For thousands of years, people have used the sky to track the passage of time. Farmers in ancient Egypt watched for the first morning appearance of the bright star Sirius to know when to prepare for planting. Stonehenge, in England, consists of a group of large stones set in a circular pattern long ago. The locations of certain stones mark the spots where the sun rises or sets on the longest and shortest days of the year. About 1000 years ago in Central America, the Mayans built observatories such as the one shown in Figure 1. They used their observations to develop calendars and even predict astronomical events.

Figure 1  The Mayan ruin of El Caracol, also known as “The Observatory,” is located on the Yucatan peninsula. It is thought that the Mayans used El Caracol for astronomical observations.