The Antikythera Mechanism

Professor Edmunds explained a difficult concept with such skill that most of us understood at least part of it, and some of us, I am sure, most of it! It was a fascinating and absorbing lecture.

The Antikythera Mechanism is a complex geared mechanism that is over 2000 years old. The remains of the device were first discovered in 1902 when archaeologist Valerios Stais noticed a heavily corroded gear wheel amongst artifacts recovered by sponge divers from a sunken Roman cargo ship. It is believed that the ship was en route from the Greek island of Rhodes to Rome when it sank off the island of Antikythera, between Kythera and Crete. The Mechanism is now thought to date from 150 to 100 BC. This is well before the time of Posidonius and it is thought that it could be the work of the great astronomer Hipparchus.

The mechanism, often described as the world's first mechanical computer, was used to calculate and display astronomical cycles. It could have been used to accurately predict lunar and solar eclipses. The mechanism is technically more complex than any known device constructed for at least a millennium afterwards. It is the first known instrument to use geared teeth. It has over 30 gears and indicated the position of the sun and moon and possibly the location of other planets.

In October 2005 the Antikythera Mechanism was imaged in Athens by the Antikythera Mechanism Research Project using a specially modified Venlo* cabinet system manufactured by Shaw Inspection Systems and fitted with a ground breaking 400kV microfocus x-ray source designed and built by X-Tek Systems. One of these superb images is shown to the right. (Article taken with thanks from Shaw Inspection Systems Web Site)

The front of the mechanism is marked with divisions of the Egyptian calendar based on the Sothic cycle. A second inner dial can be adjusted to compensate for leap years. A small spherical moon indicates the Lunar phase. A notch in the mechanism at first puzzled the team investigating the instrument, until they realised it showed the sight irregularity in the moon's orbit. There is evidence of extra gearing which may have operated the five known planets by the Greeks.

The rear of the mechanism has two dials, the upper in a spiral with 47 divisions per turn, showing the 235 months of the 19 year Metonic cycle. The latter is based on the calculations of Meton from Athens, who noticed that 235 lunar months made up almost exactly 19 solar years. Since 12 lunar months are about 11 days short of a solar year, an additional 7 lunar months were added to synchonise the cycle in the years 3,5,8,11,13,16 and 19. The Metonic cycle was extended by the Greek philosopher Callippus to 4x19 year cycles, known as the Callipic cycle. This is represented on a smaller subsidiary dial. Both these dials are important in fixing calendars.

The lower dial is also a spiral with 223 divisions depicting the Saros cycle. The latter is a period of approximately 18 years 11days 8 hours and represents the length of time between occurrences of a particular eclipse. Thus this dial can be used to predict eclipses of the sun and moon. This is an extremely powerful tool in the hands of the educated Greeks over the population as a whole. A smaller subsidiary dial displays the 54 year Triple Saros cycle.

Research into this mechanism is still ongoing as there are many other gear wheesl un-accounted for. A new paper is coming out soon with, we are told, some surprising conclusions - we all look forward to it!

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