More than 2,200 years ago, Eratosthenes, who was a Greek astronomer, geographer, and mathematician, used a simple proportion involving the distance between two ancient cities and measures of shadows cast in those cities during a summer solstice to estimate the circumference of Earth (Nicastro 2008, 25-28). Today, middle school students can use similar proportional reasoning with physical measurements easily collected by a smart phone and trundle wheel to duplicate the feat of estimating Earth's circumference. Within this hands-on discovery activity, students explore the seventh-grade Common Core standard to "analyze proportional relationships and use them to solve real-world and mathematical problems" (CCSSI 2010, p. 47) using popular technology and integrating concepts of latitude and longitude.