By Aristotle's time, the political world changes radically. Sweeping
down out from Macedonia, Phillip conquers Greece. His son Alexander the
Great (whom Aristotle had tutored) embarks on a series of military campaigns
which create an enormous eastern empire from Macedonia and Greece to Afghanistan,
to India and Egypt.
Even though Alexander's empire fragments at his death, its various parts, headed by his generals, spread Greek philosophy and arts throughout the former empire where the Greek culture evolves in contact with the conquered civilizations. Because the Greek elements dominate, this is called the Hellenistic period. By 200 BCE, a new political power, Rome, had intervened in Greek affairs and by the time of Christ was boasting that the Mediterranean Sea was "Mare Nostrum." (our sea). While Rome had conquered the Hellenistic areas politically and militarily, it is equally fair to say that the Hellenistic culture conquered the Roman empire intellectually and artistically.
Many astronomical advances took place during the hellenistic period, mostly under the influence of astronomers at the Great Library of Alexandria, Egypt.
Erathosthenes (273-192 BCE) was the second head of the Great Library. He was the first to propose a grid system to locate places on the earth, although his system was not based on angular measurements but on distances. He was more successful in measuring the size of the earth; his angular measurement was correct to the minute of arc. Unfortunately the imprecision of distance units reduced the accuracy of the size calculation to some 15%. He also calculated the angle between the ecliptic and the celestial equator.
Aristarchus (310-230 BCE) first to try to figure out the relative distances from the earth to the sun and the moon; although logically correct, his available measurements were so crude that the results were useless; advocated a heliocentric Solar System with a rotating Earth and a motionless sphere of stars so far away that the revolution of the earth would not produce any parallax. This system was rejected by other astronomers for several reasons:
2. the earth is constantly changing, and nothing is perfect; the heavens are perfect and unchanging, and therefore the earth cannot be a heavenly object
3. The notion of a moving earth contradicts the senses. Nothing gets blown off the earth as should happen if the earth were moving through aether. Nor can the earth be rotating. or the centrifugal force would throw objects off the earth.
One of the greatest of these astronomers was Hipparchus (ca. 150 BCE) who produced new star maps and the first western star catalogue of some 800 stars which he classified by a brightness scale essentially still used today), improved calculations of solar and lunar distances to the earth, invented a new sighting instrument and (re)discovered precession. Realizing the limitations of Aristotle's model of the universe, he postulated epicycles and eccentrics to allow for retrograde motion and changes in brightness. He was also the first to locate places (such as cities) by their latitude and longitude. Perhaps his greatest legacy was introducing the notion of exact, quantitative prediction and that prediction must match observation.
Unquestionably, the most famous hellenistic astronomer was Ptolemy. Born in Egypt, residing in Alexandria in the second century C.E., he had at his disposal all the knowledge that was stored in its famous library. Elaborating on Aristotle's geocentric system, and on Hipparchus' epicycles and eccentrics, Ptolemy produced the most mathematically (geometrically) sophisticated system of all antiquity The non-moving earth sat at the center of concentric spheres which carried a planet (directly or indirectly) or stars. To account for planetary retrograde motion and variations in brightness because of distance, some planets actually were carried on smaller circles (epicycles) which in turn were carried on the larger spheres (deferents). To account for the actual motion some were carried on secondary epicycles superimposed on main epicycles. Because this system described the observed motions of the objects in the sky quite well, it would last for nearly 1500 years.