Our prehistoric ancestors were very aware of the movements of celestial bodies in the heavens - especially the Sun. Ancient sacred sites were usually oriented towards significant horizonal astronomical events. Since the time of Norman Lockyer when he documented at the turn of the last century that indeed the Summer Solstice Sunrise did come up over the Heel Stone at Stonehenge (on the Wiltshire Downs in England), scientists have been finding significant astronomical orientations at more and more ancient sacred sites all over the world.
From the Peruvian Machu Picchu's 'hitching post of the Sun', the Intihuatana Stone, to the Viking labyrinths in Sweden, and at Cairn T at Loughcrew Carnbane East (seen here on the right), it was important to orient the sacred site towards the Sun, the Moon and occasionally specific stars as they rose or set along the horizon. Carnbane East is in the Loughcrew Mountains of Ireland, about twenty miles North-West of Newgrange.
Notice the shaft of light creating a circle with an eight-petalled flower inside, pecked on the back wall of the chamber. Sig took this series of photos in the late eighties, Notice how different the beam of light looks in the video from Knowth.com above. I am not clear why this is the case - perhaps one was shot one day off from the actual point of Equinox.
If you are new to Archaeoastronomy, the first book I would suggest that read is Robin Heath's Alexander Thom: Cracking the Stone Age Code.