This unique text discusses the mathematical principles behind Megalithic stone circles, and how these were used for observing lunar cycles in prehistoric times. The author, A. Thom, shows that stone circles were precisely planned and laid out in accordance with certain geometric figures in the classic Pythagorean tradition. Containing some mathematical and astronomical details, along with notes on site survey and location, this book is ideal for amateur enthusiasts and academicians of archaeology, astronomy, and mathematics.