o different gear arrangements can be used to create this ratio. First, one may simply use a more intricate combination of gears, as Archimedes did in his mechanical sphere. The second solution is one of the greatest innovations in Greek engineering; the development and incorporation of a differential gear. In addition to having been the first mechanized globe, Archimedes' sphere became a model for later Greek astronomers. For example, Posidonios of Rhodes, a contemporary of Cicero, built a mechanical globe based on Archimedes' sphere. Members of the school of Posidonios created a device to compute the positions of the sun and the moonwhat we now call "The Antikythera Mechanism." Challenged by the same, mechanical difficulty Archimedes faced in representing the synodic month, these scientists developed the first differential gear to solve the problem. Archaeological evidence suggests that after the Antikythera Mechanism was lost in a shipwreck, the differential gear essentially disappeared from western knowledge until 1575, when it reappeared in a globe clock designed by Jobst Brgi. The differential gear later became a critical component of the cotton gin, a late 18th century invention that marked the beginning of the industrial revolution. However, devices such a the Antikythera Mechanism were quite rare. The celestial sphere was the most common form of celestial representation, prompting a number of structural modifications. Because of the difficulty in imagining the position of the earth within a solid representation of the heavens, the celestial globe assumed a more skeletal appearance over time. This new model of the heavens, the armillary sphere, quickly began to replace the more ambiguous celestial globe. However, the method of locating celestial bodies remained the same. Greek astronomers continued to use an ecliptical system for specifying the position of the stars and planets. To understand how this system works it is first nec...
