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Alignment of chamber top hole at Newgrange

Like the roof box at Newgrange, the top hole of the chamber (normally covered by the capstone) could also have an archaeoastronomical meaning. Look at the following figure:

The red color depicts the measurements (the red rectangular ACDE is the projection of the top hole under the capstone on the ground plan) made during my visit at Newgrange on Dec. 20, 1996:

height (h) difference between the underside of the cap stone and top plane of the basin in the central recess (B) is around 6150 +/- 100 [mm] (including an approximated lowering of the roof in time by 500 [mm] [O'Kelly (1982), page 120]).
distance between the most southern corner of the top hole (A) and the stone C8 (B) is 4610 +/- 100 [mm].
the top hole has a dimension of 915 x 750 x 1000 x 1000 +/- 40 [mm] (AC x AD x CE x DE). Point E has not been determined directly, but by using a photograph.
AC is turned some -30^o +/- 1^o from the main passage azimuth and AD is some 65^o +/- 1^o turned from the main passage azimuth.
there is no blockage of the light going from the top hole under the capstone (A) towards C8 (B).
the passage azimuth (azi_pas) is around 135^o +/- 1^o (Tim O'Brien, 1996, pers. comm)

All the (red) dimensions have been measured by using a 2D water leveled laser pointer and detailed maps of O'Kelly.

Taking this into account, we get the following information with regard to the light patches in the central recess at the level of the top of the basin during summer solstice day on 3050 BC:

The size of the picture is: 2080 x 1400 [mm]
The error in the colored pixels in x and y direction are: +/- 150 [mm]

Description of the colors:

Green
Red
Purple
Yellow
Black

To get this effect the capstone should have been removed or just have been moved to allow for a small light patch, which could have been possible during the time the cairn material was not yet fully covering the capstone area.

So to summarize: The sun shines through this top hole under the capstone some 30 days before and after the longest day on the plane of the basin. During the longest day it will shine some 50 minutes on this plane, if there was only a small aperture through the top hole.

Just for comparison, the sun shines through the roof box (at winter solstice) some 5 days before and after the shortest day in the chamber and during the shortest day it will shine some 15 minutes in the chamber.

Another remarkable thing is; the altitude of A with regard to B is 53^o 9' has almost the same value as the latitude of Newgrange (53^o 42'), so (is there logic behind this so?) every day one can see at a certain moment, through the top hole, the ecliptic.

Conclusions

It looks like that there could be an alignment between the top hole and the basin in the central recess of Newgrange. The above information was not determined with very small inaccuracies, but even the present inaccuracies provide a good general idea.

A circular cone of stones over the chamber can be seen up to the level of the capstone. This could indicate that some time the chamber was covered with a nice circular cone of stones which let the capstone almost free.

Remember that, as somebody said, every roof is pointed towards the heaven and thus would possibly allow the high/midday summer solstice sun inside. This is true of course (you can also see this, due to the small influence in the direction of the passage and the height of the chamber), so this top hole-basin alignment is of course no proof. The likelihood of an on purpose alignment is higher, because there are more alignments in this monument and other monuments and there are alignments towards other buildings in the neighborhood.

Another increase of likelihood is if the same phenomena is seen in other corbelled roof. It is a pity that not many corbelled roofs survived the years. Looking at data on Knowth, the top hole of Knowth also seems to act in the same way. If there exist more original corbelled roofs, please let me know.

Acknowledgments

I thank Clare Tuffy, custodian of Newgrange, for allowing me to do the measurements.

Other sources

Through the custodian of Newgrange, I got into contact with Gerry Bracken, Ireland (Pers. comm. [1998]). He had done, between 1991 and 1993, comparable measurements with regard to the top hole under the capstone. The difference between his work and mine is that he thought that the light would shine in the right recess chamber (which will not happen, in my opinion).
Jacqueline I. Garnett and her husband also have investigated the top hole for Newgrange in the end 1970's (Garnett, [2005]), to determine what stars can be visible from the recesses. This to related the sky view with the megalithic art in the monument.

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Major content related changes: August 22, 1997