Think it has something to do with 'Axial Precession' which for the earth is something like a 26000 year cycle.
Precession in all it's obvious forms is explained here...
http://en.wikipedia.org/wiki/Precession
Guess 3-4000 years is only a small way through the Earths total precession cycle. Plus the moons orbit doesn't really alter much past the 18.5 year cycle in terms of differing orbital parameters (i.e it's only marginally higher/lower or nearer/distant from the Earth).
Hence Callanish still works and probably will for several thousand years as it doesn't involve the 'Precession of Stars', rather focusing on the movement of our satellite for it to work.
Though I'm no archaeology buff, so i like to leave the theory and practice discussions to others in the know.
After watching the 'Moon' docu' i was surprised to find out that it is actually 3.78CMs a year and not 1CM that the moon is spiralling out of Earths orbit. Though nothing to untoward 'axially speaking' should happen for around 1 Billion years.
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