Skip to Main Content

The Antikythera Mechanism I

With Java animations by Bill Casselman

3. The Sun-Moon Assembly

Images by Bill Casselman.

These two images give a schematic version of De Solla Price'sgeneral gear plan; on the right thecolor picks out the sub-assembly connecting the sun's motion to that of the moon.

The sun marker and the moon marker were driven by the twocentral gears (the moon axis threaded through the sun's),exactly like the hour and minute hands on a modern clock.The train of gears linking the sun's motion to that of themoon can be described by the meshing pattern and thenumbers of teeth.

The sun gear has 64 teeth. It meshes with the smaller of a 38,48 gear pair. The 48 meshes with the smaller of a 24,127 gear pair. The 127 meshes with the 32 teeth of the moon gear. The ratio of angular speeds can then be calculated as

64 48 127 254
-- X -- X --- = --- = 13.36842..
38 24 32 19

which is an excellent approximation of the astronomical ratio 13.368267.. .

Since the sun-moon linkage involves an odd number of meshings,the two gears will turn in opposite directions. For the display tobe realistic the sun and the moon must move the same way. In the device thiswas accomplished by a vertical "contrate" gear (marked A inDe Solla Price's plan) linkingthe sun gear to an identical gear above it, which thus turnedat the same speed but in the opposite direction.

This image can be java-animated. It shows the operation of the Sun-Moon assembly, with a somewhat fanciful simulation of the display. In the actual device, the zodiac constellations were represented by their Greek names ("Libra" and the end of "Virgo" are decipherable in the relic). The constellation-schemata used here are imitations of the more accurate versions in Find the Constellations by H. A. Rey, Houghton-Mifflin Co., Boston, 1988.

Java animation: GO
(by Bill Casselman, University of British Columbia).