The stones show the point on the horizon where the sun sets on the day of the new quarter in spring. The distance between the stones is 8.3m.
The gradients of the top of the stones are in good alignment with the sun’s path.
The sun’s path is exactly right compared with the stones 28 days after the spring equinox, defined as the day before spring equinox according to today’s definition. (“refraction equinox”).
The sunset on the on the day of the new quarter can be seen today. The next date on which this will occur is April 17 2015, 20:10.