(Extract from Uriel’s Machine, an extraordinary book written by Christopher Knight and Robert Lomas)
Professor Thom, a civil engineer, spent most of his retired life visiting, surveying and analysing stone circles in Britain. He found that a great proportion of the megalithic temples he measured seemed to be built using a standard unit of measurement, which he called the Megalithic Yard.

Although the unit he discovered was just over a foot in length, he doubled it to form a length closer to a yard, at 2.72 ft.
Dragon Stones Golden Sunrise
  His findings were originally doubted by archeologists and so-called experts, but now there is overwhelming evidence for this standard unit.

In my work with stone circles, I have always been drawn to use this unit of measurement. Although I have not known why, I feel it has an important role to play in the creation of sacred space. It is one factor in the many interwoven qualities of stone circles that transform the mundane into the sacred. In the back of my mind I have wondered what the megalithic yard really represents, and recently I found out, to my complete satisfaction. Christopher Knight and Robert Lomas have, in their book Uriel’s Machine, discovered the true relevance of the megalithic yard. It is a solution which is both awesome, and simple. I do not have the means to test its accuracy, so I trust those who claim they have. Below are extracts from their book. It is amazing!
A Megalithic Standard

There are 366 sunrises from one Winter Solstice to the next; let us suppose that the ancient stone builders decided that there were 366 days in the year.

Professor Thom had very skillfully demonstrated that the Megalithic Yard (2.72ft or 2’ 8.64”) was in evidence in the ancient structures throughout much of western Europe, yet was unable to identify how it had been arrived at.

Time is the key. The ancient builders divided their circle into 366 parts. How long does it take for the sun or a bright star to traverse one degree? It is relatively simple to do – place two posts one degree apart, and observe. Then create a pendulum and adjust its length until it gives exactly 366 beats for the full appearance of a star between two posts placed a megalithic degree apart.

As a 366th part of one revolution of the Earth, a megalithic degree is equal to just over 236 seconds, or 3.93 minutes of time. The length of pendulum that produces exactly 366 pulses is 16.32 inches in length.

Thom had doubled up the basic unit he had found to make his megalithic yard appear close to a modern yard. He had observed the smaller unit but chose to call it “half a Megalithic Yard”. So, doubling the length of the pendulum, it comes out to exactly 32.64”. A precise Megalithic Yard!

This is the answer to one of the greatest puzzles of prehistory. Once understood, anyone could use this principle to find the “sacred length” without reference to anyone else. It would work anytime, anywhere, and it would be totally accurate. This explains the consistency of accuracy found throughout Europe, over such a large area.

The Megalithic Yard is a truly staggering concept of measurement. It is based on three absolutely fundamental values:

1). The orbit of the Earth around the sun
2). The rotation of the Earth on its own axis
3). The mass of the Earth.

The Earth’s orbit gave the 366 split of the horizon, the rotation of the Earth gave the time span, and the mass of the planet (gravity) dictated the length of the pendulum to give 366 beats.
Simply brilliant!