The ancient astronomer Ptolemy (in Ancient Greek: Κλαύδιος Πτολεμαῖος, Klaudios Ptolemaios) was a Greek astronomer and Roman citizen who lived from around 90 AD to 168 AD in Egypt, at the time part of the Roman Empire. A prolific thinker and scientist, today Ptolemy is known chiefly for his work on astronomy, known today as The Almagest. It remains the only complete and comprehensive treatise on astronomy to survive from ancient times. Ptolemy also wrote books on optics, music and harmony, geography and even astrology.
The title of his key work the Almagest was not the title Ptolemy gave it. The first title attributed to this work was Μαθηματικὴ Σύνταξις or Mathematical Treatise. This was soon expaned to Ἡ Μεγάλη Σύνταξις (hey megalay zuntaxis) or The Great Treatise. When the manuscript was rediscovered by Arab mathematicians and astronomers centuries later, its importance as an astronomical treatise was clearly understood, and they named it The Almagest. In this case, the prefix al- is redundant, al- being the definite article the. The Arab astronomers who found it elevated it from the Great Treatise to the Greatest, and used the Ancient Greek superlative megiste, meaning greatest. Today, its name and importance converge-it is the most important early astronomical treatise, and therefore the greatest!
Image of Ptolemy’s triquetrum (or ‘parallactic instrument’) according to William Cunningham’s The Cosmographical Glasse, conteinyng the Pleasant Principles of Cosmographie, Geographie, Hydrographie, or Navigation (London: John Day, 1559), in the public domain.