It is told that the Roman Colonia Marciana Ulpia Traiana Thamugadi, that is Timgad in Algeria, had been oriented to the sunrise on the day of Trajan's birthday, that is 18 September 100 AD. Here we use software CalSKY to investigate the sunrise azimuth and compare it to the direction of the decumanus of the Roman town.
The sunrise azimuth (without considering the local natural horizon and the effect of the atmospheric refraction) was of 86.4° on 100 A, and it is of 87.0° on 2019. So, a difference of half a degree exists. The same we see for the altitude of the sun. Let us consider the azimuth 86.4° and compare to the direction of the decumanus. Here we use satellite images. Of course a local measurement is necessary for further detailed studies. From a Bing Map we have the following Screenshot 1. Screenshot 1: Satellite image courtesy Bing Maps. The azimuth of the decumanus is of 85°.
In the screenshot you can see the measurement of the angle made by means of GIMP pair of compasses. The azimuth of the decumanus, from true North, is of 85°. As a conclusion, using CalSKY, we have that it is possible that Timgad had been founded on 18 September 100 AD. On that day the sunrise azimuth was of about 86.5°. We have a difference of about 1.5°. However, it is possible to reduce this difference, including the effect of atmospheric refraction. Let us assume that the azimuth could be affected of 0.5°, then the sunrise azimuth can be estimated as 86°, and the difference reduced to a degree. This is a result which is far better than that given in [1]. Of course, the result here proposed is given for an astronomical horizon. Using Google Earth, we can see a simulated horizon without relevant hills in the direction of the decumanus. I would like to conclude in the following manner. Assuming the site of Timgad laying on a flat surface, which is the same of that observed in Screenshot 1, we could use data given above to estimate the uncertainty of the directions of the decumani, planned by means of a centuriation method. It could be of about one degree. Actually we do not know if the surveyor used the first ray of the sun he perceived or the full sun to determine the direction of the decumanus. It is meaningless to give conclusions concerning the days of the foundation, based on smaller uncertainties of the direction of decumani. Of course GPS investigations at the archaeological sites can given us astonishing results, but the precision of the modern measurements cannot be transferred to the ancient methods, without a careful evaluation. In any case, it seems to me that it is impossible to determine the specific year of the foundation by comparing the azimuths of the decumani to the sunrise azimuths. And this is clearly shown by the case of Timgad. To change the difference of half a degree we need two thousand years. Therefore, from a sunrise azimuth we cannot determine the year of foundation.
This content is AI-processed based on open access ArXiv data.