The Antikythera mechanism

First analogue computer replica

described by D Zimmermann

Back in 1900 the wreck of the Antikythera was discovered after having sunk in antiquity. One of the items recovered was left largely unnoticed for many further decades. Eventually it was realized that this mechanism, for that is what it was, was of a complexity not seen until the middle ages and the birth of the mechanical astronomical clock. It was, in fact, the first analogue computer designed to calculate astronomical positions.

It has been described as “beautiful” and “more valuable than the Mona Lisa” by Professor Michael Edmunds of Cardiff University.

The device is displayed in the National Archaeological Museum in Athens. In recent years much research has been done and replicas made.

A year or more ago, Don Unwin, who likes to describe himself as a “maker of things” started to produce a replica Antikythera mechanism which is now approaching completion. He first heard about the device 25 years earlier and has since collected every piece of published material.

Don is well known in model engineering circles for making models of all types plus many other items ranging from caravans to cine cameras. He has made a series of replicas of important horological and scientific instruments that are now housed in the Whipple Museum of Science in Cambridge. These include the only replica of Harrison’s H3 clock, the de Dondi planetarium, the Richard of Wallingford 14th Century clock and the 18th Century Grand Orrery.

It is thought that the mechanism was made in around 150 – 100 BC, possibly in Corinth. Such is the excellence of the work and brilliance of the design that it seems likely that there were forerunners, from the place where Archimedes lived some time earlier.

When Don Unwin first saw the device he was struck by the similarities to his de Dondi planetarium, the original of which dates from as recently as 1347.

The mechanism, as found, contains 30 gears with seven gears lost, although it is thought that the complete version might have contained 72. A date would have been entered via a crank and the locations of the Sun, Moon and planets  calculated.

The mechanism has three main dials, one on the front, and two on the back. The front dial has two concentric scales. The outer ring is marked off with Egyptian calendar. Inside this, there is a second dial marked with the Greek signs of the Zodiac and divided into degrees. The calendar dial can be moved to compensate for the effect of the extra quarter day in the solar year by turning the scale backwards one day every four years. The front dial probably carried at least three hands, one showing the date, and two others showing the positions of the Sun and the Moon. The front dial also includes a second mechanism with a spherical model of the Moon that displays the lunar phase.

The mechanism may have had indicators for the five planets known to the Greeks. None of the gearing for such planetary mechanisms survives. Also the front dial includes a parapegma, which was used to mark the rising and setting of specific stars. The upper back dial is in the form of a spiral, with 47 divisions per turn, displaying the 235 months of the 19 year Metonic cycle, important in fixing calendars..

The lower back dial is also in the form of a spiral, with 225 divisions and a smaller dial, used to predict solar and lunar eclipses.

Another dial displayed the less-scientific cycles of games such as the Olympics.

We can’t wait to see Don’s replica.