Keeping time was challenging for ancient civilizations, so this one built a city to do it. This dispatch is from the ancient archaeological site at Teotihuacán, in Mexico. I walked around the ruins with a guide, and learned about how the people who built the site planned it in a way that helped them track time. Scholars theorize that the structures at Teotihuacán were built to align with the cosmos on certain days of the year, which let the people know when it was time to plant crops or conduct rituals.