The Antikythera Mechanism: Improved CT images

The Antikythera Mechanism is an astronomical calculator made by the ancient Greeks in 200 BC that was capable of indicating the timing and characteristics of eclipses, positions of the sun and moon, including the moon’s phase and quite possibly the location of the five visible planets that can be seen with the naked eye. It consisted of at least 30 gears that made predictions displayed on 12 individual dials, all designed and built by the Greeks based on Babylonian knowledge. In its current state, it survives as 82 severely corroded and calcified fragments of various sizes, with its larger fragments on display in the National Archaeological Museum in Athens.

Since its recovery just off the Greek island of Antikythera from a Roman shipwreck at the beginning of the 20th century, it has since been researched extensively using various imaging modalities, mainly utilising X-rays. The direct analysis of the mechanism in addition to historical accounts of life at the time has led to our further understanding of how it works and why it was even made!

HOW THE MECHANISM WORKS

By analysing ancient Babylonian astronomical recordings, the ancient Greeks were able to establish mathematical relationships that were subsequently ‘programmed’ into gears. The user would have been able to manipulate the time input by turning the driving gear. With each rotation, the subsequent gears within the mechanism would work together to perform arithmetic calculations leading to its various outputs. The accuracy of the device turned out to be remarkable, it had managed to correct for an ‘anomalies’ in lunar orbit. By using a ‘pin and slot’ mechanism where two pairs of gears worked together, with one pinned slightly off centre. This meant that it could drive the next gear at different rates mirroring the elliptical orbit of the moon. A unique feature of the mechanism.

In addition to the sun and moon pointers, indicating their position, the moon also had a black and white ball that indicated phase. There is also evidence for another five pointers, representing the visible planets that can be seen by the naked eye, Mercury, Venus, Mars, Jupiter and Saturn. However, this remains unconfirmed due to an absence of any relevant gearing, belonging perhaps to a part of the mechanism which did not survive or has not yet been recovered.

It is predicted that the device was certainly used and very relevant to Greek life. Eclipses during this time may have been regarded as important events or omens of things to come. Some more obvious indications come from the fact that the device had a dial just for indicating the timing of various PanHellenic games such as the Olympics which, after all, are indicated by astronomical events.

X-RAY CT IMAGES

In 2005, an international team of inquisitive scientists and engineers assembled in Athens to give us more by delivery sophisticated modern imaging techniques, believing that there was still a great deal of secrets to be unlocked surrounding the mechanism and its capabilities. A key imaging technique that was deployed was X-ray Computed Tomography (CT), producing detailed 3D images of the insides of the device! These X-ray CT images went on to yield greater insight and with rigorous analysis came further interpretation of text and understanding of gear functions.

X-ray CT, which is mainly known for its use vast use in the clinic, works by taking conventional 2D X-rays while an object rotates and then computes these into 3D images, processes known as ‘capturing projections’ and ‘reconstruction’ respectively.

Given the team’s allotted time to capture their images, the number of fragments and the time to complete a single scan, it was decided that reconstruction would best be left to process later. During this CT frenzy, a difficulty presented itself regarding the iconic main fragment. Due to its larger dimensions and surprisingly lesser-than-expected content of bronze, the parameters set to image this crucial fragment had to be fine-tuned and repeated until finally, the 6th scan was captured. It promised the greatest resolution as it pushed the CT system to its limits, totalling nearly 3000 projections for a full rotation. Unfortunately, when it came to reconstruction, the 3D image appeared blurry.

IMPROVED CT IMAGES

At University College London, a team came together in an endeavour to recover the intended image quality. At first, it was suspected that the object might have moved during the scan. However, a quick comparison of two projections 360 degrees apart confirmed that this was not so and thus left another hypothesis; that some of the projections were not captured. When each projection is separated by an incremental 0.12 degrees it is no easy task to visually look through them to identify where projections are missing. A statistical computer programme was able to compare neighbouring projections, identifying that 25 projections were missing. Filling in these projections with blank images resulted in subsequent projections to be displaced into their correct angular views without introducing false information. The end result was the sharpest and most detailed images of the principal fragment of the Antikythera mechanism.

A significant feature of the Antikythera mechanism in comparison to other heritage objects is that it had a manual inscribed onto it intended for the user. In total it is estimated that there were 15,000 characters on the mechanism, 3000 of which have been transcribed. As a lot of these letters are about 1.6mm tall, it can be incredibly difficult to read them, hence the need for high-resolution scans. Tony Freeth, a prominent scholar on the workings of the mechanism, used these improved CT images to further refine his previous transcriptions by clearing up certain ambiguities in the text.

Following the publication on improved X-ray CT images of the principal fragment, the raw and processed images have become freely available in the open source domain, and others are invited to analyse and build upon them. With various groups interested in building models, especially structurally authentic ones, hopefully, these restored higher resolution scans will aide these individuals in their endeavour to replicate this ancient device.

Improved X-ray computed tomography reconstruction of the largest fragment of the Antikythera Mechanism, an ancient Greek astronomical calculator Pakzad A, Iacoviello F, Ramsey A, Speller R, Griffiths J, Freeth T, Gibson A (2018) PLOS ONE.

X-Ray CT Data Available: https://doi.org/10.7910/DVN/UCXZWU