Editing Antikythera mechanism (section)

===Operation===
On the front face of the mechanism, there is a fixed ring dial representing the [[ecliptic]], the twelve [[zodiac]]al signs marked off with equal 30-degree sectors. This matched with the Babylonian custom of assigning one twelfth of the ecliptic to each zodiac sign equally, even though the [[constellation]] boundaries were variable. Outside that dial is another ring which is rotatable, marked off with the months and days of the Sothic [[Egyptian calendar]], twelve months of 30 days plus five [[Intercalation (timekeeping)|intercalary days]]. The months are marked with the Egyptian names for the months transcribed into the [[Greek alphabet]]. The first task is to rotate the Egyptian calendar ring to match the current zodiac points. The Egyptian calendar ignored leap days, so it advanced through a full zodiac sign in about 120 years.<ref name=freeth-12/>

The mechanism was operated by turning a small hand crank (now lost) which was linked via a [[crown gear]] to the largest gear, the four-spoked gear visible on the front of fragment A, gear b1. This moved the date pointer on the front dial, which would be set to the correct Egyptian calendar day. The year is not selectable, so it is necessary to know the year currently set, or by looking up the cycles indicated by the various calendar cycle indicators on the back in the Babylonian [[ephemeris]] tables for the day of the year currently set, since most of the calendar cycles are not synchronous with the year. The crank moves the date pointer about 78 days per full rotation, so hitting a particular day on the dial would be easily possible if the mechanism were in good working condition. The action of turning the hand crank would also cause all interlocked gears within the mechanism to rotate, resulting in the simultaneous calculation of the position of the Sun and Moon, the [[Lunar phase|moon phase]], [[eclipse]], and calendar cycles, and perhaps the locations of [[List of gravitationally rounded objects of the Solar System#Planets|planets]].<ref name=freeth-09/>

The operator also had to be aware of the position of the spiral dial pointers on the two large dials on the back. The pointer had a "follower" that tracked the spiral incisions in the metal as the dials incorporated four and five full rotations of the pointers. When a pointer reached the terminal month location at either end of the spiral, the pointer's follower had to be manually moved to the other end of the spiral before proceeding further.<ref name=freeth-06 />{{rp|10}}