Previous page Page 2 of 4 Next page
Index |  History |  Highlights |  WhatWhenWhere
6th century BC
5th century BC
     Pythagoreans and astronomy
     Heavenly spheres
     The four elements
     Electricity and magnetism
     Democritus and the atom
     The birth of biology

4th - 3rd century BC
From the 2nd century BC

Bookmark and Share
The Pythagoreans and astronomy: 5th century BC

Followers of Pythagoras, in the 5th century, are the first to produce an astronomical theory in which a circular earth revolves on its own axis as well as moving in an orbit. The theory derives in part from the need to locate the great fire which they believe fuels the universe.

The Pythagoreans place this fire at the hidden centre of things, with the earth revolving round it more closely than any of the other bodies visible in the sky. The reason why we never see or are scorched by the fire is that we live on only half the sphere of the earth, and the earth revolves so that our half is always turned away from the flames.


Moving outwards from the earth in the sequence of heavenly bodies, they place the moon next, then the sun, the planets and finally the stars, which are unlike the others in being fixed on an outer sphere.


Heavenly spheres: from the 5th century BC

This theory introduces the concentric circles which become the false orthodoxy of the next 2000 years, as eventually enshrined by Ptolemy. It also starts a wild goose chase which will exercise many brilliant minds: what mechanical model can explain the erratic motion of the planets? Eudoxus of Cnidus, in the 4th century, is the first to propose a series of transparent spheres in the heavens, carrying the heavenly bodies at different speeds in linked groups with slightly varying centres.

To make such machinery conform to what can be observed in the sky, ever more complex arrangements are needed. Later in the 4th century Aristotle believes he has solved it. He requires no fewer than fifty-five transparent spheres.


The Pythagoreans are too far ahead of their time in proposing their one central grain of truth - the revolving globe of the earth. But Copernicus, developing this idea, will acknowledge them as his earliest predecessors.

For most Greek astronomers there seems to be overwhelming evidence that the earth is stationary and the heavens move. This is true even of the greatest among them, Hipparchus. Like his predecessors, he believes that it must be possible to analyze the movement of the spheres. He finds the available data inadequate, so devotes himself not to cosmology but to the prime task of an astronomer - observation of individual stars.


The four elements: c.450 BC

The Greek philosopher Empedocles, a native of Sicily, introduces a theory which will be accepted in Europe until the 17th century. He states that all matter is made up, in differing proportions, of four elemental substances - earth, air, fire and water. Not until the arrival of a 'sceptical chemist' (the title of a book by Robert Boyle in 1661) is there a serious threat to this Greek theory of the elements.

Soon an equivalently simple notion is put forward to account for the make-up of living creatures, in the theory of the four humours.


Electricity and magnetism: 5th century BC

Two natural phenomena, central to the study of physics, are observed and speculated upon by Greek natural scientists - probably in the 5th century BC, though Aristotle gives credit for the first observation of each to the shadowy figure of Thales.

One such phenomenon is the strange property of amber. If rubbed with fur it will attract feathers or bits of straw. Modern science, in its terms for the forces involved, acknowledges this Greek experiment with amber (electron in Greek). The behaviour of the amber is caused by what we call electricity, resulting from the transfer of what are now known as electrons.


The other natural phenomenon, observed in lodestone rather than amber, also derives its scientific name from Greek experiments. Lodestone is a naturally occurring mineral (formed of iron oxide), and it will surprisingly attract small pieces of iron. .

The Greeks find this mineral in a region of Thessaly called Magnesia. They call it lithos magnetis, the 'stone of Magnesia'. Thus the magnet is identified and named, though like rubbed amber it will only be a source of interest and amusement for the next 1000 years and more - until a practical purpose is found for it in the form of the compass.


Democritus and the atom: c.420 BC

In the late 5th century BC Democritus sets out an interesting theory of elemental physics. Notions of a similar kind have been hinted at by other Greek thinkers, but never so fully elaborated.

He states that all matter is composed of eternal, indivisible, indestructible and infinitely small substances which cling together in different combinations to form the objects perceptible to us. The Greek word for indivisible is atomos. This theory gives birth to the atom.


Democritus describes an extraordinary beginning to the universe. He explains that originally all atoms were whirling about in a chaotic manner, until collisions brought them together to form ever larger units - including eventually the world and all that is in it.

His theory will find few followers over the centuries. But his imagination provides an astonishing first glimpse of the Big Bang.


The birth of biology: 5th - 4th century BC

The Greek philosophers, voracious in their curiosity, look with interest at the range of living creatures, from the humblest plant to man himself. A Greek name is coined by a German naturalist in the early 19th century for this study of all physical aspects of natural life - biology, from bios (life) and logos (word or discourse). It is a subject with clear subdivisions, such as botany, zoology or anatomy. But all are concerned with living organisms.

The first man to make a significant contribution in biology is Alcmaeon, living in Crotona in the 5th century. Crotona is famous at the time for its Pythagorean scholars, but Alcmaeon seems not to have been of their school.


Alcmaeon is the first scientist known to have practised dissection in his researches. His aim is not anatomical, for his interest lies in trying to find the whereabouts of human intelligence. But in the course of his researches he makes the first scientific discoveries in the field of anatomy.

The subsequent Greek theory, subscribed to even by Aristotle, is that the heart is the seat of intelligence. Alcmaeon reasons that since a blow to the head can affect the mind, in concussion, this must be where reason lies. In dissecting corpses to pursue this idea, he observes passages linking the brain with the eyes (the optic nerves) and the back of the mouth with the ears (Eustachian tubes).


Aristotle may be wrong about the brain being in the heart, but in general he gives a far more complete and well observed account of biology than any other Greek philosopher.

He inaugurates scientific zoology in his reliance on careful observation. He is particularly acute in his study of marine life, having much to say on the habits of fishes, the development of the octopus family, and the nature of whales, dolphins and porpoises. He is also a pioneer in attempting a system of classification. Observing an unbroken chain of gradual developments, as the life of plants shades into that of animals, he acknowledges the complexity of the subject and seems almost to glimpse the pattern of evolution.


Aristotle's notes on botany are lost, but many of his observations no doubt survive in the earliest known botanical text - nine books On the History of Plants written by Aristotle's favourite pupil, Theophrastus.

Writing in about 300 BC, Theophrastus attempts to classify plants, as well as describing their structure, habits and uses. His remarks are based on observations carried out in Greece, but he also includes information brought back from the new Hellenistic empire in the Middle East, Persia and India, resulting from the conquests of Alexander the Great.


Previous page Page 2 of 4 Next page