Astronomical Myths Part 10

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 circ.u.mscribed the celestial spheres. That of the moon first, then that of Mercury, in which several constellations, as the Lyre, Ca.s.siopeia, 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 Ca.s.siopeia 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, Princ.i.p.atus, 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!

[Ill.u.s.tration: FIG. 16.--HEAVENS OF THE MIDDLE AGES.]

Another such richly illuminated ma.n.u.script in the library at Ghent, ent.i.tled Liber Floridus, contains a drawing similar to this under the t.i.tle _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 ma.n.u.script 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 pa.s.s 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 ent.i.tled _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...o...b..um Caelestium_, in which he laid down the principles of his system. We will quote the pa.s.sage 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 princ.i.p.al 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 s.p.a.ce.

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 pa.s.sing from the Greeks to the Romans, and from them to the middle ages, the doctrine of Aristarchus underwent a curious modification, a.s.similating 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 a.s.signed an immovable or rapidly-moving home--whether G.o.d 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 circ.u.mference, they appear to us to go _up_; because we so speak of the s.p.a.ce 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 ma.s.s 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, accomplis.h.i.+ng 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 a.s.sert 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 a.s.semblage 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 a.s.sign 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 circ.u.mstances, 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 ill.u.s.trious 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 ill.u.s.trious men urged me, in the interest of mathematics, to overcome my repugnance and to let the fruit of my labours see the light. They a.s.sured 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.

[Ill.u.s.tration: 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...o...b..t 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 pa.s.sed away tranquilly on the 23rd of May, 1543.

[Ill.u.s.tration: PLATE VIII.--DEATH OF COPERNICUS.]

The Copernican system required, however, establis.h.i.+ng 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:--