Episode 6: Philosopher Scientists.

The innovative ideas of the early Greeks. Miletus was, 2600 years ago, one of the most powerful cities in the eastern Mediterranean, with trade links to ancient Babylon and Egypt. It was home to one of the first Greek philosophers, Thales, who was the first person to suggest that natural forces rather than the gods might be responsible for dramatic events such as earthquakes and even eclipses. As one of the first true scientists, he successfully predicted a total eclipse of the sun in 585 BC, bringing an end to a 15 year war.

It was on the recommendation of Thales that Pythagoras travelled to Egypt, developing an interest in geometry that led him to realise that the Earth was a sphere. This was the start to a series of philosophical schools which went on to calculate the relative sizes and distances of Earth, Moon and Sun and, under Aristarchus, even to suggest that the Sun and not the Earth was the centre of the Universe.

Episode 7: Wheels within wheels.

Later Greeks measure and map the heavens.In 1900, a Greek diver exploring an ancient shipwreck recovered a corroded lumps of metal which seemed to have gears embedded in it. It has taken a century to discover the true sophistication and purpose of this "Antikythera mechanism". It is, in effect, an astronomical computer, able to calculate the movements of Sun, Moon and perhaps even certain planets and yet it is more than 2000 years old. It may owe its ingenuity to two of the greatest later Greek astronomers, Archimedes and Hipparchus, both of whom made accurate measurements of the heavenly bodies and studied the mathematics of their movements.

Around the year AD 150, Alexandria was the home to the last major sky watcher of ancient times: Ptolemy. He drew together all of the Greek knowledge of the heavens in a 13 volume masterpiece best known today by its later Arabic title, the Almagest. With its definitive list of over a thousand stars and 48 constellations, it became the standard astronomical text for the next 1400 years, even though it was based on an Earth-centred model of the universe, with the Sun revolving around us.

Episode 8: Mathematics of the Sky.

Islamic science fuels the torch of discovery. During the dark ages in Europe, knowledge of the heavens and an interest in astronomy were kept alive in Arabic and Persian lands. Omar Khayyam was not only a great poet but an astronomer, philosopher and mathematician. He measured the length of the year with great precision and devised a calendar that is more accurate than the one we use today. Many of the names of the stars as well as the number system with which we count and calculate have Arabic origins.

Centuries before the invention of the telescope, Ulugh Beg's great Samarkand Observatory was making accurate observations of the movements of the Sun and planets and the positions of the stars. Muslim scholars not only kept the torch of astronomy alight until it could be passed back to Western Europe, they also fuelled it with new knowledge.

Episode 9: The Earth Moves

How Copernicus de-centralised the Earth.In the 16th century, the world word revolution came to mean not only a circular motion such as that of a planet but also a fundamental change in order or understanding. The key person who brought about that change was Nikolaus Copernicus, a canon in a remote Polish cathedral. For centuries, Western scholars, who were almost exclusively in the church, had accepted the cosmology of Aristotle, as championed by Thomas Aquinas. That, as recorded by Ptolemy in the Almagest, focused on the Earth being at the centre of the cosmos. That had, in turn, led to more and more complex explanations to fit the observed motions of the planets into such a geocentric scheme.

Copernicus pointed out that it would be simpler if they were to take the Sun as the centre of the system and put the Earth and other planets in orbit around it. For many years he was reluctant to publish this heretical suggestion and was only persuaded to do so by a young Austrian named Rheticus. In the end, Copernicus read the final page proofs on the day that he died in 1543. But his book, On the Revolutions of the Heavenly Spheres, was indeed revolutionary.

Episode 10: The Imperfect Cosmos.

Kepler and Tycho turn the Universe on its side. One of the greatest observational astronomers in the years just before the invention of the telescope was a Danish nobleman named Tycho Brahe. He boasted a prosthetic metal nose and a pet elk! But he also set up a lavish Observatory on the Danish island of Hven. His most famous observation occurred on the 11th of November 1572 when he saw what seemed like a new star flaring up in the sky. As it faded over the next few weeks he realised that the heavens are not perfect and unchanging as some philosophers would have them be.

Some years later, he met up with a young German mathematician named Johannes Kepler. They couldn't have been less alike. Tycho was loud and domineering, Kepler was withdrawn though forceful to the point of rudeness. Tycho thought that the Earth was the centre of the universe, Kepler agreed with Copernicus that our planet circles the Sun. But Kepler needed Tycho's accurate observations. Kepler proposed that the planets were pulled around the Sun not by divine forces but by something emanating from the Sun. He reasoned that, since the planets appear to be slightly closer to the Sun at some times, they should move faster at these times. That would make their orbits not circular but elliptical. With the help of Tycho's observations he had found a way to calculate planetary orbits mathematically.

Friday 6 June 9.00pm - Omnibus edition

A special edition based on the this week's programmes.