Sentence:  Directly opposite the oikoumene was the "antichthones," or "opposite land," or "<WSD>antiearth</WSD>," also called the "periocci." It corresponded to the quarter of the globe North America occupies, and its night and day were the reverse of the oikoumene. 
Meaning of the word marked with <WSD> tags:  A hypothetical twin planet to earth, proposed by the Pythagoreans, which cannot be seen from earth.