Claudius Ptolemy, living in Alexandria in the second century, compressed the entire accumulation of ancient astronomy into a single vast work. Titled in Greek the "Mathematical Compilation," it became known by the name Arab scholars gave it, meaning "the greatest": the "Almagest." Within it are a star catalogue, methods of observation, and — most importantly — a complete mathematical model computing the motions of the Sun, Moon, and planets.

Ptolemy's model was Earth-centred: a motionless Earth stands at the centre of the universe, the heavenly bodies turning around it. But the sky could not be explained by simple circles; the planets seemed at times to slow, stop, even go backward (retrograde motion). Ptolemy solved this with ingenious geometry: each planet moves on a small circle (epicycle) that itself turns on a large circle (deferent). With enough circles added, the model predicted the sky with remarkable accuracy.

This is why the Almagest stood for fourteen hundred years: though wrong, it worked. A model that could correctly predict the sky did not have to be actually true — one of the most enduring lessons of the philosophy of science. The system was complex, but it agreed with observation and sat comfortably with the religious worldview.

The Almagest was studied, corrected, and extended in the Islamic world for centuries, and returned to Europe from there. But when Copernicus in 1543 explained the same observations far more simply by placing the Sun at the centre, Ptolemy's model was shaken; Kepler's ellipses and Newton's law of gravity buried it entirely. Yet Ptolemy's achievement cannot be belittled: his was the work that tamed the sky with mathematics and first showed, at this scale, the predictive power of theory.