Ptolemy compiles the Almagest

In the second century CE, Ptolemy assembled a systematic astronomical treatise in Alexandria, later known as the Almagest. He drew on observations and geometrical work associated with Hipparchus and other predecessors. The book connected calculations for the Sun, Moon, stars, and planets. Descriptions of observations, instruments, and procedures allowed subsequent astronomers to understand what information and mathematical arrangements stood behind a prediction.

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Ptolemy placed Earth at the centre and used eccentric circles, deferents, and epicycles to calculate motions. Planets appear to reverse direction and change speed in the sky. Geometrical combinations made such positions predictable for a specified time. A chord table supplied numerical tools for working with angles. Astronomy was therefore a practice of observation, geometry, and organized computation, as well as an account of the structure of the heavens.

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Arabic translations carried the treatise into new scholarly settings, and its Arabic name later passed into Latin. Astronomers updated observations, compiled tables, and adjusted parameters. Calendar making, navigation, and teaching used this computational tradition. When Copernicus and others changed the cosmic arrangement, they still confronted inherited observations. The Almagest preserved a long accumulation of recorded sky positions that any subsequent explanation had to address.

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