In a book entitled _De lineis angulis et figuris_ Grosseteste elaborated some general principles relating to the incidence and reflection of rays from celestial bodies. The _De natura locorum_ is an attempt to show how far these principles may be used to account for various phenomena of the earth’s surface. Grosseteste conceived of celestial rays and influences as emanating in an infinite number of cones, or “pyramids,” as he called them, the apexes of which were the celestial bodies; the longer and more oblique these pyramids, the weaker the effect of the rays upon the earth’s surface and vice versa.[721] Let us see how Robert applied the principle of the pyramids to explain conditions in the equatorial zone, in the southern hemisphere, and in the polar regions. _The Equatorial Zone_ Logically the equatorial zone should be scorched and burnt by the sun because the pyramids are there the shortest and the angles at which the rays reach the earth approach nearest to a right angle. As a matter of fact, Robert had it on the authority of Ptolemy and Avicenna that, whereas the subtropical regions are intensely hot, the subequatorial zone is not only temperate but extremely temperate (_temperatissimus_); indeed, he said, theologians place Paradise under the equator in the Orient. A modification of the principle of the pyramids was therefore necessary. In his readiness to admit such modifications of rules that he had laid down, Robert showed an open-minded and a scientific spirit. In order to allow for the circumstance of a supposedly temperate equatorial region, he stated that the heat received during the daytime must be neutralized by the coolness of the nights, since day and night between the tropics are always approximately the same length, as they are in the latitudes of Europe during spring and autumn only.[722] _The Southern Hemisphere_ The southern hemisphere, on the other hand, Robert thought to be uninhabitable on account of the intense heat of summer and bitter cold of winter. The excessive heat he ascribed to the fact that the eccentricity of the solar orbit around the earth brings the sun no less than five degrees nearer the earth during the southern summer than it approaches during the northern summer.[723] The pyramids, or lines of heat radiation, are therefore shorter and the heat is more intense.[724] Conversely the southern winter must be colder than that of the north because the sun at that season is farthest from the earth. Granting a geocentric universe, this reasoning was sound though its consequences were exaggerated. It is quite true that the earth is nearer the sun in the summer of the southern than in the summer of the northern hemisphere, yet no extreme results flow from this circumstance, and there is no great difference in the amount of heat received by each hemisphere.[725]