At this point I hope my colleague Dave Kane will step in and explain to you a very specific effect we have been experimenting with.

I'm sure you've all been told that one must never stare at the Sun for too long because it will either damage your eyes or send you mad!

Well, when you stand in the shadow of the capstone and look at the Sun through it, the effect is that the intense glare of the sky around it is almost completely removed, enabling one to look for longer periods, especially once your eyes have acclimatised. This then brings into play a phenomenon known as 'persistence of vision', whereby one can see a 'negative' image where the strong light has 'etched' an image onto the retina which persist for quite some time after. If you close your eyes just after doing this, you can effectively examine what you have been looking at with more detail than is possible otherwise. Interestingly, the shape of this aperture, when looked at with the Sun coming through it in the mid afternoon, is almost exactly like an eye!

Whether this is exactly what was intended, we may never know. ;)

p.s. Just to illustrate a point regarding the testing of our hypotheses (and that any of them being disproved is still an important lesson!), we initially theorised that perhaps the midday Sun at Summer Solstice would shine right down the aperture and move down stone 6. Well it doesn't, not by quite a bit. In fact the small beam of light doesn't come within a couple of feet of the nearest upright! So we learned a valuable lesson that day, but it's still quite possible that there could have been an area of white chalk all around the quoit with markings similar to a sundial?