The winter solstice phenomenon at Newgrange
The winter solstice phenomenon at Newgrange, Ireland: Accident or Design
by T.P. Ray, School of Cosmic Physics, Dublin Institute for Advanced Studies, Ireland
On Midwinter's Day, around four and a half minutes after sunrise, the sun shines down the
'roof-box' of the Neolithic passage graves of
Newgrange, Ireland,
and illuminates the floor of its main chamber 18m away. When the monument was constructed, however, the first light would
have occurred at sunrise in the form of a very narrow beam bisecting the chamber. Here I suggest that the width and height of the
gap in the floor of the roof-box may have been deliberate, tracing the path of the sun at the solstice. Newgrange predates the
astronomical structures of
Stonehenge by 1,000 years and as such
may be the oldest astronomically orientated structure in the world.
The white quartz glows in the Winter Solstice sunlight at Newgrange
Located some 50km from Dublin, Newgrange is the largest in a group of Neolithic passage
graves in the Boyne Valley, County Meath. Although discovered in 1699, the tomb was not properly excavated until the 1960s. It consists of a
passage and chamber off which are three smaller alcoves, giving the structure an overall cruciform appearance.
Surrounding it is a large circular mound, over 80m in diameter, of loose stones, and encircling this again is an incomplete ring
of standing stones. The mound and ring are, however, not concentric and probably not contemporaneous.
Plan of Newgrange tomb, the marked line shows the minium azimuth or 'first-light' axis of the passage.
As early as 1909, Lockyer remarked that the passage grave was approximately aligned to
the rising Sun at midwinter. He did not, however, pay much attention to the site. During recent excavations of Newgrange,
O'Kelly unearthed a curious decorated structure located above the entrance
to the tomb. This structure, which was later to be called the 'roof-box', covered a vertical gap between the first and second
roof slabs of the passage.
Conscious of a local tradition that the chamber was illuminated by the Sun at a certain
time of the year, O'Kelly investigated whether the roof-box had some solar function, and found that the midwinter Sun shone
through the roof-box and gap to illuminate the central chamber of the tomb 18m away. This event would have occurred
even if access to the tomb was blocked by the entrance stone.
Sectional elevation of the east side of Newgrange showing the path of the sun rays
Patrick later concluded that the orientation of the roof-box towards the
winter solstice was deliberate.
Heggie has pointed out, however, that on the basis of Patrick's calculations, the Sun would illuminate
the main chamber if its declination lay at any position between -22°58' and -25°53', and hence the
probability of a solsticial alignment in the corresponding range of azimuths is about 1 in 13.
Heggie concluded that this fact was 'not really significant enough to excite much interest'.
But Heggie's calculation is in error for two reasons. First, as will be discussed below,
Patrick's upper azimuth limit is about 1° too high and hence the declination range
should be reduced accordingly. Second, and more importantly, the three-dimensional aspect of this problem has been ignored.
As can be seen the solsticial light from the roof-box
meets the floor at a glancing angle within the main chamber.
If the gap between the first two roof-slabs was, say, 20 cm
lower, or equivalently the passage was a few metres longer, sunlight
would not enter the chamber. In fact, one would then observe
the Boyne Valley, rather than the local horizon, through the
gap. Correspondingly, if the gap was higher, the solsticial
light would have been projected onto the back wall. Although
such an alignment is of course admissible for statistical purposes,
it would not have as dramatic an effect as that of a glancing
beam on the floor. Thus the chance of Newgrange being accidentally
aligned with a solsticial sunrise/sunset point is smaller, at
least by a factor of two, than the figure quoted by Heggie.
It's the higher sunbeam from the Roofbox that illuminates the chamber inside Newgrange
Nevertheless the evidence still remains weak and it would not rival that found for sites
such as Stonehenge and Kintraw. To test the solar hypothesis
further, we re-surveyed the roof-box, as seen from the main chamber,
and the local horizon in the direction of sunrise on Midwinter's Day.
The passage itself is sinuous (Fig. 1), restricting the range of azimuths from
which a ray of light can enter the main chamber. We found this range to be
133°49' - 137°29 at the chamber entrance, with a typical error of about +/-3'.
