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ISAW Papers 4 (February, 2012)

The Cosmos in the Antikythera Mechanism

Tony Freeth1 and Alexander Jones2

Abstract: The Antikythera Mechanism is a fragmentarily preserved Hellenistic astronomical machine with bronze gearwheels, made about the second century B.C. In 2005, new data were gathered leading to considerably enhanced knowledge of its functions and the inscriptions on its exterior. However, much of the front of the instrument has remained uncertain due to loss of evidence. We report progress in reading a passage of one inscription that appears to describe the front of the Mechanism as a representation of a Greek geocentric cosmology, portraying the stars, Sun, Moon, and all five planets known in antiquity. Complementing this, we propose a new mechanical reconstruction of planetary gearwork in the Mechanism, incorporating an economical design closely analogous to the previously identified lunar anomaly mechanism, and accounting for much unresolved physical evidence.

1. Introduction

1.1. The Antikythera Mechanism

The Antikythera Mechanism was on board a ship otherwise laden with fine bronze and marble sculpture and glassware, which sank within a few years after 70 BC off the island of Antikythera, between Crete and the Greek mainland.3 The shipwreck site was discovered by Symiote sponge divers in 1900 and salvaged by them, under Greek government supervision, in 1900-1901.4 In 1902 fragments of the Mechanism were noticed among unsorted bronze pieces from the wreck at the National Archaeological Museum in Athens.5

1.2. The Fragments

Fig. 1 Both sides of Fragments A, B and C of the Antikythera Mechanism. ©2005 Antikythera Mechanism Research Project. All rights reserved.

Scientific data produced by Hewlett-Packard Inc. This shows Polynomial Texture Mapping (PTM) with specular enhancement of the three main fragments of the Antikythera Mechanism, Fragment A (top row), Fragment B (bottom left) and Fragment C (bottom right). PTMs enhance surface details, revealing text and features that are difficult to read from photographs. 82 fragments survive, which are probably all from the Mechanism.

When the remains of the Antikythera Mechanism were recovered from the sea, it is very likely that it was in one piece, and surely not more than two or three. There are now 82 separate fragments, all of which probably belonged to the original device. Seven of the largest fragments are labelled from A - G and the remaining smaller fragments from 1 – 75. The two sides of each fragment are designated -1 and -2. For example, A-1 is the familiar view of Fragment A with the four large spokes. This designation does not mean “front” and “back”, since there are many fragments whose orientation is not yet known. It is simply designed to distinguish the two sides of each fragment.

Fragment A is by far the largest fragment and contains twenty-seven of the surviving thirty gears. There is a single additional gear in each of Fragments B, C and D. The fragments are heavily calcified and corroded after nearly two thousand years under water. Much of the material of the fragments appears to consist of bronze corrosion products with very little free metal surviving. Despite two thousand years under water, many of the surfaces of the fragments are rich in detail, showing mechanical features as well as inscriptions, which cover some of the surfaces. The remains of about a dozen gears are visible on the surface and the rest have been identified through X-ray studies.

1.3. Scientific Investigations

There have been three major X-ray studies of the Antikythera Mechanism since the early 1970s.6 In addition, the National Archaeological Museum in Athens has undertaken X-ray studies of some individual fragments. Historically, many of the most important scientific developments have come from X-ray investigations. The most recent scientific data gathering was undertaken in 2005 by the Antikythera Mechanism Research Project (AMRP)—an informal collaboration of academics from the universities of Cardiff, Athens and Thessaloniki; staff at the National Archaeological Museum in Athens; and two high-technology companies, Hewlett-Packard (USA) and X-Tek Systems (UK) (now part of Nikon Metrology).

