4. Filtering Out Noise

In an effort to establish whether the Listoghil Samhain/Imbolc phenomenon is the creation of ancient architects or an artefact of the work of their modern-day equivalents, I created a scale 3D model of the chamber in Google Sketchup. This allowed modelling of shadow and of the progress of the sun at precise times along with spatial co-ordinates.

4.1 The dark time

Using Burenhult's total station information, I modelled the mound surrounding the chamber before the excavation and reconstruction. The model demonstrated that sunlight was blocked from entry to the chamber in the modern period prior to excavation. The sunlight did light the roof-slab face during the year, but mounds of cairn material and fieldwalls obstructed direct light to the chamber (Figure 11). Without the excavation/reconstruction, the October/February events could not have been observed. The unusual light phenomenon occurring inside the chamber can be described as an unforeseen consequence of reconstruction.

Figure 11

Figure 11: The original mound re-modelled in Google SketchUp (based on Burenhult 1998b, Fig 3.)

4.2 Removing the cairn

For a seasonal alignment to be valid, it seems a prerequisite that the sun could light the chamber unhindered for a sustained period. We can never be certain if this was the case at Listoghil. Subsequent (Neolithic) intrusions may have occurred at the Mound of the Hostages, Tara (O'Sullivan 2005, 237), and new dates from Carrowmore (Bergh and Hensey forthcoming) indicate protracted access to the chambers of the smaller sites. Admittedly the Neolithic dates from Listoghil are closely clustered.

In a second experiment, the cairn in the model was removed completely from the chamber. The 31 October and 10 February events were run. As occurs in situ, the roof-slab is illuminated (shadowed by the blocking stone), at 7.40am in October and at 8.12 in February. The computer model showed the sunlight creating an inverted triangle of light on either side of the top of the backstone, rather than only one triangle to the right. The other significant difference was that the event lasts 20 minutes, twice as long as happens today (a consequence of occlusion by the reconstructed cairn). The entire progress of the shadow across the capstone is completed in this time. Twenty minutes later the roof-slab itself falls into shadow as the sun gains height.

The computer reconstruction exercise crystallised the observation that at the seasonal cusps, two events happen at Listoghil; one outside the chamber, the other inside. The sun-in-the-saddle event on the horizon, as we have seen, serves to isolate a particular time period. The foresight of the saddle (regardless of reconstruction or chamber covering) narrows this effect to two days or so for an observer at the location of Listoghil. But the computer modelling exercise shows that the interaction of the 'gnomon stone' B, and the chamber interior has a similar pinpointing effect. And, inside the chamber, as outside, there is a secondary, wider focus, extending beyond the three-day event seen at winter-start and winter-end. With the horizon event, you can continue to watch the progress of the sun to (and back from) the rounded hill of the Cailleach a Bhérra. Inside the chamber, too, a more long-term process is observed.

4.2.1 Shadow play in the chamber

The computer model showed that for 20 weeks between October 11 and March 1, the base of the pointed shadow of stone B crosses the roof-slab belly twice, sweeping from the edge bounded by stone D to that bounded by stone F and back again. For the three days around 31 October and 10 February, the shadow falls perpendicular to the backstone, like a clock hand at the hour of twelve (Figure 12). Although it is parallel to the axis of the chamber, it is not exactly central on the ceiling, because the peak of stone B is set slightly off-centre.

Figure 12: Animation: Google Sketchup simulation of the progress of the shadow spear. Simulation 1; 31 October 2010, simulation 2; 11 October 2010 and simulation 3; 21 December 2010

Over the course of winter, the shadow on the roof-slab works like a crude seasonal sundial. For an observer lying on their back on the chamber floor with their feet pointing east-southeast towards stone B and their head at backstone (E), the alignment of the shadow and the axis of their body marks Samhain or Imbolc, the seasonal transits. When the shadow is positioned towards the edge of the chamber roof on the observer's right (south), it is either 11 October or 1 March in our calendar. When the shadow has crossed to the slab's edge on the observer's left (north), midwinter has arrived. If this effect is intentional, it is unique among Irish passage tombs (or perhaps not quite: see Prendergast 1991 and Prendergast's shadow). If it is intentional, it is the manifestation of an ancient design which is a calculated response to place, landscape and seasonality.

