Displaying astronomical alignments in academic papers

Segesta alignment
Astronomical alignment at Segesta

There’s a couple of papers which have come out recently which use different techniques for indicating astronomical alignments at archaelogical sites. The image above is one I put together for a poster to show why horizon altitude is important as well as azimuth. It’s quite tight, so it’d be no good if you wanted to see where sunrise was in midsummer for instance, and charting the paths of astronomical bodies over a site is a problem. By and large you can treat a site as a small flat area, so there’s not usually any cartographic problems in accounting for the curvature of the earth. The sky in contrast is very curved over every archaeological site, so how to you display that in a paper?

The Megalithic Portal put me on to an interesting article published in Information Visualization: A Sky Dome visualisation for identification of astronomical orientations by Georg Zotti. The abstract includes:

This paper presents a novel diagram combining archaeological maps with a folded-apart, flattened view of the whole sky, showing the local horizon and the daily paths of the Sun, Moon and brighter stars. By use of this diagram, interesting groupings of astronomical orientation directions, for example, to certain sunrise and sunset points could be identified, which were evidently used to mark certain days of the year.

Unfortunately Information Visualization isn’t a journal archaeologists get, and it costs $30 to download the paper. What I can talk about though is a conference paper on his own site: A Sky Dome Visualisation for Identification of Astronomical Orientations, which includes in the abstract:

This paper presents a novel diagram combining archaeological maps with a folded-apart, flattened view of the whole sky, showing the local horizon and the daily paths of sun, moon and brighter stars. By use of this diagram, interesting groupings of astronomical orientation directions, e.g. to certain sunrise and sunset points could be identified, which were evidently used to mark certain days of the year.

The two look as though they’re likely to be similar.

The idea is actually rather clever and I’ll go through a very simplified version of a diagram based on his method.

zotti-1

I’ll start with the blue circle. That’s the horizon around the site. Anything on that blue line is at a horizon altitude of zero degrees. The orange circle, that’s the zenith, the point of the sky directly overhead. That looks weird but in the projection it makes sense. I’ll try to explain.

If you want a similar example think about the Mercator projection of the Earth. The point at the very top of the map, where the North Pole is everywhere along that line at the top. The place, which is one point, has been distorted in the projection – but that doesn’t matter if we know how to read the map. Similarly the orange circle marks 90 degrees from the horizon. If you look west to the horizon and move your gaze up then, on the diagram above, you move from nine o’clock on the blue circle along the black spoke to the orange line. When you reach the point directly overhead you’re on the orange circle. The reason the orange circle goes around the whole diagram is that whatever direction you look in, if you move your gaze up you’ll reach that zenith point, so on this diagram the zenith has to be visible in all directions.

This is quite a handy way of drawing the sky because it’s easy to draw in the horizon. For instance if there’s a mountain to the south of the site you can shade in the sky as far as the height of the mountain. You can also draw paths through the sky. The red circle is a star that just grazes the northern horizon and culminates in the south at 45 degrees. It’s a circumpolar star. Other stars have other paths and Zotti’s paper is full of examples.

I’m quite enthusiastic about the paper from an astronomical perspective. Alas, I’m never going to use the technique, nor is anyone else bar a few people in very special circumstances because the technqiue doesn’t work. The green patch in the centre is where you would draw your archaeological plan, so below is mine.

zotti-3

I’ve put in two temples with parallel alignments. Or at least I thought they were parallel alignment when I drew them. If you look to see where they point to, one temple faces 80 degrees and the other 100 degrees. It’s a very simple diagram so why wasn’t this spotted as a problem?

In his paper Zotti demonstrates the technique with a very different sort of monument, a circulare Neolithic enclosure. The presumed viewing point is at the centre of the diagram and the alignments are radial plotted from the centre of the plan. Naturally his archaeological plan works wonderfully. If however you have an archaeological site with more than one building, or even one with one building which isn’t radial then the projection in the centre of the plan isn’t going to work. To get it to work you’d either need an infinitely horizon circle or else shrink the scale of your archaeological plan to a point, either of which knock out the whole point of the paper.

It’s a shame because in many ways it’s an elegant system, but it seems to be totally inadequate for general archaeological use. This is an example of why I’m wary when I hear the word interdisciplinary. If this is broadly the same as the paper in Information Visualisation then I doubt that they ran it by an archaeologist. If the work is going to be of any use it has to be astronomically sound and archaeologically useful. Simply being acceptable in one discipline isn’t good enough.

Another person tackling the problem of displaying several astronomical alignments in one diagram is M Wilson whose Bronze Age astronomy: new survey, new horizons is the Project Gallery in this issue’s Antiquity. The solution is a lot simpler, significant points on the horizon are marked. In this case the images look crowded in places, but that’s mainly because of the nature of the Project Gallery where you have to be concise and there is a lot of information to convey.

As for the content of the paper, I’m not convinced. Again the brevity means he doesn’t have the space to fully make his case. My problem is that it appears that points on the horizon are arbitrarily chosen to fit significant dates. There are equally interesting points on the horizon that are skipped over unlabelled, so are points being cherry-picked to fit an idea? Also is there archaeological evidence that these sight lines were used? A row of stones in a line as as good a sign as you’re going to get in this period that something interesting lies in at least one direction, so you can certainly argue for some alignments, but what of the unmarked alignments? Frustratingly ethnographic evidence suggests a lot of astronomical activity will leave no archaeological trace. It’s not whether or not sky observation occurred, it’s how much you can verify.

What is impressive is that M Wilson is putting his work in front of the eyes of archaeologists on the site of the UK’s top archaeology journal. There’s a lack of affiliation, so I don’t know if he’s an amateur or not, but if he is then he’s going exactly the right way of getting the attention of archaeologists.

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