see
the secret cause of the alternating motions that had troubled him so
much, and in the joy of that discovery he said, "If we find it
impossible to attribute the vibration to a magnetic power residing in
the sun, acting on the planet without any material medium between, we
must conclude that the planet is itself endowed with a kind of
intelligent perception which gives it power to know at each instant the
proper angles and distances for its motion." In the result Kepler was
led to enunciate to the world his three celebrated laws:--
1st. That the planets move in ellipses, of which the sun is in one of
the foci.
2nd. The spaces described by the ideal radius which joins each planet to
the sun are proportional to the times of their description. In other
words, the nearer a planet is to the sun, the faster it moves.
3rd. The squares of the times of revolution are as the cubes of the
major axes of the orbits.
Such were the laws of Kepler, the basis of modern astronomy, which led
in the hands of Newton to the simple explanation by universal
gravitation, which itself is now asking to be explained.
We are not to suppose that the system of Copernicus was universally
accepted even by astronomers of note. By some an attempt was made to
invent a system which should have all the advantages of this, and yet if
possible save the immobility of the earth. Such was that of Tycho Brahe,
who was born three years after the death of Copernicus, and died in
1601. He was one of the most laborious and painstaking observers of his
time, although by the peculiarity of fate he is known generally only by
his false system.
[Illustration: FIG. 21.--TYCHO BRAHE'S SYSTEM.]
In 1577, Tycho Brahe wrote a little treatise, _Tychonis Brahe, Dani, De
Mundi AEtherei Recentioribus phenomenis, a propos_ of a comet that had
lately appeared. He speaks at length of his system as follows:--"I have
remarked that the ancient system of Ptolemy is not at all natural, and
too complicated. But neither can I approve of the new one introduced by
the great Copernicus after the example of Aristarchus of Samos. This
heavy mass of earth, so little fit for motion, could not be displaced in
this manner, and moved in three ways, like the celestial bodies, without
a shock to the principles of physics. Besides, it is opposed to
Scripture! I think then," he adds, "that we must decidedly and without
doubt place the earth immovable in the centre of world, according to the
belief of the ancients and the testimony of Scripture. In my opinion the
celestial motions are arranged in such a way that the sun, the moon,
and the sphere of the fixed stars, which incloses all, have the earth
for their centre. The five planets turn about the sun as about their
chief and king, the sun being constantly in the centre of their orbits,
and accompany it in its annual motion round the earth." This system
perfectly accounts for the apparent motions of the planets as seen from
the earth, and is essentially a variation on the Copernican, rather than
on the Ptolemaic system, but it lent itself less readily to future
discoveries. It simply amounts, as far as the solar system is concerned,
to impressing upon all the rest of it the motions of the earth, so as to
leave the latter at rest; and were the sun only as large with respect
to the earth as it seems, were the planets really smaller than the moon,
and the stars only at a short distance, and smaller than the planets, it
might seem more natural that they should move than the earth; but when
all these suppositions were disproved, the very argument of Tycho Brahe
for the stability of the earth turned the other way, and proved as
incontestably that it moved. In the Copernican system, however, these
questions are of no consequence; if the sun be at rest, this mass makes
no difference; if the earth moves like the planets, their relative size
does not alter anything; and if stars are immovable they may be at any
distance and of any magnitude.
The objections of Tycho Brahe to the earth's motion were: First, that it
was too heavy--we know now, however, that some other planets are
heavier--and that the sun, which he would make move instead, is 340,000
times as heavy. Secondly, that if the earth moved, all loose things
would be carried from east to west; but we have experience of many loose
things being kept by friction on moving bodies, and can conceive how,
all things may be kept by the attraction of the earth under the
influence of its own motion. Thirdly, that he could not imagine that the
earth was turned upside down every day, and that for twelve hours our
heads are downwards.
But the existence of the antipodes overcomes this objection, and shows
that there is no up and down in the universe, but each man calls that
_down_ which is nearer to the centre of the earth than himself.
A variation on Tycho Brahe's system was attempted by one Longomontanus,
who had lived with him for ten years. It consisted in admitting the
diurnal rotation, but not the annual revolution, of the earth; but it
made no progress, and was soon forgotten.
More remarkable than this was the attempt by Descartes in the same
direction, namely, to hold the principles of Copernicus, and yet to
teach the immobility of the earth. His idea of immobility was however
very different from that of Tycho Brahe, or of any one else, and would
only be called so by those who were bound to believe it at all costs.
