Direct Light Shining on the Backstone at Cairn T

Around the Equinox direct sunlight shines onto the Backstone in a rectangular pattern framed by the original passage stones. As the sun rises the rectangle of light decreases in size and moves gradually to the right. It eventually diminishes to become a small dot of light before leaving the backstone. On the day of the Equinox the beam of sunlight focuses onto the sun symbol on the backstone.