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From Columbia Encyclopedia: celebrated Greco-Egyptian mathematician, astronomer, and geographer. He made his observations in Alexandria and was the last great astronomer of ancient times. Although he discovered the irregularity in the moon’s motion, known as evection, and made original observations regarding the motions of the planets, his place in the history of science is that of collator and expounder. He systematized and recorded the data and doctrines that were known to Alexandrian men of science. His works on astronomy and geography were the standard textbooks until the teachings of Copernicus came to be accepted. The mathematical and astronomical systems developed by the Greeks are contained in his 13-volume work, Almagest. With credit to Hipparchus as his chief authority, he presented in his famous book problems and explanations dealing with the known heavenly bodies and their relations to the earth. The Ptolemaic system thus evolved represented the earth (a globe in form) as stationary in the center of the universe, with sun, moon, and stars revolving about it in circular orbits and at a uniform rate. From the center outward the elements were earth, water, air, fire, and ether. Beyond lay zones, or heavens, each an immense sphere. The planets were assumed to revolve in small circles, called epicycles, whose centers revolved around the earth in the vast circles, or deferents, of the spheres. (To account for the precession of the equinoxes and other phenomena, later astronomers found it necessary to add more epicycles and to make both epicycles and deferents eccentric.) The Almagest also contains other astronomical information, including a catalog of more than 1020 stars (giving their latitudes, longitudes, and magnitudes), as well as mathematical information, including a table of chords. Ptolemy’s system of geography is founded upon the works of Marinus of Tyre; many errors stem from his underestimation of the earth’s circumference. However, his system was in use until the 16th cent. His mathematical theories, most valuable in the field of trigonometry, are preserved in his Analemma and Planisphaerium. His writings, circulated in the original Greek and in Arabic and Latin translations, include also the Tetrabiblos, a study of astrology.
From Wikipedia: Ptolemy also devised and provided instructions on how to create maps both of the whole inhabited world (oikoumenè) and of the Roman provinces. In the second part of the Geography he provided the necessary topographic lists, and captions for the maps. His oikoumenè spanned 180 degrees of longitude from the Canary islands in the Atlantic Ocean to China, and about 80 degrees of latitude from the Arctic to the East-indies and deep into Africa; Ptolemy was well aware that he knew about only a quarter of the globe.
The maps in surviving manuscripts of Ptolemy's Geography however, date only from about 1300, after the text was rediscovered by Maximus Planudes.
Maps based on scientific principles had been made since the time of
Eratosthenes (3rd century BC), but Ptolemy invented improved
projections. It is known that a world map based on the Geography
was on display in Autun (France) in late Roman times. In the 15th
century Ptolemy's Geographia began to be printed with engraved
maps; an edition printed at Ulm in 1482 was the first one printed north
of the Alps. The maps look distorted as compared to modern maps, because
Ptolemy's data were inaccurate. One reason is that Ptolemy estimated the
Earth too small: while Eratosthenes found 700 stadia for a
degree on the globe, in the Geographia Ptolemy uses 500 stadia.
It is not certain if these geographers used the same stadion,
but if we assume that they both stuck to the traditional Attic stadion
of about 185 meters, then the older estimate is 1/6 too large, and
Ptolemy's value is 1/6 too small. Because Ptolemy derived most of his
topographic coordinates by converting measured distances to angles, his
maps get distorted. So his values for the latitude were in error by up
to 2 degrees. For longitude this was even worse, because there was no
reliable method to determine geographic longitude; Ptolemy was well
aware of this. It remained a problem in geography until the invention of
chronometers at the end of the 18th century AD. It must be added that
his original topographic list cannot be reconstructed: the long tables
with numbers were transmitted to posterity through copies containing
many scribal errors, and people have always been adding or improving the
topographic data: this is a testimony of the persistent popularity of
this influential work.
In his Optics, a work which survives only in a poor Arabic translation, he writes about properties of light, including reflection, refraction and colour.
Reading
13-volume work, Almagest (in Greek Hè Megalè Syntaxis, "The Great Treatise"). It was preserved, like most of Classical Greek science, in Arabic manuscripts (hence its familiar name) and only made available in Latin translation (by Gerard of Cremona) in the 12th Century.
Geography
Analemma
Planisphaerium
Tetrabiblos
Writing available on the net
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