The goal of archaeoastronomical field surveys is to assess the astronomical potential of a set of archaeological or material remains. Often we are interested in structural alignments, asking where they point to on the horizon and what rises or sets there, or would have risen or set there at some point in the past. Sometimes we are interested in the interplay of sunlight and shadow. We might be interested to know, for example, the particular times of the year and day when sunlight suddenly streamed into the dark interior of a prehistoric tomb such as Newgrange in Ireland, or when streaks of sunlight or shadow fell in distinctive ways across the spiral rock carvings on Fajada Butte in Chaco Canyon.
It is, of course, possible to go to an archaeological site and simply observe what happens, for example, at a sunrise or sunset. Being there in this way has the advantage that you can assess the visual impact of a phenomenon first-hand and in context, and maybe spot alignments or light-and-shadow effects that might otherwise have been missed. For those lucky enough to be able to revisit a place on many different occasions, special effects only visible at certain times may suddenly show up. Some important discoveries have certainly been made in this way, including the famous midwinter sunrise alignment at Newgrange. However, there are dangers. An obvious one is that a site and its landscape may have altered significantly since earlier times: a monument may have been constructed and reconstructed in several stages or may have suffered damage more recently. Because of this, effects that are spotted now may have appeared rather different, if they appeared at all, in the past. A more subtle danger arises because people are inevitably drawn to a site on what they perceive as special days, such as the solstices. If you spot an eye-catching effect on a day such as this, it is easy to forget that equally eye-catching effects might also be present on other days, thereby decreasing the likelihood that the solstitial effects were actually intentional and meaningful to the original builders. Yet another problem is that the rising and setting position of the celestial bodies may have been different then from now. How great a problem this is depends upon the astronomical body or phenomenon in question, the precision of the alignment or effect, and the time span that has passed since it was (ostensibly) set up.
For professional work, a set of methods and techniques has been established by archaeoastronomers over the years. These form an essential part of the broader study of archaeoastronomy, just as excavation and other field techniques form a central part of archaeology as a whole.
For assessing alignments, the standard procedure is to determine the azimuth and altitude of the relevant point or range of points on the horizon using an instrument such as a theodolite or a magnetic compass and clinometer, depending upon the accuracy required. From this we are able to determine the declination, which provides an indication of what rises and sets there, or would have risen or set there at any given era in the past. In particular, it is not necessary to wait for a particular day such as the solstice in order to determine where the sun would have risen and set on that day, or on any other day in the year. Depending upon the accuracy required, it may also be possible to determine horizon data directly from large-scale topographic maps or by analyzing digital topographic data using computer software such as Geographical Information Systems (GIS). This approach is also useful in helping to overcome practical problems such as the obscuration of the horizon by modern buildings or trees, or even the bad weather that might beset a survey. There is also great potential for using computer technology to combine three-dimensional reconstructions of sites, monuments, and landscapes with visualizations of the whole sky; this is a technique that remains to be fully exploited.
For light-and-shadow effects, various forms of simulation are possible, including three-dimentional computer reconstructions. For example, at the time of writing, a simulation of a “sun dagger” that crosses one of the spiral petroglyphs at Chaco Canyon, where the user can set the day of the year and the time of day, can be viewed in the cultural astronomy gallery at the Adler Planetarium in Chicago.
It is important to be rigorous in presenting the results of field surveys so that others can objectively assess the evidence. A photograph of a pretty sunset in a horizon notch behind a standing monument is generally of little value because it is unquantitative: there can often be great flexibility in choosing a suitable spot for taking the photograph, and one often has only the photographer’s word as to when the picture was taken. Scaled horizon diagrams and associated site plans are as essential to the archaeoastronomer as carefully drawn and scaled excavation plans are to the archaeologist.
A vital, though less tangible, aspect of field procedure is selection of data. The mere existence of an astronomical alignment does not prove that it was intentional or meaningful in the past. This means that we must pay attention (some would say, strict attention) to selecting data fairly. It proves nothing to go to a site and, wittingly or unwittingly, choose the alignments that look promising astronomically while ignoring the rest. Selection criteria form an essential part of the methodology that underlies all archaeoastro-nomical fieldwork.
