CHAPTER VIII.

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.

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.

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!

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.

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.

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.

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.

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.

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.

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.

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."

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.

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.

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.

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.

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.

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.