The Winter Solstice is on December 21 and marks the shortest day of the year, which was once a very important day to many cultures. In fact, there are thousands of structures across the globe, including the impressive Stonehenge, built by our early ancestors to predict the equinoxes and solstices. So why did they make all this effort? Michael Friedlander, a professor emeritus of physics, and John Kelly, a senior lecturer in archaeology, both at Washington University in St. Louis, introduce us to the field of archaeoastronomy, which they use to examine one of the greatest pre-Columbian civilizations in the United States: Cahokia.