Complex sentence: The original prototype was designed to track the motions of the planets as a compass tracks magnetic north; the free floating needle was intended to track the movements of the heavenly bodies such as Venus or Mars instead of tracking north and south.
Split into simpler sentences: The original prototype was designed to tract the motions of the planets as a compass tracks magnetic north. I.E. the free floating needle was intended to track the movements of the heavenly bodies such as Venus or Mars instead of tracking north and south.