In far northern latitudes, hope arrives on December 21. The next night will be one merciful second shorter; a week later, night has relinquished a minute and a half to day. For ancient people, the struggle to survive until spring must have seemed just a little easier as the sun offered more of its feeble warmth each day after the winter solstice. But how to know when to start hoping, when to celebrate that momentous shift? 

About 7,000 years ago, people began building wood and stone circles to mark out the sun’s path, from its extreme northern rise in summer to its farthest southern set in winter. Cattle herders built the earliest known example of these at Nabta Playa, a dry lake bed deep in the Sahara. Over the next 2,000 years, people all over the world assembled hundreds of simila...