In the pantheon of outstanding characters from early Greece, two individuals in particular advanced complementary kinds of enquiry. Plato (429-347 BC) and Aristotle (384-322 BC), both philosophers of great stature, had opposite approaches to their study. Plato, the pupil of Socrates, sought knowledge through his mind without 'getting his hands dirty' in experiment. Aristotle, though not an experimentalist in the modern sense, observed and researched to find his answers.

Plato made the whole world the subject of his enquiry. He admired geometry, particularly spheres, and thought them significant in understanding the world. His cosmology was based on the notion that reality or truth lay in perfect and beautiful Ideal Forms, accessible to the mind but not part of the physical, ordinary world.

The visible world, according to Plato, was an imperfect reflection of this ideal reality. In that culture it is not surprising that the motions of the stars and planets were thought to be spherical orbits around a spherical earth. To make the theory of celestial motions fit with what people actually saw, no fewer than 27 spheres had to be incorporated into his cosmogony or model of the universe. Plato's urge to fit the outside world with his interior idea of orderly beauty might at first seem ill-conceived and absurd. It is worth bearing in mind that the holy grail of twentieth century science - to find a single explanation that will accurately describe all matter and the motion of that matter- is strikingly Platonic in its ambition.

There must have been something in the water. The line of distinguished thinkers following Socrates and Plato continued with Plato's pupil, Aristotle. Aristotle was accomplished in poetry, rhetoric, ethics, economics, politics, physics, metaphysics, meteorology, anatomy. He is credited with laying the foundations of both deductive reasoning and logic.

Aristotle discovered that free fall is an accelerated form of motion but believed (not unnaturally) that heavier bodies fell faster than lighter bodies. This plausible misconception lasted for 1600 years. His cosmogony consisted of an immobile earth nestled within 55 translucent spheres. Later theorists, confronted with a system bearing little relation to observed reality, were forced to invent epicycles (small cycles on the cir*****ference of larger cycles) and oddities in the orbits of the sun, stars and planets around the earth.

Aristotle had a pupil gifted in military strategy, Alexander the Great (356-323 BC). He extended the Greek empire to Egypt, built the city of Alexandria and installed a general. The general founded a dynasty which included an astronomer whose worldview was to dominate until the seventeenth century.

Ptolemy (c. AD 1OO-168) was born in Egypt, into a dynasty that died with Cleopatra, immortaliser of asps and eyeliner. His work may have lacked some moral rectitude - he fudged some data to fit the theory better, but he did lots of valiant work mapping the stars and planets. Ptolemy was less attached than his predecessors to making his system of the universe fit with a pre-conceived idea of beauty and symmetry. His model universe was complex, even ungainly. He defended his cosmogony by declaring that the complexity of his scheme reflected the complexity of the real world.

The ptolemaic cosmogony, with its stationary earth in the centre of rotating complex cycles of heavenly bodies, was not unreasonable. The notion that the sun travels across the sky seems entirely plausible. There is no immediately obvious evidence that we are rotating (on earth's axis) at a rate of 1670 kilometres per hour (at the Equator) or orbiting the sun at a rate of 108,000 kilometres per hour.

Not every thinker subscribed to this credible, but erroneous, worldview. Aristarchus (c. 310-230 BC) of Samos, a skilled geometer, put forward the idea that the sun might be the centre of the universe and that earth might revolve around the sun. Another oddball -Democritus of Abdera (c. 460-370 BC) conceived the notion that all matter was made of indivisible units he called atomos. Luckily this idea had a long shelf life. Its sell-by date was over 2000 years away.