On the day of the new quarter in the autumn the shadow of the higher stone passes across the lower stone 8.4m away, allowing the shadow to just reach the top of the lower stone.
The animation shows the shadow’s passage the day before, and exactly on the day of the new quarter:
The shadow from the higher the stone ends up in the right height on the lower stone 28 days after the autumnal equinox according to the “refraction equinox.”
There is no problem in seeing how the shadow sweeps across the stone in real time. The next dates on which this will occur is October 22nd 2015, around 9:40.