I consulted Thom's annotated survey of Avebury - which is criticised as 'fanciful', I know. It's the lunar maximum moonrise that is indicated by one of the sections of the Z-Stones, through the southern entrance, (and I know these were questionably located by Mr Marmalade). The two dirty great portal stones are shaped, I believe, to facilitate this observation, which is now barred by a copse of elm? trees. I remember watching this southern moonrise from a putative stone circle site high in the Northumbrian fells. I never realised what a grand experience it would be.
Soon be time for the first crop circles to appear - oh, freak out !