Ptolemy (c. 100 AD- c. 170 AD) was an astronomer and a mathematician whose astronomical theories and explanations dominated scientific thought until the 16th century (Ptolemaic System). Ptolemy proposed a theory to explain the motions and positions of the planets, Sun, and Moon against a background of unmoving stars.

Ptolemy began with the theory that the Earth and was located at the center of the universe. The planets and stars moved in perfectly circular orbits. Ptolemy believed the planets, Sun, and Moon moved in small circles in which the Earth was centered. In this way, he made his system fit most of the observations that astronomers had recorded.

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.

Ptolemy also constructed sundials and maps that charted the world as people of his time knew it. His maps, which used longitude and latitude, influenced mapmakers for hundreds of years. Ptolemy also explored the properties of light, especially refraction and reflection.

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