Ptolemy (ad100?-170?), astronomer and mathematician, whose astronomical theories and explanations dominated scientific thought until the 16th century (see Ptolemaic System). He is also remembered for his contributions to the fields of mathematics, optics, and geography. Ptolemy's actual name, Claudius Ptolemaeus, reflects all that is really known of him: The name 'Claudius' suggests a Roman background, while 'Ptolemaeus' hints at a possible Egyptian heritage for him. In fact, ancient sources report that for most of his life he lived and worked in Alexandria, Egypt. At a time when Egypt was ruled by Romans, Alexandria was the center of widespread Greek culture, and we know that Ptolemy wrote his works in Greek.
Ptolemy's earliest and most famous work, originally written in Greek, was translated into Arabic as al-Majisti (Great Work). In Europe, medieval Latin translations reproduced the title as Almagesti, and it has since become known simply as the Almagest. In this work, Ptolemy proposed a geometric theory to account mathematically for the apparent motions and positions of the planets, sun, and moon against a background of unmoving stars. This work did not include any physical descriptions of objects in space.
Ptolemy began by accepting the generally held theory that the earth did not move but was at the center of the universe. The planets and stars were considered, for philosophical reasons, to move continuously in perfectly circular orbits. He then elaborated on the theory in an attempt to account for the astronomical puzzles that the theory presented, such as the apparent backward motions of the planets and apparent variations in size or brightness of the moon and planets. Ptolemy proposed that the planets, sun, and moon moved in small circles around much larger circles, in which the earth was centered. In this way, he made his system fit most of the observations that astronomers had recorded.
Ptolemy used the term epicycle to describe the small circle around which he claimed objects in space move. To make his theory of epicycles appear sound, he had to introduce variations from traditional mathematics. This departure from traditional assumptions was one reason the Polish astronomer Nicolaus Copernicus rejected Ptolemy's system in the 16th century and developed his own heliocentric theory, which correctly stated that the sun was located at the center of the solar system (see Copernican System). Even so, Copernicus retained an elaborate system of epicycles.
Ptolemy also contributed substantially to mathematics by advancing the study of trigonometry, and he applied his theories to the construction of astrolabes and sundials. In his Tetrabiblos, he applied astronomy to astrology and the creation of horoscopes. Of considerable historical importance, is Ptolemy's Geography, which charts the world as people of his time knew it. This work, which employs a system of longitude and latitude, influenced mapmakers for hundreds of years, but it suffered from a lack of reliable information. Ptolemy also devoted a treatise, Harmonica, to music theory, and in Optics he explored the properties of light, especially refraction and reflection. Optics, known only from an Arabic version, stresses experimentation and the construction of special apparatus to promote the study of light and to develop a mathematical theory of its properties.