The Antikythera mechanism
is by far the most sophisticated piece of technology that survives from
the ancient world. This corroded mass of battered bronze gearwheels
languished at the bottom of the sea for more than 2000 years, before
being salvaged by sponge divers in 1901.
The device was originally a mechanical computer (some people prefer to
say calculator), which used Greek astronomers' state-of-the-art theories
to model the movements of the Sun, Moon and planets in the sky.
Well that's what scholars thought, anyway. But a new paper
on the mechanism, published earlier this year in the Journal of the
History of Astronomy, suggests that they might have got things back to
front. Jim Evans,
an expert in the history of astronomy based at the University of Puget
Sound in Tacoma, Washington, and his colleagues Alan Thorndike and
Christian Carman have made the most accurate measurements yet of the
Antikythera mechanism's zodiac dial, used to display the positions of
celestial bodies in the sky.
Evans described his work at an event held in March at the Getty Villa in
Los Angeles, at which he and I discussed the Antikythera mechanism (see video).
His analysis has various technical implications for the way that the
device displayed information - to be honest when I first heard him speak
I thought it was the kind of thing that only true Antikythera geeks
would get excited by. But when I went through the paper in more detail I
saw this knock-out sentence, right at the end:
"Finally, if the maker of the Antikythera mechanism used gears to
model Babylonian astronomical cycles, and if, as is likely, the
mechanism reflects a craft tradition going back to the time of
Archimedes, this raises the fascinating, but unprovable, possibility
that epicycles and deferents entered Greek astronomy, not because of
natural philosophical considerations, but because some geometer applied a
geometrical image of gearing to a cosmic problem."
theory of epicycles - the idea that celestial bodies moved in small
circles as they traced larger orbits around the Earth - is arguably the
most famous aspect of Greek astronomy. Although often scoffed at, it was
actually very good at explaining the apparent movements of the Sun,
Moon and planets through the sky, and it pretty much defined our view of
the cosmos (see top three pics for various examples) until Kepler came
up with the idea of elliptical orbits in the early 17th century AD.
What Evans and his colleagues are suggesting is that geared devices like
the Antikythera mechanism didn't model this theory after all. They
That's huge. It would give mechanical models a starring role in the
history of astronomy, in other words in the way that we have come to
understand the universe around us. If Evans is right then without models
like the Antikythera mechanism, there would have been no epicycles, and
for 2000 years we would not have seen the cosmos in the way that we
did. Our modern understanding of how the solar system works would
presumably still be the same, but the history of how we reached this
point would be dramatically different.
I've written a feature about this latest work in this week's edition of Nature. But here's a summary of what led Evans and his colleagues to suggest this idea.
First, a bit of background about epicycles. The Greeks generally thought
that the celestial bodies in the solar system - the Sun, Moon and five
known planets - were orbiting Earth. They saw these celestial bodies as
divine, and believed that their orbits must therefore consist of perfect
this isn't what you see when you look at the sky. The Sun and Moon
(because the orbits of the Earth and Moon are actually ellipses, not
circles) appear to speed up and slow down. And the planets (because
they're orbiting the Sun, not the Earth) have a rather inconvenient
habit of changing direction.
To explain this, the Greeks came up with the idea that celestial orbits were made up of different circles
superimposed on one another. For example they reckoned that each planet
traced a small circle - an epicycle - at the same time as moving around
its larger orbit - the deferent. Similar theories of the Sun and Moon's
motion involved superimposing one circle onto another with a slightly
When researchers who had X-rayed the surviving pieces of the Antikythera device published a reconstruction of its workings
in 2006, they noted a crucial piece of gearing that was used to drive
the Moon pointer. A "pin-and-slot" mechanism allowed one gearwheel to
drive another around a slightly different centre, giving an undulating
variation in speed. This pin-and-slot mechanism was itself mounted on a
bigger 9-year turntable, effectively modelling how the orientation of
the Moon's ellipse rotates around Earth.
