sigs. a6v-a8r Concerning the immobility of the Earth. {The Earth is immobile.} That the Earth is held immobile in the midst of all since it is the heaviest. Its heaviness is seen to be proved thus: every heavy thing tends naturally toward the centre. Now the centre is a point in the middle of the firmament. Therefore, the Earth, since it is heaviest, naturally tends toward that point. Also, whatever is moved from the middle toward the circumference ascends to the heavens. If the Earth were moved from the middle, it would be ascending, which is recognised as impossible. Concerning the total size of the Earth. {The size of the Earth.} The total circuit of the Earth on the authority of the philosophers Ambrose, Theodosius, Macrobius and Eratosthenes is defined as comprising 252,000 stades, by allocating 700 stades to each [degree] of the 360 degrees of the zodiac. Taking an astrolabe on a clear starry night and, sighting the pole through both apertures in the indicator, note the number of degrees in which the indicator is. Then let our measurer of the cosmos proceed directly north until on another clear night, observing the pole as before, the indicator stands a degree higher. After this let the extent of his journey be measured, and it will be found to be 700 stades. Then, given this number of stades for each of 360 degrees, the total circuit of the Earth will be found. From these data the diameter of the Earth can be found thus by the rule for the circle and diameter. Subtract the twenty-second part from the circuit of the Earth, and a third of the remainder - that is, 80,181 stades and a half and third part of one stade - will be the diameter or thickness of the terrestrial orb. Chapter two, concerning the circles from which the material sphere is composed and that supercelestial [sphere] of which this is the image, is understood to be composed. Of these circles some are larger, some smaller, as sense shows. {What the larger circle in the sphere is.} For a great circle in the sphere is one which, described on the surface of the sphere about its centre, divides the sphere into two equal parts, {The lesser circle.} while a small circle is one which, described on the surface of the sphere, divides it not into two equal but into two unequal portions. {The equinoctial circle.} Of the great circles we must first mention the equinoctial. The equinoctial is a circle dividing the sphere into two equal parts and equidistant at its every point from either pole. And it is called 'equinoctial' because, when the Sun crosses it, which happens twice a year, namely, in the beginning of Aries and in the beginning of Libra, there is equinox the World over. Wherefore it is termed the 'equator of day and night', because it makes the artificial day equal to the night. And it is called the 'belt of the first motion'. {First motion of the primum mobile.} Be it understood that the 'first motion' means the movement of the primum mobile, that is, of the ninth sphere or ultimate heaven, which motion is from east through west, returning to east again, which also is called 'rational motion' from resemblance to the rational motion in the microcosm, that is, in man, when thought goes from the Creator through creatures to the Creator and rests there. {The second motion of the firmament and planets.} The second motion is of the firmament and planets contrary to this, from west through east, returning to west again, which movement is called 'irrational' or 'sensual' from resemblance to the movement of the microcosm from things corruptible to the Creator, returning to things corruptible. {Zone of the prime motion.} It is called the 'zone of the first motion' because it divides the primum mobile or ninth sphere into two equal parts and is itself equally distant from the poles of the World. {Artic pole} /It is called either 'septentrional' or 'boreal'./ It is to be noted that the pole which always is visible to us is called 'septentrional', 'Arctic' or 'boreal'. It is called 'septentrional' from septentrio, that is, from Ursa Minor, which is derived from septem and trion, meaning 'ox', because the seven stars in Ursa move slowly in the manner of an ox, since they are near the pole. Alternatively, those seven stars are called septentriones as if septem teriones, because they tread the parts about the pole. 'Arctic' is derived from arthos, which is Ursa Maior, for it is near Ursa Maior. The boreal pole is so called because it is where the wind Boreas comes from. {Antarctic pole.} /Either 'meridional' or 'austral'./ The opposite pole is called 'Antarctic' as opposed to 'Arctic'. It also is called 'meridional' because it is to the south, and it is called 'austral' because it is where the wind Auster comes from. The two fixed points in the firmament are called the 'poles of the World' because they terminate the axis of the sphere and the World is turned on them. One of them is always visible to us, the other always hidden. {Virgil.} Whence Virgil: this vertex is ever above us, but that Dark Styx and deep Manes hold beneath our feet [Virgil, Georgics, Book I, 240].