Let us now gather into one picture the details of mediæval astronomy which Dante has taught us, and try to see the universe as it presented itself to men’s minds in his days.
Motionless, at the centre of all things, is the earth, a massive globe 20,400 miles in circumference.[756] Only the northern part of one hemisphere is inhabited; the rest is covered by ocean. It is surrounded by air, into which rise the exhalations from land and sea which cause winds, rain, etc. Above the height to which these rise, and above the tops of the highest mountains, the air sweeps uninterruptedly round the earth from east to west, sharing the movement of the skies. Above this again is the region of pure fire: all fire on Earth’s surface tends to rise into it, and hot dry vapours ascending here burst into flame, and are seen from Earth as shooting stars and comets.
Bounding the sphere of fire is the lowest of the celestial spheres, that of the moon; and here we pass abruptly from the world of the four elements, transitory, changing, to the celestial world, eternal, changeless. Sphere rises above sphere, carrying moon and sun and planets, until we reach the vast orb in which are set all the stars.
All these bodies are composed of a mysterious imponderable substance called æther; all are perfectly spherical, and all their movements are circular. The diurnal rotation which we see is caused by an outermost sphere which contains no star: it communicates its own motion to all the spheres within it. The zodiacal motions of sun, moon, and planets, and the precession of the equinoxes, are caused by the proper motion of each sphere, rotating from west to east. The oscillating movements of the five planets and the varying speed of the moon are caused by small rotating spheres called epicycles, on which planets and moon are fixed, the epicycles in their turn being attached to the large spheres.
The distance from the earth to the nearest heavenly body, which is the moon, is 64⅛ times the radius of the earth; the distance of the fixed stars, more than 20,000 times the radius of Earth.
Mercury, the smallest of the heavenly bodies, is only 232 miles in diameter,[757] the moon and Venus are both rather more than a quarter the size of the earth, Mars is a little larger than Earth; next in order of size come the stars, from the faintest visible increasing up to those of the second magnitude; Saturn and Jupiter are much larger than Earth, and the fifteen brightest (first-magnitude) stars larger still; the sun is the largest body in the whole universe, having a diameter 5½ times Earth’s diameter. It is immensely bright and hot, is the source of light to the whole universe, and on Earth is the chief giver of light, heat, and life, and the chief measure of time.
The moon and stars are also useful in this way to Earth, and upon her are poured all the influences of the celestial spheres, the sun, moon, and planets: these are the instruments by which every event on Earth is brought to pass.
It is their nature to move in circles, as it is Earth’s nature to remain motionless at the centre of the universe; but all their motions are controlled by the wills of immortal angels, which are subject to the Will of the First Mover.
The sensible universe is eternal, but it is finite and measurable, and of definite form,[758] being bounded by a sphere. Beyond this in every direction is infinity.
Such was the mediæval conception of the Universe, as we gather from learned mediæval books, and as we find it vividly pictured in Dante’s glowing verse. How do we conceive it now?
The heavens are the same to our eyes as to Dante’s. The blue dome of the sky still arches over our heads, and in it we may see the stars shine out after the sun has set; we may see Venus making all the orient smile at early dawn, and the light of the moon not yet risen paling the stars of Scorpio. We may also see with our bodily eyes what was only visible to the keen eyes of Dante’s mind, the rising sun travelling to our left, and the southern stars circling round the southern pole, while the stars of the north sink below the ocean floor.[759]
But to one who knows anything of modern astronomy the ideas suggested are widely different. Most striking is the absence of that sharp contrast between Earth and heaven which our forefathers felt. Earth is not to us small, dark, and inert, made of the basest material and sunk, like the dregs of the Universe, to its lowest depths. Earth is a heavenly body, as the Pythagoreans rightly guessed, a beautiful big planet, shining brightly by reflected sunlight and moving swiftly, like her sister-planets. To Venus she is the brightest star in the sky; to the moon she shows a far larger disc than the moon shows to us, and markings more beautiful and wonderful, brilliant caps of snow at either pole, oceans coloured blue-green, and continents varying in tint, whenever the dazzling white clouds part to reveal her surface.
Earth is the largest planet among her neighbours, Mercury, Venus, and Mars; just as Jupiter is largest among the giant planets—Saturn and the more recently discovered Uranus and Neptune. In the gap between these two groups modern astronomers have discovered hundreds of tiny planets, to which Earth is a monster, for many are only a few miles in diameter. The meteorites which shoot across our view, “startling quiet eyes,” we still regard as a kind of conflagration in our upper atmosphere, but instead of vapours rising from Earth’s surface we recognise them as visitants from inter-planetary space, caught by Earth’s great mass as they pass too near, and flaming up with the heat caused by their sudden rush through the air. They exist in shoals of thousands and millions, some small as pebbles, some like great rocks, but all belonging to our system and pursuing definite paths like planets. The same must be said of comets, which have just as little to do with a hypothetical fire-sphere round Earth, but travel in regular periods, some long, some short; several small ones are seen every year, and occasionally a great splendour like Halley’s draws near to us in its orbit, yet even these seem to consist of very small quantities of matter. Other members of our celestial family are the moons of Mars, Jupiter, and the rest, of which more are still being discovered. The Earth-Moon system is, however, unique in that our satellite is much nearer our own size than the proportionately tiny companions of other planets and the pair must look like a beautiful “double” moving through the stars and continually revolving round one another.
