For centuries, stone circles have excited the imagination of their visitors, while archaeologists, astronomers and anthropologists have argued about the purpose of these mysterious abandoned rings. In recent years, accurate surveys of many sites have revealed that these monuments were not simply roughly arranged stone circles, but instead elegantly designed ritual centers. Modern excavations have shown that the earliest circles were erected over five thousand years ago and that sightlines were often built into them towards the sun or moon. Whether in northern Scotland, western Ireland, Wales,...