Claudius Ptolemy was a widely influential astronomer, geographer, and mathematician in Ancient Greece whose ideas were the standard in those fields for hundreds and hundreds of years.

As is the case with many people from antiquity, little is known of his early life. He is thought to have born in or around the year 100 and to have been a Roman citizen. He first came to prominence in the city of Alexandria, in Egypt. That city was founded by Alexander the Great a handful of centuries before Ptolemy was born. The Roman Republic conquered Alexandria and then made it into the Empire's second-largest city. Alexandria was known around the world for its expansive Library.

Ptolemy studied the writings of scientists from Ancient Greece and from the Ancient Middle East. Building on their work, he wrote a handful of works that remained standard texts in their subject areas for many generations. The most famous of these is likely Almagest, a 13-volume treatise on astronomy that incorporates the theories of others as well as his own ideas on the various celestial bodies known to have existed at that time and their relationship to Earth. He was a dedicated astronomer and made careful notes of observations he made during a 15-year period ending in 141 (including cataloging more than 1,000 stars) before starting to write Almagest. He finished it in 150. Among his predecessors whom he credited were Aristotle and Hipparchus. Notably, Ptolemy wrote in this work that Earth was the center of the universe and that every other observable heavenly body rotated around Earth. He also felt compelled to describe the movements of those heavenly bodies, as they moved around Earth, by including in Almagest a series of complicated mathematical tables. What he observed, as did many others who devoted time to observing the night sky, is that the Sun and the planets don't move in perfect circles or at a constant speed. How, then, to account for this? He supported Hipparchus's idea of the epicycle, a sort of nested circle that has a smaller circle within in. Introducing this idea made his calculations add up and kept intact the geocentric theory (Earth as the center of the cosmos) and Aristotle's conception of the orbits as perfect spheres. In clinging to these ideas of Aristotle, Ptolemy ignored the claims of Aristarchus, who had lived in Alexandria a few centuries earlier and had put forward the idea that it was the Sun that was the center of the universe. Aristarchus had no evidence to support his theory, and Ptolemy rejected it.

Another of Ptolemy's monumental works was Geography. One of his ambitions in this work was to present a map of the entire known world. Building on the work of an earlier cartographer, Marinus of Tyre, he did so, approximating a drawing of all that he knew about the observed world and roughly keeping the relative size and position of everything the same. He did make a few mistakes, as modern geographers discovered, but he did also include a list of 8,000 places around the world, along with their coordinates of latitude and longitude.