At the equinox, the rays pass either side of the standing entrance stone and travel through into the chambers. A local historian, Christy Roche has a lovely clip on You Tube showing this happening.
"Equinox at the Hag's Bed"
This photograph was taken at 16.57 hours on the 20th March 2014, the exact moment of the equinox.
I am standing with my back to the sun at the front of the tomb. The beams to the right disappeared quickly, the short shaft beam didn't seem to move very much beyond the stone, the tiny dot in the pinnacle holding up the capstone remained for a long time( still there at 17.57 hours) the goblet beam stretched along the side wall and the tiny bell beam was the only beam to reach the back wall.
I am standing with my back to the sun at the front of the tomb. The beams to the right disappeared quickly, the short shaft beam didn't seem to move very much beyond the stone, the tiny dot in the pinnacle holding up the capstone remained for a long time( still there at 17.57 hours) the goblet beam stretched along the side wall and the tiny bell beam was the only beam to reach the back wall.
LABBACALLEE - MARCH 2014
Hover over the photographs and you can see what's happening from 16.49 hours to 17.57 hours.