Claudius Ptolemaeus, commonly known as Ptolemy, was a Greco-Egyptian mathematician, astronomer, and geographer who lived in the 2nd century AD. He is best known for his work "Almagest," which was a comprehensive treatise on astronomy and the motion of celestial bodies. Ptolemy's geocentric model of the universe, which placed the Earth at the center with the sun, moon, and planets orbiting around it, was widely accepted for over a thousand years. In addition to his astronomical work, Ptolemy also made significant contributions to geography, creating maps and a guide to drawing maps that remained influential for centuries. His work had a lasting impact on the fields of astronomy, geography, and cartography.