in Alexandria, on the outskirts of the empire, a school of Egyptian astronomers led by Ptolemy and Hipparchus flourished four centuries after Aristotle. Their calculation of the earth’s circumference (twenty-five thousand geographical miles) was surprisingly accurate. They also partitioned the globe into 360 degrees, lined its surface with parallels of latitude and meridians of longitude, and, with their invention of the astrolabe, which measured latitude by “shooting the sun,” provided an instrument which was to be used by mariners, including Magellan, until the Elizabethan Age.
