Ptolemy, << TOL uh mee >> (A.D. 100?-165?), was one of the greatest astronomers and geographers of ancient times. He was also known as Claudius Ptolemaeus. Almost nothing is known about his life. But scholars have determined that he made his astronomical observations in Alexandria, Egypt, about A.D. 150. Ptolemy’s observations and theories are preserved in a 13-part work entitled Mathematike Syntaxis, or Mathematical Composition. This work was so admired that it became known as the Almagest, meaning the greatest.
In the Syntaxis, Ptolemy rejected the idea that the earth moves. He pointed out that the earth is spherical and claimed that everything in the universe moves either toward or around the earth’s center. For example, heavy objects fall in straight lines toward the earth’s center. And, according to Ptolemy, the moon, sun, planets, and stars move around the earth at various rates of speed. Ptolemy believed that the stars were fixed points of light in a rotating sphere. Against this background of stars, Ptolemy traced the motions of the planets and worked out a theory for each of them. He stated that the planets are much closer to the earth than the stars, but are farther away than the moon. Ptolemy developed his system of astronomy largely from the ideas of the Greek astronomer Hipparchus (see Hipparchus ).
Ptolemy’s system of astronomy was accepted as authoritative throughout Europe until 1543. That year, the Polish astronomer Nicolaus Copernicus formulated his theory that the earth is a moving planet. Copernicus showed that Ptolemy’s system was based on questionable assumptions and arguments (see Copernicus, Nicolaus ).
Ptolemy devoted two parts of the Syntaxis to a catalog of the stars. He described a mathematical arrangement of the stars and gave the celestial latitude and longitude, as well as the magnitude (brightness), for each of them. This catalog included 1,022 stars grouped into 48 constellations. Ptolemy also discovered the irregularity of the moon in its orbit.
Ptolemy dealt with certain technical aspects of astronomy in his other writings. His serious treatment of astrology helped to spread belief in astrology as a scientific field. In his Optics, Ptolemy provided an extensive treatment of light and vision. He also dealt with reflection from various types of mirrors and studied the refraction (bending) of light as it passes from one medium to another.
Ptolemy’s Geography opens with an excellent theory of map projection. The book lists places with their longitudes and latitudes. It also contains a map of the world, including Europe, northern Africa, and most of Asia, as well as 26 maps of specific areas. Ptolemy exaggerated the land mass from Spain to China and underestimated the size of the ocean. This mistake encouraged Christopher Columbus to make his famous voyage in 1492.
