Inside the rising sun shining into my fictional Dolmen in The Luna Legacy, I used the full moon.
Here at the Newgrange Dolmen in Ireland, once a year, at the winter solstice, the rising sun shines directly along the long passage into the chamber for about 17 minutes and illuminates the chamber floor. This alignment is too precise to be widely considered to be formed by chance. Professor M. J. O'Kelly was the first person in modern times to observe this event on December 21, 1967.
The sun enters the passage through a specially contrived opening, known as a roof box, directly above the main entrance. Although solar alignments are not uncommon among passage graves, Newgrange is one of few to contain the additional roof box feature.
The alignment is such that although the roof box is above the passage entrance, the light hits the floor of the inner chamber. Today the first light enters about four minutes after sunrise, but calculations based on the precession of the Earth show that 5,000 years ago first light would have entered exactly at sunrise.
Read the full article here: https://history-ireland.blogspot.com/2012/12/newgrange-winter-solstice.html