Hellenistic Astronomy is the study by ancient Greeks based from the Babylonians astronomical observations and used the information for practical goals and to develop cosmological framework to base their philosophical ideas.
Thales of Miletus (620 B.C.E. – 546 B.C.E.)
Thales, known as “father of philosophy”, tried to provide rational explanations for the astronomical events without involving supernatural beings. His explanation of heavenly phenomena was the beginning of Greek philosophical tradition, astronomy and scientific method. It is believed that he predicted the total eclipse. He was the founder of Milesian School of Natural Philosophy to promote the scientific approach and logical deduction from the observational facts. Thales was so committed to his work that he fell into a well while he was observing the night sky.
Aristotle (384 B.C.E. – 322 B.C.E.)
Aristotle by observing stars at different place concluded that the earth is spherical as quoted bellow:
“Indeed there are some stars seen in Egypt and in the neighborhood of Cyprus which are not seen in the northerly regions; and stars, which in the north are never beyond the range of observation, in those regions rise and set. All of which goes to show not only that the earth is circular in shape, but also that it is a sphere of no great size: for otherwise the effect of so slight a change of place would not be quickly apparent.” Aristotle: Book 2, Chapter 14, p. 75
Aristarchus of Samos (310 B.C.E. – 230 B.C.E.)
Aristarchus was a great astronomer and mathematician revolutionized the prevailing idea about position of the earth and the sun in our solar system. He put forward the hypothesis that sun was the center of the universe, the Earth, along with other planets, revolved around the Sun. He believed that the universe is much bigger and the stars are suns far away from us.
This Sun-centered view of the universe is often referred to as “heliocentric”. In fact, a rotating Earth was believed by Heraclides Ponticus (390 B.C.E. – 310 B.C.E.) before him and the Pythagorean tradition also believed that the Earth was not the center of the universe, but that earth revolved around the “Central fire”, an imaginary body believed by them to be the actual source of the light of the universe.
Eratosthenes (275 B.C.E. – 192 B.C.E.)
Eratosthenes, in 240 BCE, calculated the size of the Earth in close proximity to what we know today it. He derived it by measuring the angle of the shadow that the sun made over a vertical pole at Alexandria at noon and observing that at the same time, the sun light fell straight into a well in Syene, a town in southern Egypt. His conclusion of about 45,460 kilometers is very close to the real number. He was the first to realize that our planet was a sphere and used the power of observation, deduction and mathematics to calculate its size.
Hippaarchus of Nicea (190 BCE – 120 BCE)
Hipparchus created the discipline of trigonometry. He also improved the main astronomical instruments of his time (the astrolabes and quadrants). Hipparchus concluded that the geocentric model better explained the observations than did heliocentric model of Aristarchus. The sun-centered model could only stand mathematical conclusion was by guessing that the earth rotates sun in a elliptical orbit, and this guessing was something that Hipparchus was not willing to accept, since the consensus among astronomers was that the planetary orbits were circular. On the other hand, Hipparchus improved the calculations of Aristarchus regarding the sizes and distances of the sun and moon. He calculated the distance of the moon from the earth with an error of only five per cent.