Two non-destructive investigatory techniques were used: Polynomial Texture Mapping (PTM) to enhance surface details of the fragments and Microfocus X-ray Computed Tomography (X-ray CT) to examine the interiors of the fragments at high resolution.7 PTM enables a sample to be interactively “re-lit” in software to enhance the surface. It has the ability to factor out confusions of colour and texture to reveal the essential form of the surface. This dramatically improves the interpretation of surface details. X-ray CT makes possible the reconstruction of high-resolution 3D X-ray volumes of the fragments. X-ray viewing software, VGStudio Max by Volume Graphics, enables “slices” to be viewed at different angles through the sample. We have found that this is the most useful tool of analysis. In this way, the data in a single plane can be isolated, examined and measured. The software also enables the brightness and contrast to be adjusted. Both PTM and X-ray CT have proved invaluable in studying the Antikythera Mechanism. Though the X-ray CT was initially designed to probe the mechanical structure of the Mechanism, it has also enabled the reading of inscriptions inside the fragments, which are not visible on 2D X-rays. All 82 fragments were subjected to both techniques. Subsequent scientific analysis resulted in a new interpretation of the gears and their functions as well as a marked increase in the number of inscriptions that have been read—many discovered using X-ray CT.8 In recent years, the new data and scientific results have created considerable international research activity focused on the Antikythera Mechanism.

1.4. The Functions of the Antikythera Mechanism

1.4.1. External Architecture

The Antikythera Mechanism was contained in a wooden box, which had bronze Front and Back Covers. A small portion of the wooden box, as well as a wooden sub-frame, survive in Fragments A, F and 14.9 We infer the existence of a wooden sub-frame from our own observations of the X-ray CT data. It appears to have encased all the gears, while the outer box carried the front and back plates. The evidence for the Front Cover is from Fragment G and a number of other small fragments. These establish that the Front Cover had inscriptions facing outwards. The Front Cover may have covered the whole of the front or just the central dial—the evidence appears to be insufficient to settle this issue. The Back Cover appears to have covered the whole of the back dials and to have been fixed to the Mechanism with sliding catches, since our observations of the X-ray CT of Fragment F establish that there was a sliding catch in the bottom right-hand corner of the Back Cover. Evidence for the Back Cover can be found in Fragments A, B, E, F and 19. The Back Cover had inscriptions on its inside face and none that we can find on its outside face.

The front plate was divided into three sections. A central dial system displayed outputs from the Mechanism on a Zodiac Dial, marked with 360º scale divisions and a Calendar Dial, marked with 365 days. The Calendar Dial was designed to be moveable, so that the Mechanism could accommodate the fact that four Egyptian calendar years fall short of four 365.25 day solar years by one day.10 Above and below the dials, were plates covered in inscriptions in the form of a Parapegma (star calendar).11 At the right-hand side of the Mechanism there was an input, and we assume that this was turned by hand with some sort of handle or crank, though only the keyway for the input remains. Beneath the removable Back Cover, there were two major dial systems (top and bottom) in the form of spirals, divided into lunar months, with subsidiary dials inside them.12 The top dial showed a 19-year Metonic calendar, divided into 235 lunar months.13 Inside this dial was a subsidiary dial, showing the 4-year panhellenic games cycle and (conjecturally) a dial showing the 76-year Callippic cycle.14 The bottom dial showed a 223-month eclipse prediction dial, based on the Saros cycle.15 This dial included glyphs that indicated information about the predicted eclipse possibilities, including time of the eclipse.16 Inside this dial was a subsidiary Exeligmos Dial, designed to adjust the eclipse times for successive turns of the Saros Dial.17

The Antikythera Mechanism is an astronomical calculating machine that predicted phenomena involving the Sun, Moon, stars and probably the planets—the latter being the focus of considerable debate and the subject of much of this current study. Our conclusion in this study is that the Antikythera Mechanism almost certainly calculated the motions of all five planets known in ancient times.

1.5. The Cosmos in the Antikythera Mechanism

Improved readings of an inscription that was on the Mechanism's back cover and that described its external features and displays leave little room for doubt that the front display incorporated revolving pointers bearing little spheres to represent all five planets known in antiquity making their apparent motions around the Earth.18