At all events, so far as we know, only one method was employed by the Greeks for determining the size of the earth. This consisted of finding on the same day of the year the meridian altitudes of the sun at two places supposed to be on the same meridian of longitude, the distance between which was known through itineraries. The angle between the two meridian altitudes was then assumed to bear the same relation to the circumference of the heavens as the distance between the two points of observation bore to the circumference of the earth. Cleomedes[57] and Martianus Capella[58] described how Eratosthenes carried out such observations in Egypt. The figure determined by Eratosthenes is surprisingly accurate. Whether the stade used by him was 157.50[59] or 168[60] meters, as different modern scholars contend, the circumference according to his estimate would be 39,375 or 42,336 kilometers. In either case the error is seen to be very slight, the true circumference of the earth being about 40,000 kilometers. _THE DISTRIBUTION OF HABITABLE REGIONS; ZONES; THE DISTRIBUTION OF LAND AND WATER_ We see, then, that the writers of antiquity whose opinions were destined to mold the thought of the medieval period believed that the earth is a sphere, immovably fixed in the center of the universe. We must now examine their theories regarding the distribution of phenomena on the surface of the globe and the interaction of these phenomena. Of prime importance were their views concerning the distribution of habitable areas of land, but these were so closely bound up with the theory of climatic zones that it is absolutely necessary to understand what this theory was before going further, even though the subject of zones might more properly be included in the study of the atmosphere. ZONES