The Sphere of the World

On the Absolute Size of the Earth.

¶ The total circumference of the earth, by the authority of the philosophers Ambrose Theodosius Macrobius A 5th-century Roman provincial and author whose commentary on Cicero’s Dream of Scipio was a primary source for medieval geography and Eratosthenes original: "Euristenis"; Eratosthenes (c. 276–194 BC) was the Greek polymath who first calculated the Earth’s circumference with remarkable accuracy, is defined as containing 252,000 stadia an ancient Greek unit of distance; based on the author's later calculations, this equates to roughly 25,000 to 30,000 miles. This is calculated by assigning 700 stadia to each of the 360 parts of the zodiac The 360 degrees of the celestial circle. For if an astrolabe a portable instrument used by astronomers to determine the position of celestial bodies is taken during the clarity of a starry night, and the pole is sighted through both holes of the sighting arm original: "medicliniu[m]"; the movable sighting rule or alidade on an astrolabe, the number of degrees where the sighting arm stands should be noted. Then, let a "world-measurer" travel directly north from the south until, in another clear night, the pole is sighted as before and the sighting arm stands one degree higher. After this, let the distance of this journey be measured, and it will be found to be 700 stadia. Then, by giving 700 stadia to each of the 360 degrees, the circumference of the earthly globe will be found. From these figures, according to the rule for the circle and diameter, the diameter of the earth can be found in this way: subtract the twenty-second part from the circumference of the earth, and a third part of the remainder—which is 80,181 stadia and a half and a third of one stadium—will be the diameter or the thickness of the earthly globe.

¶ Chapter Two: On the circles from which the material sphere is composed, and that celestial sphere which is understood to be composed by imagining it through this one.

A decorative woodcut initial 'H' with a floral and vine pattern in the background, set within a square frame.

Of these circles, some are greater and some are lesser, as is evident to the senses. For a great circle in a sphere is said to be that which, described on the surface of the sphere above its center, divides the sphere into two equal halves. A lesser circle is that which, described on the surface of the sphere, does not divide it into two equal halves, but into unequal portions. Among the greater circles, we must first speak of the equinoctial the celestial equator. The equinoctial is, therefore, a certain circle dividing the sphere into two equal halves, which is equidistant from both poles in every one of its parts. And it is called "equinoctial" because when the sun passes through it—which happens twice a year, namely at the beginning of Aries The spring equinox, around March 21 and the beginning of Libra The autumnal equinox, around September 23—there is an equinox equal length of day and night across the entire earth. For this reason, it is also called the "equalizer of day and night," because it makes the artificial day equal to the night. It is also called the belt of the first motion original: "cingulus primi motus"; in geocentric cosmology, the "first motion" is the daily rotation of the outermost sphere of the universe, which carries all other spheres with it from east to west. From which it should be known that the first motion...