While many concepts on astronomy of the ancient Greeks are no longer relevant, many of their ideas are used to guide modern astronomers in establishing concepts more valid. The ancient Greeks had no clear division of science and philosophy. Neither did they have separate and developed fields such as physics, chemistry, biology, and astronomy. The formerly deficient range of information the ancient Greeks had, allowed an individual to become an expert in several fields whereas nowadays, there is a tendency for an individual to have an intensive knowledge in a more defined and rather limited field.One of the most influential theories the ancient Greeks had, was what the centre of the universe was. Contrary to what has been proven today, the ancient Greeks believed that the Earth was the center of our Universe opposed to the Sun — referred to as a geocentric universe. The ancient Greeks have considered the Moon and the Sun as planets in addition to Mercury, Venus, Mars, Jupiter, and Saturn. The days of the week were later named after the gods who were believed to have been represented by these “planets”.In the third century B.C.E., Aristarchus of Samos presented the first known model in which the Sun was at the centre of the universe, and the Earth revolved around it — referred to as a heliocentric universe. However, this perception was generally unaccepted and it was not until the 16th century that Nicolaus Copernicus revived the idea. Following the 16th century, the invention of the telescope in the 17th century aided 17th-century astronomers to realize that Earth, among the other planets, did in fact orbit around the Sun. Furthermore, it was recognized that the Moon is a satellite of the Earth — not a planet.Though the ancient Greeks lacked the technology to fly out into space, they were nonetheless able to clarify that the Earth is spherical. The earliest argument for why the Earth is round was provided by Pythagoras in 500 B.C. According to Pythagoras, the Moon was a sphere; thus, making sense that the Earth is spherical as well. Aristotle presented a century and a half later, the earliest physical evidence that the Earth is round. Aristotle indicated that ships sailing over the horizon would disappear hull first and that during a lunar eclipse, a round shadow is cast on the Moon by Earth. Additionally, at different latitudes, different constellations are visible. It was during this time that Greek philosophers had begun discovering scientific explanations of the world rather than resorting to the gods.Early astronomers then began making physical measurements partially to better comprehend the seasons experienced throughout the year. Eratosthenes of Cyrene was the first person to determine the size of the Earth, combining geometrical calculations with physical observations to produce a reasonably accurate measurement. Though the details of this measurement were in a now-lost manuscript, his technique has been recorded by other Greek historians and writers.Thales of Miletus, an ancient Greek philosopher, had many theories about the Earth and the space around it, including the eclipse and equinox. Thales is believed to have anticipated the eclipse of the Sun on the 28th of May B.C. While it is unknown how Thales got to his conclusion of the solstices, he was mentioned to be the first to have determined the course of the Sun from solstice to solstice. Presumably with the exploration of solstices and equinoxes, the length of a solar year was simultaneously recognized by Thales. Although the Egyptians have known about the seasons for millenniums, Thales was the first to explain the seasons scientifically, for his understanding of the solar year.Understanding the movement of the Sun had many wondering what the Sun was composed of. Anaxagoras, an ancient Greek philosopher and scientist, was notorious for his materialistic views, including his argument of the Sun being a fiery rock, contrary to the angry god many others believed the Sun to have been. Additionally, he had determined the cause of an eclipse. He saw that depending on whether it is a lunar or solar eclipse, the Moon would come between the Sun and the Earth or the Earth between the Sun and the Moon. Hipparchus created a star catalog of each of the star’s names along with his measurements of their positions. However, the evidence of this catalog is poor and is particularly according to Pliny the Elder, a Roman scholar. Hundreds of years later, Ptolemy composed a 13-volume summary of Greek astronomy. Translated into Arabic and collectively known by their Arabic title “Almagest”, this piece of work included a star chart many believed to have been predominantly derived from Hipparchus’s catalog. In addition to the star chart, this summary included texts on trigonometry, complete information on the Sun and Moon, and most significant for prevailing astronomy, an intricate model for predicting the positions of the planets in the future. While ancient Greek astronomy may have included many inaccurate discoveries, concepts such as our heliocentric solar system and Earth being spherical, have encouraged astronomers today to accurately make new breakthroughs. Without the knowledge left by the ancient Greek astronomers, astronomers today may still believe the Earth to be the centre of our universe and provoke inaccurate calculations of the planets and stars around us and the composition of the Sun may yet to be determined.