The Back Dial and Back Plate Inscriptions

TitleThe Back Dial and Back Plate Inscriptions
Publication TypeJournal Article
Year of Publication2016
AuthorsAnastatsiou, Magdalini, Bitsakis Yanis, Jones Alexander, Steele John M., and Zafeiropoulou Mary
Journal TitleAlmagest
Start Page138
Journal Date05/2016

The rear face of the Mechanism consisted of a rectangular “Back Plate“ dominated by two large spiral dials. The upper five-turn Metonic Dial represented a 235-lunar-month calendrical cycle while the lower four-turn Saros Dial represented a 223-lunar-month eclipse prediction cycle. A subsidiary quadrant “Games“ dial was situated inside the Metonic Dial, and a subsidiary three-sector Exeligmos Dial inside the Saros Dial. Preserved text inscribed around the dials (from the lower right quarter of the plate), probably representing about a quarter of the original inscription, provided further information associated with the predictions of eclipses. This paper describes the reconstruction from the Mechanism’s fragments of the surviving parts of the text on the plate and its dials, giving transcriptions and translations. The Metonic Dial inscriptions imply a calendrical scheme similar to that described by Geminos. It was intended to be a version of the calendar of Corinth as it was practiced either at Corinth itself or in some locality of Epirus. The Games dial shows six competitions, four Panhellenic (Olympics, Pythian, Isthmian, and Nemean) plus Naa (Dodona) and very probably Halieia (Rhodes). On the Saros dial there were probably originally about 50 or 51 month cells with a lunar and/or solar eclipse prediction, each carrying a “glyph“ and an index letter. Predicted eclipse times (in equinoctial hours) on the glyphs were calculated as times of true syzygy according to solar and lunar models that both involved anomaly, with the simple Exeligmos dial extending the predictions over three or more Saros cycles. We are reluctant to base a firm construction date on interpretation of the eclipse cycles. The additional information referred to by index letters from the Saros dial was grouped into paragraphs; that for lunar eclipse prediction probably ran down one side of the plate, and that for solar eclipse prediction down the other. Statements about direction may imply a meteorological aspect by referring to predictions of winds attending the eclipses. Five references to colour and size at eclipse are the only Greco-Roman source known to us that suggests prediction of eclipse colors, and might conceivably be linked with astrology.