Sentence:  They understood not the motion of the eighth <WSD>sphear</WSD> from West to East, and so conceived the longitude of the Stars invariable. 
Meaning of the word marked with <WSD> tags:  Any of the concentric hollow transparent globes formerly believed to rotate around the Earth, and which carried the heavenly bodies; there were originally believed to be eight, and later nine and ten; friction between them was thought to cause a harmonious sound (the "music of the spheres"). (from 14th c.)