Sky & Telescope - November 2023 - 22

FRONT AND EXPLODED VIEWS: © 2021 TONY FREETH (2)
ASTRONOMY HISTORY by Tony Flanders
A Clockwork Cos
I
The Antikythera Mechanism is a remarkable meld of mathematics and precision engineering.
n 1900 sponge divers discovered an ancient shipwreck
off the tiny Greek island of Antikythera. Over the
course of the following year, in the first major underwater
archaeology ever undertaken, they salvaged numerous
objects from the wreck. Among them were what turned out
to be the remains of a device that computed and displayed
the positions of the Sun, Moon, and planets. Coins recovered
from the same shipwreck indicate that it sank around
60 BC, meaning that the Antikythera Mechanism, as it's
now known, contains the oldest gears ever found - at least
30, and originally there were probably more than twice that
number. It's by far the most complex machine known to have
been built before the late Middle Ages, when it was surpassed
by astronomically themed clocks.
The Mechanism attracted little notice at first, possibly
because its innards were hidden inside a single, encrusted
lump of corroded bronze. The first photograph, taken in
1902, shows four fragments with many eye-catching gears.
The original lump may well have broken apart when it was
exposed to air since most of the original bronze had been
replaced by fragile corrosion products during the device's two
millennia under water. In addition to several large pieces,
there are now many tiny fragments resulting either from
intentional dissection or accident. Fortunately, most of the
remnants have preserved their original shape and internal
structure surprisingly well.
Scholars made considerable progress understanding the
Mechanism over the years, but their conclusions remained
tentative until three rounds of X-ray imaging revealed
numerous details inside the fragments. The last of these
sessions in 2005 used advanced tomography techniques
that proved successful beyond any reasonable expectation.
Together with state-of-the-art optical imaging, it revealed a
wealth of inscriptions, some on the surface and others buried
deep inside the fragments. The multidisciplinary team assembled
to perform the 2005 study published their results in a
2006 paper in Nature, which included a detailed description
of much of the original gearwork together with the device's
overall layout and function. All the team's major conclusions
are now universally accepted.
It turns out the Mechanism included two bronze sheets,
which presumably protected it in transit. These sheets are
filled with writing - what Tony Freeth (University College
London), the lead author of the paper, refers to as its
" owner's manual. " Much of the text is too degraded to read,
22 NOVEMBER 2023 * SKY & TELESCOPE
but the part that has been deciphered gives a pretty clear idea
how the device worked. The latest reconstruction (presented
below, right, and on page 26) by Freeth and others appeared
in a 2021 article in the journal Scientific Reports.
Calendars and Eclipses
The Antikythera Mechanism was roughly 33 cm (13 inches)
high, 18 cm wide, and perhaps 10 cm thick. It had a knob or
crank on the side that advanced the date by about 78 days per
rotation. The user, in theory, could go indefinitely far into
the future or past, though this would get tedious at almost
500 cranks per century.
Most of the back plate was occupied by two spiral grooves.
Each spiral was marked off in increments corresponding to
synodic months (new Moon to new Moon). The upper and
lower spirals showed 235 months (the Metonic period) and
223 months (the saros period), respectively.
A stylus mounted on a gear-driven arm tracked outward
along each spiral as the date advanced. The instructions note
that when the stylus reaches the end of the groove, the operator
should lift it out and re-insert it at the beginning, much
Sky & Telescope - November 2023

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