Counting Months and Years: The Upper Back Dial of the Antikythera Mechanism 1

Abstract

The surviving piece of the upper back dial itself constitutes most of Fragment B, as seen in Figures 3 and 4. At axis N, its centre, there is the stub of an arbor but no wheel; at axis O, the centre of its subsidiary dial, there is an arbor bearing the wreck of a sin- gle wheel and what is probably a tiny frag- ment of the frame plate on which most of the wheelwork was planted.6 I suggest that there must have been a further intermedi- ate arbor, carrying two wheels to complete a compound train transmitting motion from a second wheel on axis N to the wheel on O. By analogy with the arrangement of the train behind the lower back dial, we would expect the arbor to have pivots working in holes both in the frame plate and in the dial plate, but the remains of these parts