On both the autumnal blot and the winter blot the shadow of the higher stone passes across the lower stone 7.5m away, allowing the shadow to just reach the top of the lower stone. By coincidence the last possible days for autumnal and winter blot almost exactly occur at the same solar altitude.
The animation shows the shadow’s passage the day before, and exactly on the last possible day of the autumnal blot and the winter blot:
The gradients of the top of the stones are in good alignment with the sun’s path.
The shadow from the higher the stone ends up in exactly the right height on the lower stone on the last possible days for both the autumnal blot and the winter blot. The days are exactly one day before 1 1/2 moon cycle after the autumnal equinox and the winter solstice.
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 February 4th 2015, between 9:40 and 9:52 and November 8th between 9:10 and 9.22.