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I sometimes see mention of this idea of solstice alignments at Pentre Ifan but it points mostly southward unless it is meant to catch the light near the meridian when the sun is low, but I don't know what the vertical measurements are or how the entrance would have originally been situated. (maybe it has to do with the spread of the forecourt?) Still might make a great picture with the moon low in the sky during a lunar proxy.
The idea of the moon simulating the sun's path as quoted in the original post back in 2001 came from here:
http://www.archaeocosmology.org/eng/moonsun.htm
Years later I did a lot of work on this concept of lunar proxies during which I learned about interest in the earlier 2001 part of the lunar cycle (what I call an equalized lunar standstill):
https://www.exploreglobe.net/equalized-lunar-standstill.html
Some of the best events this time around have passed in the last year or so, but while sharing info about an upcoming opportunity I found the above message from 20 years ago!
The next opportunity is in the early morning hours of 17 September 2021 to see the moon at declination value very close to -24 as it sets most anywhere in Britain and Ireland.
As for Pentre Ifan, someone could observe on the previous evening of the 16th well before setting in the early morning hours of the 17th (the declination value then is closer to -24.5 which is a bit lower than the sun reached even at the last minimum). Later on toward the early morning hours of the 17th the moon will set at -24 and maybe it would be visible from an angle from 'within' the tomb.
I have seen alignments like that where the solstice was just visible from one back corner of a passage/chamber instead of centered, but I don't know if that applies here. Could be worth checking out while standing under the capstone looking southwest and moving around to get pictures as the moon goes down to estimate if that position might have fallen within the original chamber. It would be a good idea to have a diagram of the site to mark exactly where one was standing (by best estimate) as the moon touched the horizon.
It might even be worth performing this investigation from sunrise to sunset at the actual winter solstice even though the sun no longer reaches quite the same extremes, it could still establish the basic angles for further refinement/assessment.
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