I had an opportunity to visit Knowth to photograph sunset at the entrance to its western passage on Sunday 13th October 2019. What unfolded was sublime. Weather conditions were among the most incredible I have seen. A huge, dark shower cloud rolled in towards sunset, and just when hope was fading of seeing the sun, it appeared through a wall of rain between the bottom of the cloud and the horizon, creating this truly dazzling and awesome scene.
This is the time of year when the sunlight reaches its maximum penetration into the western passage of Knowth – or should I say the bend in the passage, where the old passage and the new meet when Knowth was modified and enlarged. Contrary to popular assertion, the sun does NOT shine into the passages of Knowth exactly at the equinoxes. It shines into Knowth west about 18 days before spring (vernal) equinox and 18 days after autumn equinox. Ideally, I should have been there on October 11th. But I could only manage two days later, on the 13th.
The photo is taken from just in front of the gate at the entrance to Knowth's Western passage, BEHIND kerb stone 74, the entrance kerb stone, which you can see at the bottom of the photo. The "gnomon" stone and the large "egg" stone are illuminated by flash. The sky was doing its own incredible thing!
Fascinatingly, the day upon which the sun attains its maximum reach into Knowth West in early March, is 384 days or 13 synodic lunar months ahead of the following vernal equinox. Because the sun is moving along the horizon rapidly at this time of year – by approximately one solar diameter per day – it would appear that on the day upon which the sun attains maximum penetration into the western passage, the sun would be blocked by the gnomon stone at the moment of sunset. Does this mean that the shadow of the gnomon is cast into the passage? Does it also mean that the sunlight is effectively split into two beams – one illuminating the orthostats on the northern wall of the passage and the other the southern wall? Some interesting questions.
I suspect the large egg-stone to the south (left) of the gnomon is also important. What does its shadow do in relation to kerb 74? Perhaps the point of its shadow lines up with the vertical groove in K74 on the day of Samhain? More investigation needed.
I'm grateful for the assistance of the OPW in helping make this observation possible.