Complex sentence: By contrast, the Earth (where objects seem to have motion only when moved) and things on it were composed of substances that were heavy and whose natural state was rest--thus the Earth was a "lazy" body that was not readily moved.
Split into simpler sentences: By contrast, the Earth (where objects seem to have motion only when moved) and things on it were composed of substances that were heavy and whose natural state was rest. Consequently, the Earth was considered to be a "lazy" body that was not readily moved.