Greek and Hellenistic astronomy Introduction The Greek and Hellenistic astronomy developed from the initial steps taken by Egyptians and Mesopotamians during 2nd and 3rd century BC. The Egyptians and especially the Mesopotamians had done a lot of scientific observations and had named the constellations of stars, and many individual stars. The early Greek contribution to astronomy was not so much in observation as it was in applying logical thinking and geometry to these observations. That is how Greek scientists figured out that the earth went around the sun, calculated the size of the earth, and understood that the moon went around the earth. Some famous Greek astronomers were Anaxagoras, who figured out what caused eclipses, Aristarchus, who figured out that the earth went around the sun, and Thales, who figured out that the earth was round. Eratosthenes Eratosthenes of Cyrene (275-195 BC) was born in Cyrene which is now in Libya in North Africa. Although a brilliant scholar in so many fields, he was never considered the best in any one subject. This earned him the nickname ―Beta,‖ for the second letter in the Greek alphabet. Famous for his 'Sieve of Eratosthenes' to detect the prime numbers Eratosthenes is probably best known for the surprisingly accurate measurement of the circumference of the Earth. Comparing the noon shadow at midsummer between Syene (now Aswan on the Nile in Egypt) and Alexandria, assuming that the sun was so far away that its rays were essentially parallel, using the distance between Syene and Alexandria, he calculated the length of the circumference of the Earth as 250,000 stadia (about 46,250 km). The actual distance at the equator is 40,075km. Eratosthenes‘ other contributions to astronomy include measurements of the distances from the Earth to the moon and Earth to the sun. These were far less accurate than his Earth measurement. He also determined the obliquity of the Ecliptic, measured the tilt of the earth's axis with great accuracy obtaining the value of 23° 51' 15" and created a star map containing 675 stars. Aristotle‘s Universe The Ancient Greeks developed astronomy, which they treated as a branch of mathematics, to a highly sophisticated level. The first geometrical, three-dimensional models to explain the apparent motion of the planets were developed in the 4th century BC by Eudoxus of Cnidus and Callippus of Cyzicus. Their models were based on nested homocentric spheres centered upon the Earth. Nevertheless such ideas soon died out. A different approach to celestial phenomena was taken by natural philosophers such as Plato and Aristotle. In his Timaeus Plato described the universe as a spherical body divided into circles carrying the planets and governed according to harmonic intervals by a world soul. Aristotle, drawing on the mathematical model of Eudoxus, proposed that the universe was made of a complex system of concentric spheres, whose circular motions combined to carry the planets around the earth. This basic cosmological model prevailed, in various forms, until the Sixteenth century. One of the earliest accepted models of the universe is credited to the Greek philosopher Aristotle, who lived in the 4th century BC. Aristotle's cosmos is based around the ideas of natural place and natural motion. A stone thrown up falls down. Water seeks to move to lowlands. On the other hand fire tends to go up. Aristotle reasoned that every object has its ‗natural place‘. There is a centre to the cosmos. Earth and water have their natural place there and their natural motion is towards the centre. Fire and air have a natural motion away from the centre. Not only are the heavens arranged around a central, focal point, the central point is also key to the physics of motion. Heavens are entirely unchanging, according to Aristotle. He disagreed with Heraclides Ponticus who proposed that the Earth rotates around its axis. He argued that humans could not inhabit a moving and rotating Earth without violating common sense perceptions. Aristotle reasoned that if indeed the Earth were moving we would all fall over. Aristotle postulated that the primary four ‗elements‘— earth, water, air and fire were naturally disposed in concentric spheres. Earth, composed of rocks, was at the lowest, that is center. It was surrounded by water and then by air and then by fire- sun. Outside these were the invisible spheres on which the celestial bodies rotated. This was the cosmic model proposed by Aristotle. Ptolemy‘s System One of the main problems with the geocentric, or Earth-centred, model of the universe was that it could not explain all the observed motion of the celestial bodies satisfactorily. Seasonal change in the star position could not be easily explained by Aristotle‘s naïve model. It could not also explain why the Moon waxed and waned over a period of one month. But the apparent motion of the planets were quite puzzling. Stars made an apparent westward movement every day completing the circle once a year. The moon also moved eastward every day completing the circle once in 27/28 days. The planets appeared to move westward. However, occasionally they would appear to stop in their paths and then move eastward (which astronomers call ‗retrograde motion‘ or Vakram in India). Again, they would appear to stop and again start moving westward, as in the beginning. This cycle would be repeated after different intervals of time for different planets. The simple Aristotelian model of celestial bodies revolving around the Earth could not explain such unusual motion. Greek astronomers contrived weird mechanisms to account for the apparently wayward motion of the planets. Spheres within concentric spheres, each having differential rotation and so on. Greek geometrical astronomy developed away from the model of concentric spheres to employ more complex models in which an eccentric circle would carry around a smaller circle, called an epicycle which in turn carried around a planet. The first such model is attributed to Apollonius of Perga and further developments in it were carried out in the 2nd century BC by Hipparchus of Nicea. Hipparchus. During the 2nd century Greek astronomer Claudius Ptolemaeus – or popularly known as Ptolemy, who lived in Alexandria, proposed a complicated system of planetary motions to account for the known motions of the planets, sun, moon and stars. Like others of his time, Ptolemy believed the Earth to be stationary at the centre. The planets, the Sun, Moon and the stars revolved around Earth. Instead of the simple circular path contrived by Aristotle, Ptolemy proposed epicycle model. He imagined the planets to be revolving around small circles called epicycles at a uniform rate, while the centres of the epicycles moved around in a larger circle whose centre was the Earth. In this model, the stars were thought to be fixed to the inside of an invisible sphere- called celestial sphere. The celestial sphere rotated westwards. Ptolemy‘s classic comprehensive presentation of geocentric astronomy, the Megale Syntaxis (Great Synthesis), better known by its Arabic title Almagest had a lasting effect on astronomy up to the Renaissance. Ptolemy‘s epicyclic model could explain the observed motions of the planets quite satisfactorily for some time. However more accurate observations poured in. As Europe came into contact with Arabia the situation became quite complicated. Accurate observations from the observatories around the world- India and China were collected and compiled by Arabs. This data provided a much more accurate positions. Greek astronomers were compelled to modify Ptolemy‘s model. They added epicycle after epicycle to make the model accurate enough to fit with the date. Eventually the model became too cumbersome and complicated and many astronomers started finding it unconvincing.