Astronomical Images : Cosmological spheres and orbs (the order of spheres in the Ptolemaic worldview)

Johannes Sacrobosco

Astronomical Images

<p style='text-align: justify;'>Very little is known about Johannes Sacrobosco except that he was probably British, taught astronomy at Paris University, and died there in the second quarter of the thirteenth century. <i>Sphaera mundi</i>, his major work, was an extraordinarily popular astronomical textbook for several generations. Manuscripts of it circulated through all the main European centres of learning. It was first published in 1472 in Ferrara, and went through dozens of editions up to the mid-seventeenth century. In introducing the Sphere of the World, Sacrobosco held that the Sphere was divided by substance and by accident. This figure shows the division of the sphere by substance. (By division by accident, Sacrobosco meant the division of the World according to its orientation relative to the poles of the World, from the perspective of people dwelling at different latitudes.) According to Sacrobosco, the Sphere was divided by substance into nine spheres. The outermost of these was called the Prime Mover, or <i>primum mobile</i>. Inside this sphere was the sphere of the fixed stars, indicated here by the circle featuring asterisk-like symbols. Inside the sphere of the fixed stars were the seven spheres of the planets, with Saturn having the largest sphere and the Moon having the smallest. In this figure, each of the spheres of the planets is labelled with the symbol for the planet. After this figure, Sacrobosco went on to contrast the aethereal region, composed of the spheres described above, with the elementary region, depicted at the centre of this figure. The aethereal region was made of the single Element of the quintessence and was immutable, while the elementary region was composed of the four Elements ' Earth at the centre, surrounded by Water, Air and Fire in turn ' and was subject to change. Interestingly, the Earth region at the centre of this figure resembles a T-O map, a traditional representation of the terrestrial world borrowed from cartography. The table below the concentric circles in this figure provided a guide to the symbols that would be used in this and other figures in the work. As well as giving the symbols for the signs of the zodiac, which were shown in the sphere of the <i>primum mobile</i>, and the symbols for the planets, this table gave the symbols for astrological features ' the aspects, i.e. the angle between planets in a horoscope, and the head and tail of the dragon, i.e., the points where the Moon's orbit crosses the ecliptic ' which were not used in the concentric circles figure.</p>