The upper limit is ~1° greater than that quoted by Patrick. We measured the light
beam to be just under 34 cm wide near the entrance although it narrows towards
the back recess, largely because stone L20 leans inwards. Close to the entrance
to the chamber, however, L20 restricts the width of the light beam by 8 cm at
most, so that if L20 were uprighted the minimum azimuth would be less than the quoted value by only 20'.
The chamber floor is slightly lower than the gap in the roof-box and hence at low altitudes,
the light of the Sun is spread across the floor. The minimum altitude at which sunlight
can enter the chamber is given by the height of the local horizon, 55' +/-2', whereas
the maximum altitude, confirmed by actual timing; measurements of when the
Sun disappeared (F. Prendergast, private communication) from the main chamber,
is 2°11' +/-2'. Using height measurements of the gap behind the roof-box and
the chamber floor, we found that light at minimum elevation would reach at least 1 meter into the back recess.
However, sunlight is now seen to extend only as far as the edge of this alcove, because
the minimum azimuth axis, or 'first light' axis, of the passage does not point towards
midwinter sunrise. In fact, light from the right-hand limb of the Sun first enters
the main chamber four and a half minutes after sunrise (F. Prendergast, personal communication),
and at that time the Sun is almost completely above the horizon.
The Newgrange roof-box over the entrance at the Winter Solstice
The question then arises as to what would have been the solar alignment when the monument was built.
Carbon dating of two charcoal samples from between the roof-slabs yielded radiocarbon ages of
4,450 +/-40 and 4,460+/-45 BP. To convert this to a calendar date, we have used the
dendrochronological record provided by samples of Irish Oak. This indicates that
the probable construction period for the site is 3,150+/- 100 BC.
For these dates the obliquity of the ecliptic would have been 24°2' +/-1' and
the midwinter Sun would rise at an azimuth of 133°54 +/-4'. Here we have assumed,
for calculating refraction, a pressure of 1,010 mbar, a temperature of 0°C and
the height of the local horizon to be 55° +/-2'. Variations in temperature
and pressure make very little difference to the result. We see that the Sun
would have risen just within the limits of the roof-box as seen from the main
chamber. In other words, the 'first-light axis of the passage corresponds,
within 5' or so, to midwinter sunrise 5,150 years ago.
If, at that time, one assumes that stone L20 was in an upright position, the accuracy would still be better than 25'.
It follows that when Newgrange was built, the first beam of sunlight on the floor
was less than 10 cm wide, approximately 2M long, and bisected
the chamber. Rather curiously, this beam would have
entered the back recess and indirectly illuminated a three-leafed
spiral figure on its wall, as mentioned in legend.
As the Sun's altitude and azimuth increased, the beam would have become gradually broader.
The height of the gap behind the roof-box varies between 18 and 22 cm, being 20 cm on average.
As seen from the main chamber 18m away, this gap subtends an angle of roughly 38', so that
the risen Sun is framed in the vertical direction. Finally, the lower edge of the Sun would
have left the roof-box, as seen from the main chamber, at an azimuth of 137°42' +/-5'.
Again this is very close to the maximum azimuth of the passage, implying a symmetry
with respect to the Sun's passage at the winter solstice. Thus not only the height but
also the width of the gap may be significant.
Newgrange predates the astronomically orientated structures (phase III) of Stonehenge
by about 1,000 years. The evidence presented here supports the
theory that the orientation of Newgrange was deliberate, which
would make it therefore the oldest megalithic structure known
for certain to have an astronomical function. The alignment
at Newgrange was within a fraction (that is, within .5°)
of the Sun's disk although it is doubtful whether the accuracy
extends to the level of a few arcminutes. Newgrange should be
seen in the light of large-scale studies such as those of Ruggles,
which show definitive evidence that megalithic man was interested
in marking the southern limits in declination of the Sun and
Moon, albeit approximately. Such low accuracies suggest that
ancient man's interest in these bodies may have been ritualistic
rather than for the purpose of calendar construction.
We acknowledge the assistance of Muiris De Butleir, Senior Surveyor at the National Monuments
Section, Office of Public Works and the support of the Office
of Public Works, Dublin. Thanks are also due to
Tim O'Brien for his help.
The winter solstice phenomenon at Newgrange, Ireland: Accident or Design by T.P. Ray.
Reprinted from Nature, Vol. 337, No. 6205, pp. 343-345, 26th January, 1989 © Macmillan Magazines Ltd., 1989.
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