On the winter blot the shadow of the higher stone passes across the lower stone 10 m away, allowing the shadow to just reach the top of the lower stone.
The animation shows the shadow’s passage the day before, exactly on the first possible day of the winter blot, and the day after.
The gradient of the top of the two stones are in good alignment with the sun’s path. The sun’s path is exactly right a half moon cycle after the summer solstice.
The slope of the higher stone makes it impossible to find a day when the shadow falls in a proper way.