Ptolemy (aka Claudius Ptolemaeus, Ptolomaeus, Klaudios Ptolemaios, Ptolemeus) was a scientist and philosopher who lived in Alexandria, Egypt nearly 2,000 years ago. He played an important role in the history and development of astronomy and geography. We know very little of his early life, including his birth and death dates. We know more about his observations. The first observation made by him which we can date exactly was on March 12, 127.

His last recorded observation was February 2, 141. Some experts believe his life spanned the years 87 – 150. However long he lived, Ptolemy did much to advance science and appears to have been a very accomplished observer of the stars and planets.

We get a few clues about his background from his name: Claudius Ptolemy. It's a mixture of the Greek Egyptian "Ptolemy" and the Roman "Claudius". Together, they indicate that his family was probably Greek and they had settled in Egypt (which was under Roman rule) for some time before his birth. Very little else is known about his origins.

### Ptolemy, the Scientist

What we *do* know more about is Ptolemy's work. He was an astronomer, mathematician, and geographer. Among other topics. Ptolemy wrote about the Greek geocentric view of the universe (which put Earth at the center of everything), and calculated the apparent motions of the known planets. He did this by synthesizing and extending the work of Hipparchus of Rhodes, an astronomer who came up with a system of epicycles and eccentric circles to explain why Earth was the center of the solar system.

Epicycles are a small circles whose centers move around the circumferences of larger ones. He used at least 80 of these tiny circular "orbits' to explain the motions of the Sun, the Moon, and the five planets known in his time. Ptolemy expanded this concept and made many fine calculations to fine-tune it.

This system came to be called the Ptolemaic System. It was the linchpin of the theories about objects' motions in the sky for nearly a millennium and a half. It predicted the positions of the planets accurately enough for naked-eye observations, but it turned out to be wrong in the long run.

### Ptolemy the Writer

Ptolemy described his system in his books that make up the *Almagest* (also known as *Mathematical Syntaxis*). It was a 13-volume mathematical explanation of astronomy, containing information about the mathematical concepts behind the motions of the Moon and known planets. He also included a star catalog that contained 48 constellations (star patterns) he could observe, all with the same names still in use today. As an example of some of his scholarship, he made regular observations of the sky at the time of the solstices and equinoxes, which allowed him to figure out the lengths of the seasons. From this information, he then went on to try and describe the motion of the Sun around our planet. Of course, he was wrong, but his systematic approach was among the first scientific attempts to explain what he saw happening in the sky.

The Ptolemaic System was the accepted wisdom about the motions of the solar system bodies and the importance of Earth in that system for centuries.

In 1543, the Polish scholar Nicolaus Copernicus proposed a heliocentric view which put the Sun at the center of the solar system. The heliocentric calculations he came up with for the movement of planets were further improved by Johannes Kepler's laws of motion. Interestingly, some people doubt that Ptolemy truly believed his own system, rather he merely used it as a method of calculating positions.

Ptolemy was also very important in the history of geography and cartography. He was well aware that the Earth is a sphere and was the first cartographer to project the spherical shape of the planet onto a flat plane. His work, *Geography* remained the principal work on the subject until the time of Columbus. It contained amazingly accurate information for the time and given the difficulties of mapping that all cartographers raced.

But it did have some problems, including an overestimated size and extent of the Asian landmass. The maps he created may have been a deciding factor in Columbus's decision to sail west for the Indies.