Early astronomer-astrologers subscribed to the theory that the Earth is at the center, with planets in motion around the ecliptic. 2nd-century Hellenistic (a.k.a. Greek) astronomer Ptolemy published a comprehensive work called Almagest, with this geocentric model as the foundation. The Earth-as-center theory was challenged around the 17th-century by Copernicus and Gallileo. The geocentric model was replaced with the heliocentric model, one with the Sun at the center.