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Laputan Logic*
Fanciful. Preposterous. Absurd.
Sky Disc

Posted on Friday 10 March 2006

A handsome object to be sure, the Nebra Sky Disc.

Excavated illegally in 1999 from a mound in eastern Germany the item was recovered by police in 2002 in a sting operation in which the item was to be sold off to a private collector for around US$400,000.

Being made of metal (bronze [1] inlaid with gold), a precise dating has been impossible to obtain although the patina and patterns of corrosion indicate that the item is likely to be very old and not a modern forgery. Assuming the account provided by the looters is accurate the object was discovered next to various bronze weapons which have been independently dated to 1600 BC or earlier.

But herein lies the rub. When the artefact's discoverers were convicted of looting an ancient site their appeal hinged on the argument that the disc really was a modern fake and therefore it was legal to attempt to sell it (presumably they were happier to go with a charge of attempted fraud rather than illegal antiquities trading).

The fact is that whether or not this item is genuine, nothing like it of this age has even been found in Germany, or indeed the world. If it really is 3600 years old, it will be the oldest depiction of a star map in the world. Identifying many of the stars has been difficult however experts have agreed that the cluster of seven starts perched between the sun and moon represents the Pleiades constellation.

Various theories have attempted to explain the disc's possible use in ancient astronomy. Most have viewed it as some kind of calender, the importance of the Pleiades to the map is their connection with the agricultural cycle, the constellation first appears during the early northern spring (planting season) and then disappears from sight at the time of the harvest. Much has also been made of the two strips on the edges which measure an arc of 82 degrees which is also number of degrees along the horizon which the sun travels during the winter solstice at the latitude where the disc was supposedly found.

However the latest, and for me most appealling theory of what the disc may have been used for, is that it represents the configuration of moon and Pleiades that should precede to insertion of a thirteenth month into lunar year and thus synchronise it with the solar year. This is a compelling idea because ancient calenders relied on the lunar cycles for counting the months, however lunar years of twelve moons are eleven days shorter than their solar equivalent and so need to be recalibrated every few years. According to this article, the association of a five day old crescent moon with the Pleiades constellation is an event that only happens every two to three years. Apparently this algorithm for inserting an inter-calary or "leap" month was also used in the Babylonian calender as documented in cuneiform wriiten nearly a thousand years later in a clay tablet known as the MUL.APIN.

More on lunar calenders here and here.

Note 1 - the bright green colouration of the disc is from an oxide called verdigris (Greek green) caused by the action of acetic acide on copper. It is thought that the disc originally was a vivid purple colour an effect which was achieved by rubbing the surface with rotten egg.

UPDATE: alun who is a specialist in this very subject of archeoastronomy makes some very interesting points about the problems with Hansen's theory.
The latest astronomer to work on the disc is Ralph Hansen from Hamburg who said: “I wanted to explain the thickness of the crescent on the sky disc of Nebra because it is not a new moon phase.” ... [but] If Hansen is right, the disc is irrelevant to his theory. Hansen started by looking for a specific moon phase. There's two assumptions in this. One is that the phase is accurately depicted on the disc. The other is that the phase can also be accurately identified when observing the moon. It's not the phase that I'd look for. Over the course of two years the Moon would pass by the Pleiades (in various phases) twenty-five times. The easiest phase to observe would be first sighting of the New Moon passing by the Pleiades, and here's my first problem: this method would work just as well. It would be out of step with Hansen's method, but no more inaccurate. It's a bit like the difference between an hourly bus service which leaves once an hour on the hour and one that leaves once an hour at five minutes past the hour.

Another issue is that the Moon doesn't just pass the Pleiades. It travels through the whole zodiac over twenty-nine days. What happens if, like me, you don't think the seven rivet cluster is the Pleiades? Let's say it's Praesepe in Cancer. Once again because we're talking about fixed cycles it makes no difference to the accuracy. The intercalary months would fall in different years, so this system would be out of step with Hansen's model, but no less accurate.

What you have then is a use for the disc which works just as well if the Moon phase is wrong and whatever the cluster is. Hansen's method doesn't tell us much about the disc, but says a lot about how intelligent modern astronomers are. Is knowing when to insert an extra month really an astronomical problem? I don't think so. It's a social problem, and that means that astronomical methods are inappropriate.
In the comments to his post he was asked what he thought the disc might have actually been used for.
That's difficult to answer. There might not ever be enough data to make more than a rough guess. If it dates from the Bronze Age, then I think it is most likely to be a symbol of power and prestige. It's common for magicians / shamans to wear symbols showing where their power comes from like totem animals or stars. I think this would have been a very intricate thing to make, so only a successful shaman could have worn it. This doesn't mean there isn't a complex story to explain why certain features are on the disc, but I can't see how we can find such a story from the disc alone.

But it does look similar to a part of a Siberian shamanic drum. Additionally it looks nothing like the other dramatic finds from the region which are the Golden Hats.

The reason the German team examining the disc think it's thousands of years old rather than hundreds of years old is down to an analysis of the corrosion of the surface. At the moment the team seem to be finding astronomical features in the disc that they want to see. I don't know enough about corrosion to say whether the chemists are doing the same. I would like it to be genuine, but that's not enough.

If the disc has been properly excavated then it would be easier to say things about it. Unfortunately the German team are left with the testimony of the looters. I don't envy their task in trying to prove the age of the disc.
One of the experts called by the defence in the appeal hearing, Professor Peter Schauer from Regensburg University, suggested that the disc was either a fake or possibly a shamanistic ritual object from Siberia at most only a few centuries old.