---
title: "Antikythera mechanism"
chunk: 5/12
source: "https://en.wikipedia.org/wiki/Antikythera_mechanism"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T03:38:28.502947+00:00"
instance: "kb-cron"
---

The Saros dial is the main lower spiral dial on the rear of the mechanism. The Saros cycle is 18 years and 11+1⁄3 days long (6585.333... days), which is very close to 223 synodic months (6585.3211 days). It is defined as the cycle of repetition of the positions required to cause solar and lunar eclipses, and therefore, it could be used to predict them—not only the month, but the day and time of day. The cycle is approximately 8 hours longer than an integer number of days. Translated into global spin, that means an eclipse occurs not only eight hours later, but one-third of a rotation farther to the west. Glyphs in 51 of the 223 synodic month cells of the dial specify the occurrence of 38 lunar and 27 solar eclipses. Some of the abbreviations in the glyphs read:

Σ = ΣΕΛΗΝΗ ("Selene", Moon)
Η = ΗΛΙΟΣ ("Helios", Sun)
H\M = ΗΜΕΡΑΣ ("Hemeras", of the day)
ω\ρ = ωρα ("hora", hour)
N\Y = ΝΥΚΤΟΣ ("Nuktos", of the night)
The glyphs show whether the designated eclipse is solar or lunar, and give the day of the month and hour. Solar eclipses may not be visible at any given point, and lunar eclipses are visible only if the Moon is above the horizon at the appointed hour. In addition, the inner lines at the cardinal points of the Saros dial indicate the start of a new full moon cycle. Based on the distribution of the times of the eclipses, it has been argued the start-up date of the Saros dial was shortly after the astronomical new moon of 28 April 205 BC.
The Exeligmos dial is the secondary lower dial on the rear of the mechanism. The exeligmos cycle is a 54-year triple Saros cycle that is 19,756 days long. Since the length of the Saros cycle is to a third of a day (namely, 6,585 days plus 8 hours), a full exeligmos cycle returns the counting to an integral number of days, as reflected in the inscriptions. The labels on its three divisions are:

Blank or o ? (representing the number zero, assumed, not yet observed)
H (number 8) means add 8 hours to the time mentioned in the display
Iϛ (number 16) means add 16 hours to the time mentioned in the display
Thus the dial pointer indicates how many hours must be added to the glyph times of the Saros dial in order to calculate the exact eclipse times.

=== Doors ===

The mechanism has a wooden casing with a front and a back door, both containing inscriptions. The back door appears to be the 'instruction manual'. On fragment 19, it is written "76 years, 19 years" representing the Callippic and Metonic cycles. Also written is "223" for the Saros cycle. On fragment E, it is written "on the spiral subdivisions 235" referring to the Metonic dial.