Sentence:  The old imaginary <WSD>antiearth</WSD> (άγτιχϑωγ) of the Pythagoreans does not belong to the sphere of these conjectures. It and the Earth were supposed to have a parallel concentric movement; it was an idea devised to spare the Earth, which was supposed to perform around the central fire a planetary revolution in 24 hours, from having also to execute a movement of rotation, and, indead, represented no doubt the opposite hemisphere, or the antipodal half of our planet. 
Meaning of the word marked with <WSD> tags:  A hypothetical twin planet to earth, proposed by the Pythagoreans, which cannot be seen from earth.