With the reëstablishment of the belief in a spherical earth we find men again making conjectures about its size, though there is no evidence that attempts were made in Christendom (as in the Moslem world) actually to measure the circumference. In the ninth century John Scot Erigena gave, from Martianus Capella, a full explanation of the famous Eratosthenic measurements.[232] An unknown author of the ninth or tenth century of a work on geometry often attributed to Gerbert also explained, from Capella, Eratosthenes’ method of measuring a degree;[233] and the eleventh-century mathematician Hermann of Reichenau[234] had learned (possibly from Macrobius) how the length of a degree could be ascertained from observations of the pole star. His result, 700 stades, was the same as that of Eratosthenes, a fact which alone indicates that he did not himself undertake any measurement. Thus we see that as a result of the Platonic movement between the ninth and eleventh centuries knowledge of one of the most magnificent achievements of classical geographical investigation had been revived. _ZONES AND THE ANTIPODES_ ZONES Most Greek thinkers had agreed in dividing the earth’s surface into five zones, though they differed as to whether or not the equatorial zone was habitable. By Ptolemy’s time the discovery of countries in the heart of the tropical regions and possibly beyond had exploded the old idea of an equatorial ocean and fiery belt around the middle of the globe. Unfortunately the broader regional knowledge which had been at Ptolemy’s disposal was lost in the Middle Ages, and older views reappeared. The maps of the period show us the encircling ocean in which Homer had believed, and nearly all writers of the patristic age thought that Africa has a very limited extension toward the south.[235] Beyond Africa, they said, lies an equatorial ocean and an equatorial zone uninhabitable on account of heat. THE ANTIPODES