The Antikythera Mechanism: The Construction of the Metonic Pointer and the Back Plate Spirals

Abstract

The Antikythera Mechanism, the ancient mechanical computer of unique technological sophistication dated to the 2nd century B.C., was housed in a wooden case and had dials at its front and back side as well as lots of inscriptions covering its front and back doors. In its back side there were two main dials, both in the form of spirals. About one third of the upper dial is nowadays preserved on one fragment (Fragment B). The lower dial is preserved in three fragments (Fragments A, E and F), forming about half of the initial spiral. Only the pointer of the upper dial has partially survived, with a few remains of the mechanism that supported and rotated it. These remains, however, give enough information and allow for the original form of the pointer mechanism to be reconstructed. The reconstruction described in this paper fits perfectly the description of the pointer mechanism that has survived in the inscriptions of the back door of the Antikythera Mechanism. The type of the two spirals was also investigated: were they Archimedean spirals or Half Circles spirals? Our results show that both spirals were Half Circles spirals, drawn from two different centres. The unwanted eccentricity in the reading of the divisions that would be produced from the pointer being placed at one of the centres is proven to be ingeniously avoided by the mechanic with the appropriate drawing of the cell divisions.