Alignment of eastern chamber top hole at Knowth

Like the top hole at Newgrange, the top hole of the eastern chamber at Knowth (normally covered by the cap stone) could also have an archaeoastronomical meaning.

 The projection of the top hole on the ground plain and the basin in the right recess can be seen in the following picture:

 The red color depicts the measurements (the polygonal ABCDE is the projection of the top hole under the capstone on the ground plan) made during my visit at Knowth on August 18, 1997:

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

Taking this into account, we get the following information with regard to the light patches in the right recess at the level of the top of the basin during summer solstice day on 3100 BCE (18/7/3100 BCE):

The size of the picture  is: 2800 x 2950 [mm]
The error in the colored pixels in x and y direction are: +/- 150 [mm]

Description of the colors:

  1. Green
  2. Light patch around 12.42 GMT
  3. Red
  4. Light patch around 13.02 GMT
  5. Purple
  6. Light patch around 13.32 GMT
  7. Yellow
  8. Everything that will be covered with light during the above time period (including the inaccuracy of +/ 150 [mm]).
  9. Black
  10. The contours of the basin in the right recess.
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 47 days before and after the longest day on the top 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.


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

According to G. Eogan (pers. comm., 1997), no evidence is available that the space above the capstone was held open for a long time. It seems that it will have been blocked at the same moment the whole cairn was finished.

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 Newgrange, the top hole of Newgrange also seems to act in the same way. If there exist more original corbelled roofs, please let me know.


I thank Grainne Kelly and professor George Eogan for allowing me to do the measurements. Furthermore I would like to thank Michael Walsh for introducing me to George Eogan.

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Major content related changes: August 25, 1997