On the vernal blot the shadow of the higher stone passes across the lower stone 2.2m away, allowing the shadow to just reach the top of the lower stone.
The animation shows the vernal blot based on the spring equinox as defined by 1) Height Model Equinox and 2) Refraction Equinox. See definitions in Days-of-the-calendar.
The gradients of the top of the stones are in good alignment with the sun’s path.
The shadows movement across the stone is exactly correct based on 31st March 645, exactly a half moon cycle after the spring equinox, defined as the day before spring equinox according to today’s definition. That is to say the day when day and night are of equal length under the influence of sunlight’s refraction in earth’s atmosphere (“refraction equinox”).
The sun line is completely unimpeded by shading trees and can therefore be seen in the present day. The next opportunity is on 4th April 2015, between 09:15 and 09:40.