Solar Anomaly and Planetary Displays in the Antikythera Mechanism

Abstract

Recent scholarship has greatly improved our understanding of the structure and function of the Antikythera mechanism.1 One unresolved issue is whether the mechanism included a display of the variability (or anomaly) in the angular motion of the Sun. In Price’s original reconstruction, the motion of the Sun was treated only in the mean. A single pointer indicated the day of the year in the Egyptian calendar and the position of the uniformly-moving mean Sun in the zodiac. Of course, the actual position of the Sun can depart from that of the mean Sun by up to 2° at some times of year (up to 2°23′ in Hipparchos’s solar theory). Wright seems to have been the first to propose that the mechanism treated the solar anomaly.2 And since the recognition that the mechanism includes a representation of the Moon’s variable motion,3 probably based on an epicycle model like that of Hipparchos, a number of similar proposals for the solar anomaly have been made (although no gear work for the Sun survives except a simple set of gears that produced a uniform angular motion). Generally these proposals involve separate pointers for the mean and true Sun. We will show that the mechanism did, indeed, treat the solar anomaly, but in a more economical fashion. A single pointer sufficed to give the date in the Egyptian year (indicative of the mean Sun) and the place of the true Sun in the zodiac. A second unresolved issue is the nature of the mechanism’s planetary displays. The inscription on the front cover appears to include detailed references to the synodic behaviour of Aphrodite (Venus).4 Some scholars have therefore proposed that the mechanism gave a complete, kinematic display of the motions of all five known planets around the zodiac. Freeth, Jones, Steele and Bitsakis, for example, wrote, “It seems likely that the Antikythera Mechanism also displayed some or even all of the five planets known in ancient times, but there is considerable debate about this”.5 Michael Wright had made a similar proposal and, in a mechanical tour de force, built a working model of the Antikythera mechanism that included pointers for the zodiacal positions of the five planets, along with the Sun and Moon, all with co-axial movement.6 While this proves that such a design is technically possible, it does not, of course, demonstrate that this is what the builder of Antikythera mechanism actually did (as Wright himself freely grants). It is important to remember that no gear