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

The main dials of the back face of the Antikythera Mechanism have partially survived together with the pointer of the upper dial and a few remains of the mechanism that supported and rotated it. The reconstruction of this mechanism, described in this article, fits perfectly its description in the Mechanism’s inscriptions. Our results also show that both spirals were Half Circles spirals, drawn from two different centres. The unwanted eccentricity that would be produced from the pointer’s being placed at one of the centres is proven to have been ingeniously avoided with the appropriate drawing of the cell divisions.