All these bodies are made, not of a mysterious ether, but of something just as wonderful, beautiful, and incomprehensible—the same stuff as Earth. Most probably all developed in long-past ages out of a nebula, or mass of gas which for some unknown reason was intensely hot. Small bodies, like the Moon and Mars, cooled off the most quickly, and have lost or are losing their air and their water; larger bodies, like Jupiter, have still some heat, and are probably in a more or less fluid condition. On the sun an inconceivable heat still rages; everything is in a gaseous state, and this globe which men used to think eternally changeless is the scene of the fiercest turmoil; currents are always ascending and descending, clouds of fire are flung up thousands of miles high, and patches which look black compared with the surrounding surface and indicate a mysterious commotion are constantly forming, developing, and disappearing. The sun is enormously larger than Dante thought, larger than all the planets put together, large enough for our moon’s whole orbit to fit comfortably inside. It is hot enough and brilliant enough to light all the planets: this, which was only a guess of mediæval astronomers, we know as a fact to-day, and the planets which are suitably placed show it to us, when seen in the telescope; Venus, for instance, changing her phase “according as the sun looks upon her,” exactly like the moon. And Earth must show similar phases as seen from Mars.
For Aristarchus was right: the sun is the centre of the planetary movements, including those of the planet Earth.
We saw how Ptolemy’s system hinted at this, in the significant connection between each planet’s movement and the movement of the sun; but a strong argument against it was that if Earth were really moving round the sun we ought to see a yearly displacement in the positions of all the stars. Modern instruments have at last made it possible to perceive this displacement. About a hundred stars, our nearest neighbours, describe a very minute orbit in the sky, which in every case is an exact reflection of the sun’s apparent orbit round the earth. Unless, therefore, we are going to provide all these stars, as well as the planets, with epicycles, which must all revolve in the same period, in the same direction, and in parallel planes, we must conclude that the movement is really Earth’s yearly movement round the sun. These miniature star-orbits also show us the stupendous distances which lie between Earth and the nearest stars, while by far the greater number are too remote to show any displacement at all.
Aristarchus and those other ancient astronomers were also right who thought that Earth was turning on her axis, and thus producing by her own motion the alternation of day and night, and the apparent circling of the stars. When we look now at stars hastening across the sky, they no longer present themselves to our thought as fixed on a sphere with a radius of 20,000 Earth-radii, which is revolving round us at a tremendous speed; we feel that we are looking into space at glowing globes which are at various and almost inconceivable distances from us; also that all those we see, all those that Hipparchus numbered, are but an insignificant fraction of all those now known to exist. All have their own real motions, as well as the apparent motions produced by Earth, but these are in different directions, towards what goal we cannot at present tell. Compared with them, Earth is indeed insignificant, in size and also in brightness, for while she shines with reflected light they glow as the sun does and are his peers.
Nevertheless, with them too we have a feeling of kinship, for analysis of their light shows that they too are made of earth-stuffs: they are not eternal nor changeless, for very many vary constantly in their light, and they are at different stages of development, some appearing very young, while others are beginning to burn dim and low, and some can only be inferred to exist, for they seem to be wholly dark.
More wonderful perhaps than anything else, we find that the force which binds the whole of our sun’s family together is nothing strange and unknown, but the same familiar force of gravity, which the Greeks knew so well but thought only applicable to Earth. Its rule includes the seemingly unruly comets and meteors, and it extends as far as our search can reach, to the uttermost star.
This sense of unity throughout the world, which is always growing stronger, is comforting. And we allow ourselves to wonder whether intelligent life has not developed elsewhere, as well as on this globe that we inhabit. Perhaps each planet that attends our sun is fitted at some period to be the abode of life: surely among the millions of stars many must have attendant planets, and on some of them even now are living, thinking, beings.
Dante’s world was easier to think about than ours. Encompassed by an infinity which gave the imagination free scope, the material universe within was neatly rounded off, as it were, complete, finished. Only in the central spot was any development taking place, and thither were directed all effects of the circling spheres. Our universe is vague, vast, mysterious, without known limits or centre, offering problem after problem to the thinker. Dante’s marvellous moon-substance is simple compared with the baffling nature of our æther; Dante’s Angel-Movers are intelligible compared with the amazing mystery of universal gravitation.
Yet when we lift our eyes from earth to heaven, we share the feeling of our mediæval forefathers, of the ancient Greeks, of the earliest men whenever and wherever they became men. The unerring courses of the stars speak to us, “the unperturbed to the perturbed,” of perfect harmony, untouched by chance or arbitary conflicting wills. Now, however, we do not believe this is because heaven is essentially different from earth: it is because we see there only the grand outlines, only the working of great laws, of a system which includes our own earth. Could we but lose sight of the details here, and view the history of man as we view the stars, the harmony would be as grand.
HYMN TO ZEUS.