His Theory of Vortices, as it is called, will be best given in his own
words as contained in his _Les Principes de la Philosophie_, third part,
chap. xxvi., entitled, "That the earth is at rest in its heaven, which
does not prevent its being carried along with it, and that it is the
same with all the planets."
"I adhere," he says, "to the hypothesis of Copernicus, because it seems
to me the simplest and clearest. There is no vacuum anywhere in
space.... The heavens are full of a universal liquid substance. This is
an opinion now commonly received among astronomers, because they cannot
see how the phenomena can be explained without it. The substance of the
heavens has the common property of all liquids, that its minutest
particles are easily moved in any direction, and when it happens that
they all move in one way, they necessarily carry with them all the
bodies they surround, and which are not prevented from moving by any
external cause. The matter of the heaven in which the planets are turns
round continually like a vortex, which has the Sun for its centre. The
parts that are nearest the Sun move faster than those that are at a
greater distance; and all the planets, including the earth, remain
always suspended in the same place in the matter of the heaven. And just
as in the turns of rivers, when the water turns back on itself and
twists round in circles, if any twig or light body floats on it, we see
it carry them round, and make them move with it, and even among these
twigs we may see some turning on their own centre, and those that are
nearest to the middle of the vortex moving quicker than those on the
outside; so we may easily imagine it to be with the planets, and this is
all that is necessary to explain the phenomena. The matter that is round
Saturn takes about thirty years to run its circle; that which surrounds
Jupiter carries it and its satellites round in twelve years, and so
on.... The satellites are carried round their primaries by smaller
vortices.... The earth is not sustained by columns, nor suspended in the
air by ropes, but it is environed on all sides by a very liquid heaven.
It is at rest, and has no propulsion or motion, since we do not perceive
any in it. This does not prevent it being carried round by its heaven,
and following its motion without moving itself, just as a vessel which
is not moved by winds or oars, and is not retained by anchors, remains
in repose in the middle of the sea, although the flood of the great
mass of water carries it insensibly with it. Like the earth, the planets
remain at rest in the region of heaven where each one is found.
Copernicus made no difficulty in allowing that the earth moves. Tycho,
to whom this opinion seemed absurd and unworthy of common sense, wished
to correct him, but the earth has far more motion in his hypothesis than
in that of Copernicus."
[Illustration: FIG. 22.--DESCARTES' THEORY OF VORTICES.]
Such is the celebrated theory of vortices. The comparison of the
rotation of the earth and planets and their revolution round the sun to
the turning of small portions of a rapid stream, may contain an idea yet
destined to be developed to account for these motions; but as used by
Descartes it is a mere playing upon words admirably adapted to secure
the concurrence of all parties; those who believed in the motion of the
earth seeing that it did not interfere with their ideas in the least,
and those who believed in its stability being gratified to find some way
by which they might still cling to that belief and yet adopt the new
ideas. This was its purpose, and that purpose it well served; but as a
philosophical speculation it was worthless. When former astronomers
declared that any planet moved, whether it were the earth or any other,
they had no idea of attraction, but supposed the planet fixed to a
sphere; this sphere moving and carrying the planet with it was what they
meant by the planet moving: the theory of vortices merely substituted a
liquid for a solid sphere, with this disadvantage, that if the planet
were fixed to a solid moving sphere, it _must_ move; if only placed in a
liquid one, that liquid might pass it if it did not have motion of its
own.
[Illustration: FIG. 23.--VORTICES OF THE STARS.]
A variation on Descartes' system of vortices was proposed in the
eighteenth century, which supposed that the sun, instead of being fixed
in the centre of the system, itself circulated round another centre,
carrying Mercury with it. This motion of the sun was intented to explain
the changes of magnitude of its disc as seen from the earth, and the
diurnal and annual variations in its motion, without discarding its
circular path.
[Illustration: FIG. 24.--VARIATION OF DESCARTES' THEORY.]
We have thus noticed all the chief astronomical systems that have at any
time been entertained by astronomers. They one and all have given way
before the universally acknowledged truth about which there is no longer
any dispute. Systems are not now matters of opinion or theory. We speak
of facts as certain as any that can be ascertained in any branch of
knowledge. We have much to learn, but what we have settled as the basis
of our knowledge will never more be altered as far as we can see.
