Knowra Ptolemy Ptolemy Ptolemy was a second-century Greco-Egyptian astronomer and mathematician whose Almagest set out a geocentric system using circles and epicycles to model planetary motion.
Almagest : Ptolemy’s astronomical treatise presents a mathematical account of the apparent motions of the Sun, Moon, and planets. This is the work that gives Ptolemy’s geocentric astronomy its fullest form.
Alexandria : An Egyptian city founded by Alexander the Great that became a major center of Hellenistic scholarship. Ptolemy worked in the intellectual world of Roman-era Alexandria.
Geography (Ptolemy) : Ptolemy’s second-century treatise on mapping the known world with coordinates and projection methods. It applies mathematical ordering to places on Earth, paralleling his treatment of the heavens.
Islamic Golden Age astronomy : Astronomical research in the medieval Islamic world, including observation, mathematical modeling, and the revision of inherited works. Scholars translated, criticized, and developed Ptolemaic astronomy across centuries.
Deferent and epicycle : A deferent is a large circle carrying a smaller epicycle on which a planet moves in a geocentric model. Ptolemy used these circles to reproduce planets’ changing apparent speeds and retrograde loops.
Hellenistic astronomy : The mathematical study of celestial phenomena developed in the Greek-speaking Mediterranean after Alexander’s conquests. Ptolemy’s astronomy belongs to this tradition of geometric models and quantitative prediction.
Tetrabiblos : Ptolemy’s treatise on astrology, interpreting celestial configurations as influences on terrestrial events and human affairs. It presents the astrological counterpart to the astronomical calculations in the Almagest.
Copernican heliocentrism : The model proposed by Nicolaus Copernicus that places the Sun near the center and Earth in motion around it. Copernicus reorganized planetary motion around the Sun rather than accepting Ptolemy’s Earth-centered arrangement.
Equant : A point offset from a circle’s center from which uniform angular motion appears to be measured. Ptolemy used the equant to improve predictions while departing from uniform motion centered on the deferent.
Babylonian astronomy : A tradition of observing and predicting celestial phenomena in ancient Mesopotamia, recorded extensively on cuneiform tablets. Babylonian observations and numerical methods contributed to the wider inheritance available to Greek astronomers.
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