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.
TITLE BAR
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, pg. 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:
MSS(R/S)
- Midsummer Sun(rise/set)
MWS(R/S)
- Midwinter Sun(rise/set)
CQBL - Cross-quarter
day; Celtic festivals of Beltane/Lughnasad (ie early May/early August)
CQSI - Cross-quarter
day; Celtic festivals of Samhain/Imbolc (ie. early November/early February)
EQ(R/S) -
Equinox Sun(rise/set)
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.
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). Prof Alexander Thom referred to the position
± (E + i) as the major standstill and
± (E - i) as the minor standstill
(Thom, 1971, pg. 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.
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), 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).
THOM, Alexander
1967 Megalithic Sites in Britain (Oxford University
Press, Oxford)
1971 Megalithic Lunar
Observatories (Oxford University
Press, Oxford)
TILLEY, Christopher
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