Of course there have been always fantastic fancies put forth about the
solar system, but they are more amusing than instructive. Some have said
that there is no sun, moon, or stars, but that they are reflections from
an immense light under the earth. Some savage races say that the moon
when decreasing breaks up into stars, and is renewed each month by a
creative act. The Indians used to say that it was full of nectar which
the gods ate up when it waned, and which grew again when it waxed. The
Brahmins placed the earth in the centre, and said that the stars moved
like fishes in a sea of liquid. They counted nine planets, of which two
are invisible dragons which cause eclipses; which, since they happen in
various parts of the zodiac, show that these dragons revolve like the
rest. They said the sun was nearer than the moon, perhaps because it is
hotter and brighter. Berosus the Chaldean gave a very original
explanation of the phases and eclipses of the moon. He said it had one
side bright, and the other side just the colour of the sky, and in
turning it represented the different colours to us.
Before concluding this chapter we may notice what information we possess
as to the origin of the names by which the planets are known. These
names have not always been given to them, and date only from the time
when the poets began to associate the Grecian mythology with astronomy.
The earlier names had reference rather to their several characters,
although there appear to have been among every people two sets of names
applied to them.
The earliest Greek names referred to their various degrees of
brilliancy: thus Saturn, which is not easily distinguished, was called
Phenon, or _that which appears_; Jupiter was named Phaeton, _the
brilliant_; Mars was Pysois, or _flame-coloured_; Mercury, Stilbon, _the
sparkling_; Venus, Phosphorus; and Lucifer, _the light-bearer_. They
called the latter also Calliste, _the most beautiful_. It was also known
then as now under the appellations of the morning star and evening star,
indicating its special position.
With the ancient Accadians, the planets had similar names, among others.
Thus, "Mars was sometimes called _the vanishing star_, in allusion to
its recession from the earth, and Jupiter the _planet of the ecliptic_,
from its neighbourhood to the latter" (Sayce). The name of Mars raises
the interesting question as to whether they had noticed its phases as
well as its movements--especially when, with reference to Venus, it is
recorded in the "Observations of Bel," that "it rises, and in its orbit
duly grows in size." They had also a rather confusing system of
nomenclature by naming each planet after the star that it happened to be
the nearest to at any point of its course round the ecliptic.
Among less cultivated nations also the same practice held, as with the
natives of South America, whose name for the sun is a word meaning _it
brings the day_; for the moon, _it brings the night_; and for Venus, _it
announces the day_.
But even among the Eastern nations, from whom the Greeks and Romans
borrowed their astronomical systems, it soon became a practice to
associate these planets with the names of the several divinities they
worshipped. This was perhaps natural from the adoration they paid to the
celestial luminaries themselves on account of their real or supposed
influence on terrestrial affairs; and, moreover, as time went on, and
heroes had appeared, and they had to find them dwelling-places in the
heavens, they would naturally associate them with one or other of the
most brilliant and remarkable luminaries, to which they might suppose
them translated. Beyond these general remarks, only conjectures can be
made why any particular divinity should among the Greeks be connected
with the several planets as we now know them. Such conjectures as the
following we may make. Thus Jupiter, the largest, would take first rank,
and be called after the name of the chief divinity. The soft and
sympathising Venus--appearing at the twilight--would well denote the
evening star. Mars would receive its name from its red appearance,
naturally suggesting carnage and the god of war. Saturn, or Kronos, the
god of time, is personified by the slow and almost imperceptible motion
of that remote planet. While Mercury, the fiery and quick god of thieves
and commerce, is well matched with the hide-and-seek planet which so
seldom can be seen, and moves so rapidly.
These were the only planets known to the ancients, and were indeed all
that could be discovered without a telescope. If the ancient Babylonians
possessed telescopes, as has been conjectured from their speaking, as we
have noticed above, of the increase of the size of Venus, and from the
finding a crystal lens among the ruins of Nineveh, they did not use them
for this purpose.
The other planets now known have a far shorter history. Uranus was
discovered by Sir William Herschel on the 13th of March, 1781, and was
at first taken for a comet. Herschel proposed to call it Georgium Sidus,
after King George III. Lalande suggested it should be named Herschel,
after its discoverer, and it bore this name for some time. Afterwards
the names, Neptune, Astroea, Cybele, and Uranus were successively
proposed, and the latter, the suggestion of Bode, was ultimately
adopted. It is the name of the most ancient of the gods, connected with
the then most modern of planets in point of discovery, though also most
ancient in formation, if recent theories be correct. Neptune, as
everybody knows, was calculated into existence, if one may so speak, by
Adams and Leverrier independently, and was first seen, in the quarter
indicated, by Dr. Galle at Berlin, in September, 1846, and by universal
consent it received the name it now bears.
