Ancient Greek astronomy

Ancient Greek astronomy is the astronomy written in the Greek language during classical antiquity. Greek astronomy is understood to include the Ancient Greek, Hellenistic, Greco-Roman, and late antique eras. Ancient Greek astronomy can be divided into three phases, with Classical Greek astronomy being practiced during the 5th and 4th centuries BC, Hellenistic astronomy from the 3rd century BC until the formation of the Roman Empire in the late 1st century BC, and Greco-Roman astronomy continuing the tradition in the Roman world. During the Hellenistic era and onwards, Greek astronomy expanded beyond the geographic region of Greece as the Greek language had become the language of scholarship throughout the Hellenistic world, in large part delimited by the boundaries of the Macedonian Empire established by Alexander the Great. The most prominent and influential practitioner of Greek astronomy was Ptolemy, whose Almagest shaped astronomical thinking until the modern era. Most of the most prominent constellations known today are taken from Greek astronomy, albeit via the terminology they took on in Latin.

Greek astronomy was influenced heavily by Babylonian astronomy, as well as Egyptian astronomy to a lesser degree. In later centuries, Greek-language astronomical works were translated into other languages, enabling their further spread. Most notably, Arabic translations of these works benefitted astronomers and mathematicians throughout the Muslim world during the Middle Ages.

Many Greek astronomical texts are known only by name, and perhaps by a description or quotations. Some elementary works have survived because they were largely non-mathematical and suitable for use in schools. Books in this class include the Phaenomena of Euclid and two works by Autolycus of Pitane. Three important textbooks, written shortly before Ptolemy's time, were written by Cleomedes, Geminus, and Theon of Smyrna. Books by Roman authors like Pliny the Elder and Vitruvius contain some information on Greek astronomy. The most important primary source is the Almagest, since Ptolemy refers to the work of many of his predecessors.

The main features of Archaic Greek cosmology are shared with those found in ancient Near Eastern cosmology. They include (a flat) Earth, a Heaven (firmament) where the Sun, Moon, and stars are located, an outer ocean surrounding the inhabited human realm, and the netherworld (Tartarus), the first three of which corresponded to the gods Ouranos, Gaia, and Oceanus (or Pontos).

Thales of Miletus was a primary figure of the Ionian school of Greek philosophy. He is also usually credited for initiating the tradition of Greek science in modern historiography, although others have challenged this attribution. Like his predecessors, such as Hesiod and Homer, Thales accepted that the Earth was flat and rests on a primordial and endless ocean. He also notably proposed that the universe was fundamentally composed of water. The most famous successors of the tradition begun by Thales were Plato and Aristotle; while much thought continued to rely on intuition, the lasting legacy of this work was that it offered non-supernatural explanations for the normal operations of the universe; mathematics (especially geometry) was significantly developed and applied on the problems that were worked on; and it was thought that observation could disqualify candidate explanations for how the world worked.

Anaximander, a student of Thales and another prominent member of the Ionian school, realized that the northern sky seems to turn around the North star, which led him to the concept of a Celestial sphere around Earth. And, as the sky seems to vary with latitude, he also considered that Earth's surface may be curved as well. However, he incorrectly thought that the Earth was a cylinder as opposed to a sphere. The notion of a spherical Earth first found an audience with the Pythagoreans, but this was due to philosophical as opposed to scientific reasons: the sphere was considered a perfectly geometrical figure.

According to Ptolemy in his Almagest (1.2), Greek astronomy was predicated on the following assumptions (or hypotheses in Greek terminology):

The first book of the Almagest included a chapter dedicated to the defense of each of these assumptions and refuting alternative positions, using both philosophy and astronomical observation.