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					                           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.