There are now also known a long series of what are called minor planets,
all circulating between Mars and Jupiter, with their irregular orbits
inextricably mingled together. Their discovery was led to in a
remarkable manner. It was observed that the distances of the several
planets might approximately be expressed by the terms of a certain
mathematical series, if one term was supplied between Mars and
Jupiter--a fact known by the name of Bode's law. When the new planet,
Uranus, was found to obey this law, the feeling was so strong that there
must be something to represent this missing term, that strong efforts
were made to discover it, which led to success, and several, whose names
are derived from the minor gods and goddesses, are now well known.
All these planets, like the signs of the zodiac, are indicated by
astronomers by certain symbols, which, as they derive their form from
the names or nature of the planets, may properly here be explained. The
sign of Neptune is [symbol: neptune], representing the trident of the
sea; for Uranus [symbol: uranus], which is the first letter of Herschel
with a little globe below; [symbol: saturn] is the sickle of time, or
Saturn; [symbol: jupiter] is the representation of the first letter of
Zeus or Jupiter; [symbol: mars] is the lance and buckler of Mars;
[symbol: venus] the mirror of Venus; [symbol: mercury] the wand of
Mercury; [symbol: sun] the sun's disc; and [symbol: moon] the crescent
of the moon.
[Illustration: PLATE IX.--THE SOLAR SYSTEM.]
The more modern discoveries have, of course, been all made by means of
the telescope, and a few words on the history of its discovery may fitly
close this chapter.
According to Olbers, a concave and convex lens were first used in
combination, to render objects less distant in appearance, in the year
1606. In that year the children of one Jean Lippershey, an optician of
Middelburg, in Zealand, were playing with his lenses, and happened to
hold one before the other to look at a distant clock. Their great
surprise in seeing how near it seemed attracted their father's
attention, and he made several experiments with them, at last fixing
them as in the modern telescope--in draw tubes. On the 2nd of October,
1606, he made a petition to the States-General of Holland for a patent.
The aldermen, however, saw no advantage in it, as you could only look
with one eye instead of two. They refused the patent, and though the
discovery was soon found of value, Lippershey reaped no benefit.
Galileo was the first to apply the telescope to astronomical
observations. He did not have it made in Holland, but constructed it
himself on Lippershey's principle. This was in 1609. Its magnifying
power was at first 4, and he afterwards increased it to 7, and then to
30. With this he discovered the phases of Venus, the spots on the sun,
the four satellites of Jupiter, and the mountains of the moon.
[Illustration: PLATE X.--THE DISCOVERY OF THE TELESCOPE.]
Kepler, in 1611, made the first astronomical telescope with two concave
glasses.
Huyghens increased the magnifying power successively to 48, 50, and 92,
and discovered Saturn's ring and his satellite No. 4.
Cassini, the first director of the Paris Observatory, brought it to 150,
aided by Auzout Campani of Rome, and Rives of London. He observed the
rotation of Jupiter (1665), that of Venus and Mars (1666), the fifth and
third satellites of Saturn (1671), and afterwards the two nearer ones
(1684); the other satellites of this planet were discovered, the sixth
and seventh, by Sir William Herschel (1789), and the eighth by Bond and
Lasel (1848).
We may add here that the satellites of Uranus were discovered, six by
Herschel from 1790 to 1794, and two by Lassel in 1851, the latter also
discovering Neptune's satellite in 1847.
The rotation of Saturn was discovered by Herschel in 1789, and that of
Mercury by Schroeter in 1800.
The earliest telescopes which were reflectors were made by Gregory in
1663 and Newton in 1672. The greatest instruments of our century are
that of Herschel, which magnifies 3,000 times, and Lord Rosse's,
magnifying 6,000 times, the Foucault telescope at Marseilles, of 4,000,
the reflector at Melbourne, of 7,000, and the Newall refractor.
[Illustration: PLATE XI.--THE FOUNDATION OF PARIS OBSERVATORY.]
The exact knowledge of the heavens, which makes so grand a feature in
modern science, is due, however, not only to the existence of
instruments, but also to the establishment of observatories especially
devoted to their use. The first astronomical observatory that was
constructed was that at Paris. In 1667 Colbert submitted the designs of
it to Louis XIV., and four years afterwards it was completed. The
Greenwich Observatory was established in 1676, that of Berlin in 1710,
and that of St. Petersburg in 1725. Since then numerous others have been
erected, private as well as public, in all parts of the world, and no
night passes without numerous observations being taken as part of the
ordinary duty of the astronomers attached to them.