The device is remarkable for the level of miniaturization and for the complexity of its parts, which is comparable to that of 19th-century clocks. It has more than 30 gears, although Michael Wright (see below) has suggested as many as 72 gears, with teeth formed through equilateral triangles. When a date was entered via a crank (now lost), the mechanism calculated the position of the Sun and Moon or other astronomical information such as the locations of planets. Since the purpose was to position astronomical bodies with respect to the celestial sphere, with reference to the observer's position on the surface of the Earth, the device was based on the geocentric model.
The mechanism has three main dials, one on the front, and two on the back. The front dial has two concentric scales. The outer ring is marked off with the days of the 365-day Egyptian calendar, or the Sothic year, based on the Sothic cycle. Inside this, there is a second dial marked with the Greek signs of the Zodiac and divided into degrees. The calendar dial can be moved to compensate for the effect of the extra quarter day in the solar year (there are 365.2422 days per year) by turning the scale backwards one day every four years. Note that the Julian calendar, the first calendar of the region to contain leap years, was not introduced until about 46 BC, up to a century after the device was said to have been built.
The front dial probably carried at least three hands, one showing the date, and two others showing the positions of the Sun and the Moon. The Moon indicator is adjusted to show the first anomaly of the Moon's orbit. It is reasonable to suppose the Sun indicator had a similar adjustment, but any gearing for this mechanism (if it existed) has been lost. The front dial also includes a second mechanism with a spherical model of the Moon that displays the lunar phase.
There is reference in the inscriptions for the planets Mars and Venus, and it would have certainly been within the capabilities of the maker of this mechanism to include gearing to show their positions. There is some speculation that the mechanism may have had indicators for all the five planets known to the Greeks. None of the gearing for such planetary mechanisms survives, except for one gear otherwise unaccounted for.
Finally, the front dial includes a parapegma, a precursor to the modern day almanac, which was used to mark the rising and setting of specific stars. Each star is thought to be identified by Greek characters which cross reference details inscribed on the mechanism.
The upper back dial is in the form of a spiral, with 47 divisions per turn, displaying the 235 months of the 19-year Metonic cycle. This cycle is important in fixing calendars.
The lower back dial is also in the form of a spiral, with 223 divisions showing the saros; it also has a smaller subsidiary dial which displays the 54 year "triple saros" or exeligmos. (The saros, discovered by the Chaldeans, is a period of approximately 18 years 11 days 8 hours—the length of time between occurrences of a particular eclipse.)
The Antikythera Mechanism Research Project, with experts from Britain, Greece and the United States, detected in July 2008 the word "Olympia" on a bronze dial thought to display the 76-year Callippic cycle, as well as the names of other games in ancient Greece, and probably used to track dates of the ancient Olympic Games. According to BBC news:
The four sectors of the dial are inscribed with a year number and two Panhellenic Games: the "crown" games of Isthmia, Olympia, Nemea, and Pythia; and two lesser games: Naa (held at Dodona) and a second game which has not yet been deciphered.
Hublot recreates the Antikythera Mechanism, in a wristwatch
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That is spectacular.