It should be noted however that:

The most convenient way in which the computer model of Listoghil could be manipulated and images of the reconstructed phenomenon recorded (Figure 12) was upside down, looking from inside the earth up into the chamber. It echoed my experience in February 2010, when I watched the shadow of stone B cross the roof-slab while lying on my back in the chamber. Did a ritual specialist once lie in this way to observe inside the chamber? Or was the entire event, or sequence of events, arranged not for the living, but for the dead?

4.3 A late October morning in the deep past

Did the chamber of Listoghil point to winter-start/winter-end sunrise around 3550 BC? The familiar four-season cycle which prevails at temperate latitudes today remains essentially unchanged over five and a half millennia. Temperatures may have been somewhat warmer in the Middle Neolithic (Zagwijn 1994; but see brief discussion of climatic changes). This is important to interpretation, because it illustrates that then, as now, the same boundaries between seasons, the same sets of seasonal markers in the human, animal and plant worlds flagged the onset of spring, summer, autumn and winter. Two million mornings ago, the climatic experience of early morning in Carrowmore at the commencement of winter may broadly have resembled that of today.

A number of websites and software packages provide calculators for calculating azimuth and sun position. I used; the webmaster, Victor Reijs, confirmed my results independently using the Skymap programme and web page and planisphere (see Table 1). The setting for Listoghil is N 54° 14'56.1006" E -8° 31' 5.6238", eye level 60m above sea level with a distance to object (the saddle) 6.15km. (Temperature 1°C, air pressure 1013.25 Millibars.) The Ballygawley saddle is 193m above sea level, giving an apparent altitude of 1.22°. Event dates are in Gregorian calendar terms. Declination is geo-centric.

Placing the emphasis on the event rather than on a fixed time may permit the monument (rather than our preconceptions) to lead us in our interpretation. In 3550 BC the sun rose in the saddle in late autumn four days earlier than in 2008 (on the 27 October, Gregorian Calendar). Interestingly, in early spring the difference over five and a half thousand years is smaller. The saddle event has been repeated annually on 10 February for five and a half thousand years (the programme has a margin of error of ±1 day). In Neolithic times, the interval between the twice-yearly congruence of sunrise, saddle and chamber was approximately four days longer; it took 107 instead of 103 days for the sun to return to the saddle.

Figure 13

Figure 13: Sun positions in 2010 AD (purple arrows) and in 3550 BC (white arrows) compared at the same time of year; left, on 31 October (Gregorian calendar) and right, during the winter solstice

I also compared horizon positions at the same times in different epochs (Figure 13). Ferdinando Patat and Victor Reijs both confirmed the results, based on Patat's methodology (2011). According to the reconstruction, the late October sun centre rising in the saddle occurred in 2008 over 1° declination different (-15.5 in 3550 BC and -14.38 in 2010) to its position at the time of active use at Listoghil. Five and a half thousand years ago the midwinter sunrise standstill occurred about 0.63° declination south of where it happens today (1.4° azimuth); from Listoghil the sun would have appeared at the foot of Cailleach a Bhérra rather than about half-way down the flank of the hill. The winter solstice in 3550 BC was on 19 December as opposed to 21 December in 2008. (See Table 1 and Figures 13 and 14).

Figure 14: Artistic impression showing the difference in sun position at winter solstice as viewed from Listoghil. Horizon synthesis kindly provided by Ferdinando Patat for this paper.

4.4 Astronomical cross-quarter day comparison

The Geniet calculator gives the variation between the saddle event and 'true sun' cross-quarter day in 2008 as approximately 4.25° azimuth and 2.26° declination. The astronomical cross-quarter day occurs about a week after the saddle event at winter-start, and about a week before it at the end. The astronomical cross-quarter day coincides with sunrise near the peak of Aughamore Far.


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