This seemed to be a lovely demonstration of an epicyclic lunar theory
used by the Greeks, translated into wheels of bronze. This type of
gearing, in which gear wheels ride round on other wheels, is still
described as epicyclic.
the relevant gearing for the Sun and planets does not survive,
researchers assumed that if the mechanism was using epicyclic gearing to
model the motion of the Moon, it was probably doing the same thing for
these other bodies too. The photo on the left shows the epicyclic
gearing that models the motions of the planets in a reconstruction made
by Michael Wright (see how it works in this video).
So here's the new bit. Evans has now shown that the Antikythera
mechanism may not have worked this way after all. He used X-ray images
to accurately measure the divisions on the device's main dial. This dial
has two concentric scales, one showing the 360 degrees of the zodiac,
and one showing the 365 days of the year, so that pointers moving around
it can show both the date, and the position of celestial bodies in the
Just less than a quarter of this dial survives. The 360 zodiac divisions
should of course be very slightly wider than the 365 day divisions. But
Evans found that although evenly spaced, the zodiac divisions in this
surviving portion are actually closer together. To make a full circle,
other parts of the zodiac scale must compensate by being extra widely
This was done on purpose, Evans believes, to model the uneven progress
of the Sun through the sky. Instead of the Sun pointer moving at varying
speed around an equally divided dial, it moved at constant rate around
an unequally divided dial.
Evans' analysis suggests that half of the zodiac dial had extra-narrow
divisions - a "fast zone" - and half had extra-wide divisions - a "slow
zone". This scheme would have modelled the Sun's motion reasonably
accurately and is identical to an arithmetic theory that Babylonian
astronomers used for the Sun, known as System A. The Greeks borrowed
other Babylonian astronomical theories, so it's not a huge stretch to
think that they used this one too.
If Evans is right (and others in the field are taking his suggestion
seriously) then the Antikythera mechanism did not use epicyclic gearing
to model the movement of the Sun after all. It used conventional
geartrains to model much older astronomical theories.
This may therefore be the case for the planets too. Evans thinks that
they were shown on five individual dials, perhaps showing the timings of
events in their cycles rather than their position in the sky - again,
no epicycles required.
As discussed above, the Antikythera mechanism did use epicyclic gearing
to model the varying motion of the Moon. But Evans points out that the
amplitude of variation encoded in the pin-and-slot mechanism is closer
to that used in older arithmetic theories than in the epicyclic theory
used by the Greeks.
He believes that rather than modelling epicycles directly, a mechanic
looking for a way to represent an older, arithmetic theory of the Moon's
motion may have hit upon the idea of using gearwheels mounted on other
wheels to produce the cyclic variation that he was after. In other words
the inventor of epicycles was not an astronomer, but a mechanic.
Once astronomers realised that epicyclic gearing could closely model
what was going on in the sky, they could have borrowed the idea of
superimposed circles, and incorporated it into their own theories of how
the cosmos was actually arranged. The clockwork universe was born.
Not much is known about when and how the idea of epicycles first arose,
but the credit is traditionally given to an astronomer called Apollonius
of Perga who lived in the third century BC. Geared astronomical devices
seem to have arisen at around the same time - although the Antikythera
mechanism itself dates from the second or first century BC, the Roman
author Cicero wrote that Archimedes made one in the third century BC. So
the timing is about right for such machines to have inspired the idea
Over the following centuries, there could have been an ongoing
interaction between mechanics and astronomers as the theory was
developed and refined. "Maybe we need to rethink the connection between
mechanics and astronomy," says Evans. "People think of it as purely one
way, but maybe there was more of an interplay."
If this had happened, wouldn't somebody have written about it somewhere?
Not necessarily, says Evans. He points out that the history of
astronomy has generally been written by philosophers, who would have
downplayed the role of mechanics.
Greek astronomy, he says, combined "a low road of nitty gritty
arithmetical calculations", with "a philosophically-oriented high road"
that was based on aesthetically pleasing geometric theories. "The people
who wrote the history were philosophers of the high road. If there were
the influence of something mechanical, it's not surprising that it
wouldn't be there in the history. The historians emphasised the clean,
the pure, the philosophical."
As Evans admits, it is impossible to prove where the idea of epicycles
came from. But his analysis is fascinating food for thought. And a
reminder, if we needed one, not to take anything about the Antikythera
mechanism for granted.