The science of astronomy is one of humanity's oldest sciences. No one knows quite when the first people looked up and began to study the sky, but we do know that very early people began noting the sky thousands of years in the past. Written astronomical records were recorded in ancient times, often on tablets or walls or in artwork. That was when observers began charting what they saw in the sky. They didn't always understand what they observed, but realized that the sky's objects move in periodic and predictable ways.Claudius Ptolemy with an armillary sphere he used to predict solstice dates and other celestial sights. Public domain, via Wikimedia Commons.
Claudius Ptolemy (often called Claudius Ptolemaeus, Ptolomaeus, Klaudios Ptolemaios, and simply Ptolemeus) was one of the earliest of these observers. He systematically charted the sky to help predict and explain the motions of the planets and stars. He was a scientist and philosopher who lived in Alexandria, Egypt nearly 2,000 years ago. Not only was he an astronomer, but he also studied geography and used what he learned to make detailed maps of the known world.
We know very little of Ptolemy's early life, including his birth and death dates. Historians have more information about his observations since they became the basis for later charts and theories. The first of his observations that can be dated exactly occurred on March 12, 127. His last recorded one was February 2, 141. Some experts think 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
Ptolemy's work was quite advanced, considering that he didn't have the types of tools that astronomers rely on today. He lived in a time of "naked eye" observations; no telescopes existed to make his life easier. Among other topics. Ptolemy wrote about the Greek geocentric view of the universe (which put Earth at the center of everything). That view seemed to quite nicely put humans at the center of things, as well, a notion that was hard to shake until Galileo's time.
Ptolemy also 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 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 drawing by astronomer Jean Dominique Cassini was influenced by the epicycles that Ptolemy refined by his mathematics and observations of the sky. public domain
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 and too complicated. As with most other scientific ideas, simpler is better, and coming up with loopy circles wasn't a good answer to why planets orbit the way they do.
Ptolemy the Writer
Ptolemy was also a prolific writer in the subjects and disciplined he studied. For astronomy, he 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 numerical and geometrical 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 that are still in use today.
As a further 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 because the Sun does not orbit Earth. But, without more knowledge of the solar system, it would have been very difficult for him to know that. However, his systematic approach to charting and measuring sky events and objects was among the first scientific attempts to explain what happens 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.A page of Ptolemy's "Almagest" translated and reproduced by Edward Ball Knobel. public domain
Ptolemy was also very important in the history of geography and cartography. He was well aware that 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. Some scholars think that the maps Ptolemy created may have been a deciding factor in Columbus's decision to sail west for the Indies and ultimately discover the continents of the western hemisphere.
Fast Facts about Ptolemy
- Not much is known about Ptolemy's early life. He was a Greek citizen living in Alexandria, Egypt.
- Ptolemy was a cartographer and geography, and also worked in mathematics.
- Ptolemy was also an avid skygazer.
- Claudius Ptolemy, www2.stetson.edu/~efriedma/periodictable/html/Pm.html.
- “Claudius Ptolemy.” Ptolemy (about 85-about 165), www-groups.dcs.st-and.ac.uk/~history/Biographies/Ptolemy.html.
- “Notable People.” Who Was Claudius Ptolemy, microcosmos.uchicago.edu/ptolemy/people.html.?
Edited and updated by Carolyn Collins Petersen