Ptolemy
c. 100-c. 170
Greek Astronomer and Geographer
Ptolemy is known historically through his written works. His earliest and most noted treatise is the 13-volume set commonly known as the Almagest, which he probably wrote around 150. In these books, he pinpointed the location of more than 1,000 stars, identified the so-called "classical 48" constellations, explained how to calculate latitude and longitude, and predicted solar and lunar eclipses. He also used often complicated mathematical models to help explain the movements of the various celestial bodies. Thecomplexity, in part, derived from his belief that Earth was at the center of the universe, and all stars and planets revolved around Earth.
For example, Ptolemy developed an interesting system to explain why the planets usually, but not always, appear to move forward in their paths across the night sky. Under the erroneous assumption that the planets revolve around Earth, Ptolemy resorted to planetary movements known as epicycles to explain the path abnormalities.
Ptolemy's epicycle hypothesis persisted for well more than 1,000 years. Eventually, however, astronomers understood that the planets only appear to backtrack when viewed from Earth. This illusion results because the planets revolve around the Sun in different orbits and at different speeds. Earth passes a planet in an outer orbit much like a race car passes another in an outer lane. To bystanders in the stands surrounding the race track, it is clear that both cars are moving forward in their paths around the track. The view from the inner car is different. If a video camera were mounted on the car's fender, it would record the outer car apparently slowing as the inner car approached. As the inner car reached and passed the outer car, the video would show the outer car momentarily stopping before beginning to go backward. As the distance between the two cars increased, the camera would eventually see the outer car apparently cease its reversal and begin again to gain forward speed. Likewise, when viewed from Earth, other planets move across the sky at a relatively constant speeds most of the time, but occasionally appear to slow down, remain still, and then backtrack before regaining a forward trajectory.
While the underlying notion of Earth as the middle of the universe was incorrect, Ptolemy's intricate mathematical models were very precise in predicting celestial movements as viewed from this planet.
Ptolemy was also very interested in astrology and the impact of planetary position on human society. His four-volume Apotelesmatica became a primary reference for horoscope readers.
More notable from a scientific perspective, however, were Ptolemy's contributions to geography. His Geography, an eight-volume set, listed the latitude and longitude for many major localities, included a wealth of regional cultural information, and also presented mathematical models describing how to depict the spherical Earth on a two-dimensional map.
Ptolemy also used mathematics in music theory and optics, but the most influential by far were his contributions to astronomy and geography. Ptolemy's work left an imprint on these fields for hundreds of years. His conclusions prevailed well into the sixteenth and seventeenth centuries, when—largely through the studies of Nicolaus Copernicus—the scientific community finally stripped Earth of its central placement in the universe and modern astronomy began.
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