Here, as often elsewhere, Dante identifies the heavens with their Movers, and speaks as if the spheres themselves were moved by love and longing. In the tenth heaven, or Empyrean, this longing was completely fulfilled, hence this heaven was for ever at rest.[755]
VIII.
MEDIÆVAL AND MODERN VIEWS
OF THE UNIVERSE.
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.