Thirty-five sites are described in the book, analysed for their remarkably precise and ingenious application of astronomical and geometrical knowledge. "The discipline necessary could not have arisen out of nothing," writes Professor Thom. "There must have been behind it a school or system of mathematical reasoning..." Based on his own experiences of sailing off the west coast of Britain, especially from the Mull of Kintyre to the Butt of Lewis, Professor Thom suggests that one reason for the development of lunar observatories must have been people's observations of the links between tides and the movements of the moon. "It is the principal object of this book to show that he [= 'Megalithic man'] carried this study much further, until he was able to predict which full or new Moon would give rise to an eclipse of the Moon or the Sun". Professor Thom also suggests, incidentally, that the lunar solstice ("such an important concept") should be called the lunar standstill. As the PBFA Frome Book Fair takes place this year on the date of the summer standstill, ah, let's give it a go.