Thanks go to Orbis Quintus and Laputan Logic for posting on the Nebra Sky Disc recently. I haven’t been reading any blogs recently and only browsed yesterday to get some sites for Carnivalesque. The latest news from Nebra has spurred me into writing a paper on the disc which was completed in first draft form in about three hours. The final version will take some polishing because it’s a bit of a tightrope act.
On the one hand I do strongly admire the engagement with the public on the disc. They’ve also been rapid in getting out a detailed description for the academics. After getting the disc they published a detailed description in just a couple of years, which is quick by archaeological standards. When you add in that they didn’t have the original excavation data, it’s a Herculean effort. On the other hand I strongly disagree with their astronomical conclusions. I’ll hope to be talking about this at the National Astronomy Meeting in April and I’m keen to show why I to strongly disagree without impugning the other excellent work they’ve done. I think the recent news that the disc was used to insert an extra lunar month is, sadly, an excellent example of one of these pieces of work which is rigorous, thoughtful, imaginative and also completely irrelevant. Like most other theories it relies on this cluster of seven being the Pleiades.
The Pleiades on the Star Disc of Nebra?
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.” He argues that when the Moon passes the Pleiades in the phase shown, it’s time to insert an intercalary month. Extra months are necessary every two or three years in a lunar calendar if you want your calendar to stay broadly in step with the Sun. There are various problems with Hansen’s method which I don’t have time to write up today, but there’s one big problem which might best demonstrate why I’m sceptical. 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.
This gets a brief mention in the paper, most of it tackles the question about whether these seven rivets are likely to the Pleiades. I think it’s a possible explanation, but there are others which I’ll talk about when I have time to go into depth.
I don’t know where it’ll go yet. The Journal for the History of Astronomy is attractive because it’s been mentioned on HASTRO-L that it’s now available in part from the ADS Abstract Service. You can now pick up excellent papers like Archaeoastronomical analysis of Assyrian and Babylonian monuments: methodological issues [PDF] by Stanislaw Iwaniszewski.
