Shaped by the stones of the passage sides, the sill stones in the passage and chamber and orthostats at the opening of the rear recess, the vernal equinox sunbeam is shaped slightly differently from that on the autumnal equinox. The place of sunrise also moves along the horizon at its quickest at this time of year and this has prompted specualtion that the carvings were designed to measure this casted sunbeam to determine the day of equinox with great accuracy.

The dark square on the backstone is the shadow of the fixtures on the gates to the passage and a large kerbstone to the right of the opening creates the sloped shadow near the bottom of the sunbeam.

Shaped by the stones of the passage sides, the sill stones in the passage and chamber and orthostats at the opening of the rear recess, the vernal equinox sunbeam is shaped slightly differently from that on the autumnal equinox. The place of sunrise also moves along the horizon at its quickest at this time of year and this has prompted specualtion that the carvings were designed to measure this casted sunbeam to determine the day of equinox with great accuracy.

The dark square on the backstone is the shadow of the fixtures on the gates to the passage and a large kerbstone to the right of the opening creates the sloped shadow near the bottom of the sunbeam.