Here I have copied an aerial view of Avebury from Google Earth. I have superimposed a diagram of the bank and ditch, together with possible numbered locations for all the missing stones. Onto this I have plotted my construction in an accurate drawing program, at a much larger scale than shown here.
To see the construction in better detail in its environment, I have provided a Google Earth overlay for you to download. Some of you may be reading this page because you have linked in from Google Earth, so you presumably have this already.
The complete henge construction
Mouse over the diagram to see a larger version of it.
There follows a commentary, sector by sector, showing how I arrived at my figure.
The Eastern and Southern arc
It is apparent that a single arc provides a surprisingly good fit for the stones. At either end of this arc there is an abrupt change in direction.
The diagram above also shows the bisector of this arc as a blue dotted line.
The South Western arc
This arc has a centre at some distance outside the henge. Its bisector, shown dotted, is remarkable because it is at exact right angles to a line from the centre of the monument to Windmill Hill, a prominent and important feature to the North West of Avebury henge. This bisecting line also crosses a large and prominent stone on the henge's perimeter.
The Northern arc
A good fit is found for the stones and again the bisector of this arc is constructed. Curiously, this bisector goes through a missing stone to the north of the Cove, one of the large Cove stones and through a linear feature of stones in the lower small circle.
It is tempting to suggest that this line continues to and is connected in some way with Silbaby, the disputed chalk mound to the East of Silbury Hill, which has not been confirmed as ancient, but is presumably pre-Roman, as the Roman road sliced across an edge of it, as Stukeley noticed and recorded on his drawings. Silbaby is also on a direct line between Silbury Hill and the Sanctuary on Overton Hill.
The North Western arc
This arc has a centre that lies upon the bisector of the Northern arc - see the diagram above. This circle has a peculiar property, which will is discussed below.
The Cyclic Quadrilateral
It is extraordinary that the continuation of the North Western arc's circle passes through the other two corners of the henge, thus providing a true centre to the complex monument. All four bisectors of the four arcs meet at one single point.
Thus we have four points on one circle, defining a cyclic quadrilateral.
Intersecting circles and inner circles 
We can regard the figure as being defined by the intersection of the two outer circles.
In my construction, the circles are very nearly the same radius as each other. Further work may prove this to be the original intent.
Also, when plotting the two inner circles, I found that a line connecting the two inner circles intersects the mid-point of the line connecting the Northern arc.
The other bisecting line at right angles, running between the two inner circles meets the outer circle of stones very near to a corner point of the construction. Further refinement of the diagram may prove this also to be intentional.
Professor Alexander Thom's construction
For many years Thom's construction of Avebury has gone unchallenged. Here I have superimposed Thom's construction, the purple lines, with my construction, the red and blue lines.
When I set out to look at the geometry for myself, I discarded what I knew of Thom's construction and looked for the simplest, most obvious fit to the stones.
Thom's construction seems to me to be a little over-complicated and in places not a very good fit either.
It is remarkable that Thom's point D, the centre of his 3-4-5 triangle, is in very close accord with my centre as defined by my cyclic quadrilateral. There also appears to be a close relationship with my arc bisector line that crosses through Thom's point A and D.
Copyright John Smout – JS Design, 2009