Archaeoastronomy in South Wales:
Explanation of Table Data
Archaeoastronomical Study of
Prehistoric Sites in South Wales
Part 2: Explanation of Data Tables
by Martin J. Powell
The sites are seperated into categories according to type. In the abscence of clear dating evidence for many of the sites, this is the best approach that can be taken at the present time. It is useful on the grounds that similar types of monument were probably constructed within similar time frames, although there is no doubt that certain types of structure (eg. round cairns) were constructed throughout the Neolithic and Bronze Age periods. It is no co-incidence that many similar types of site are found to occur within specific regional boundaries. When putting the astronomical results presented here into context, it would be wise to take into account both factors of location and likely construction date.
All sites in the table have a title bar giving basic details of the site, followed by line data giving details of the possible alignments it contains.
Column 1 - (Black Background) the common name of the site. In the case of Neolithic tombs, this is followed by the site designation as set out by Daniel (1950) and standardised by Corcoran (1969).
Column 2 - the Ordnance Survey Grid Reference, given to eight figures.
Column 3 - the height of the site (in metres) above Ordnance Datum (ie. sea level), rounded to the nearest metre.
Column 4 - the source of the plan used for the assessment. In many cases a plan was not available and so the most comprehensive reference is given. Royal Commission references (RCAHMW) are followed by the site number in the inventory. On a few occasions the author drew up an approximate plan of the site and this is stated wherever applicable. Where the site has undergone a substantial excavation, the reference is followed by Exc in brackets.
Column 5 - the date on which the author's site survey was carried out. Specifically, this is the date on which the compass bearings were taken for comparison with the available plans. In cases where bearings were taken directly from a plan, no plan was available, or the few instances where the site was not visited, the date of the actual assessment is shown in square brackets.
LINE DATA BARS
Column 1 - description of the line in question. The author has attempted to be as descriptive as possible here, given the limited space available in the table.
Column 2 - the azimuth (bearing measured clockwise from True North) of the line in question. It is given to the nearest tenth of a degree, with an associated error range. This error is entirely subjective, based on the condition of the site and/or the ease with which the line can be obtained.
Column 3 - the horizon elevation in the direction of the line, given to the nearest tenth of a degree. The elevation was calculated using the formula given by Thom (1967, 25) which makes allowance for the curvature of the earth over large distances, and the effect of atmospheric refraction. The calculations assume an observer's eye height of 1.55 metres above the site's ground level. An error range is given in some instances where the horizon is close, determined from the contour interval used in the maps. In all cases the horizon is assumed to be free of tall vegetation cover. Where the horizon is very distant or across sea, the elevation can be negative. This is often the case in South Wales, where many sites stand on high ground overlooking the sea towards the South.
Column 4 - the name of the horizon feature indicated by the line (referred to as the foresight). This will normally be the name of a hill, town or other feature labelled on the Ordnance Survey map. Where a name is not given for a particular area on the map, a six figure Grid Reference is given. Below the name, in brackets, is the distance from the site in question to the foresight, in kilometres (rounded to the nearest 10 metres).
Column 5 - the declination (angle North or South of the celestial equator) indicated by the line. The value in this column applies to the Sun and stars, ie. there is no allowance made for parallax. The error range given takes into account the possible errors in both azimuth and horizon elevation. In all cases, allowance has been made for atmospheric refraction. The results given here make no distinction between centre disk, upper or lower limb observation (although this information was specified in the author's original program). In most cases the imprecision of the line makes identification of the observed part of the disk a pointless objective. Below the declination, significant astronomical alignments are listed in bold italics. These are listed whenever they fall within the declination error range given. Where the declination falls within the range of the Sun, the Gregorian calendar dates on which the Sun attains this declination are given. Two dates appear because, with the exception of the solstices, the Sun will attain any given declination twice during the course of the year. Significant solar alignments are denoted as follows:
Stellar alignments are then shown wherever applicable. Only the brightest stars have been selected and these are only shown if the date that the star attained the given declination falls within the likely construction period of the site.MSS(R/S) - Midsummer Sun(rise/set)
MWS(R/S) - Midwinter Sun(rise/set)
CQBL - Cross-quarter day; Celtic festivals of Beltane/Lughnasad (i.e. early May/early August)
CQSI - Cross-quarter day; Celtic festivals of Samhain/Imbolc (i.e. early November/early February)
EQ(R/S) - Equinox Sun(rise/set)
Column 6 - the lunar declination indicated by the line. This has been calculated allowing for the lunar parallax of 0º.95. The parallax results from the fact that celestial calculations are referred to the centre of the Earth, and not from its surface, which of course is where we view the lines from. The Sun and stars are so distant that parallax has little effect on the calculations, however the Moon is much closer and parallax must be taken into account. Significant alignments on the Moon are shown using the notation + (E + i) etc., where E is the obliquity of the ecliptic (the tilt of the Earth's axis in space) and i is the inclination of the Moon's orbit to the ecliptic (5º.1). Professor Alexander Thom referred to the position ± (E + i) as the major standstill and ± (E - i) as the minor standstill (Thom 1971, 18). Allowances for the small lunar 'wobble' called delta have been ignored.
Column 7 - other relevant information. In some cases a photograph of the site can be seen by clicking on the word Photo on the right of the table.
Archaeoastronomy in South Wales:
Explanation of Table Data
CORCORAN, J. X. W. P.
1969 "The Cotswold-Severn Group" in Megalithic Enquiries in the West of Britain, ed. by T. G. E. Powell et al (Liverpool University Press, Liverpool), pgs. 13-106.
DANIEL, G. E.
1950 The Prehistoric Chamber Tombs of England and Wales (Cambridge University Press, Cambridge)
ROYAL COMMISSION ON ANCIENT & HISTORICAL MONUMENTS IN WALES (RCAHMW)
1976 An Inventory of the Ancient Monuments in Glamorgan Vol. 1. Pre-Norman, Pt. 1: The Stone and Bronze Ages (HMSO, Cardiff).
1967 Megalithic Sites in Britain (Oxford University Press, Oxford)
1971 Megalithic Lunar Observatories (Oxford University Press, Oxford)
1997 A Phenomenology of Landscape: Places, Paths and Monuments (Berg Publishers, Oxford, UK / Providence, USA)
Copyright Martin J Powell 2001-2; minor revisions Feb 2006