In short, Antikythera's user interface is deceptively simple, operated by a hand crank and requiring only minor adjustments from its operator. This conceals the intricacy within, amounting to a complex mathematical model, tracking the movements of planetary bodies and incorporating a series of sub-machanisms to account for the eccentricities of their rotation.
http://blog.wired.com/gadgets/2006/1...thera_con.html