The Newgrange stone age passage tomb on the Winter Solstice is described by the site:
Above the entrance to the passage at Newgrange there is an opening. This opening allows sunlight to penetrate the passage and chamber at sunrise around the Winter Solstice. A narrow beam of light penetrates the opening and reaches the floor of the chamber, "gradually extending to the rear of the passage. As the sun rises higher, the beam widens within the chamber so that the whole room becomes dramatically illuminated. After 17 minutes the sunbeam leaves the chamber and retreats back down the passage. When Newgrange was built over 5000 years ago, the winter solstice sunbeam would have made its way to the back recess of the central chamber. Due to changes in the tilt of the Earth's axis the sunbeam now stops 2 metres from the back recess".
What is this tilt? It is the obliquity of the ecliptic. Here in the figure we can see it.
The angle of the Earth's axial tilt varies with respect to the plane of the Earth's orbit. These slow 2.4° obliquity variations are roughly periodic, taking approximately 41,000 years to shift between a tilt of 22.1° and 24.5° and back again. Currently the Earth is tilted at 23.44 degrees, decreasing.
At the time of Newgrange building, the til was of about 24 degrees.
If we want to evaluate the effect of this tilt shift on the sunrise and sunset azimuths on solstices, at the Newgrange latitude, the influence of the tilt on the sunrise azimuths on solstice is about a degree. This explains the two metres back from recess.