This image photographed inside Cairn T at 6:51am shows the shaped beam of equinox sunlight illuminating the back stone. Over a period of about 30 minutes the beam moves to the bottom right corner of the backstone. Click Here for a larger view.

This image photographed at 7:15am shows the beam of equinox sunlight illuminating the bottom right corner of the backstone. Click Here for a larger view.

This image photographed at 7:18am shows the last of the sunbeam on the backstone, the stones to the right of the back recess are now illuminated.  There is a sun symbol at the bottom of the stone in the centre of the image. Click Here for a larger view.

 

Light Years Ago by Tim O'Brien shows that the symbols of the sun engraved on the backstone of Cairn T at Loughcrew are precisely positioned to measure solar movement. A beam of light from the rising sun around the time of the March and September equinoxes is mapped on the backstone. Purchase book at Amazon.com, purchase book at Amazon.co.uk