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6th century BC
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From the 2nd century BC
     Atmospheric devices
     Influential errors of Ptolemy
     Influential errors of Galen
     The Greek legacy

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Hipparchus, a scientific astronomer: 2nd century BC

An observatory is erected by Hipparchus on the island of Rhodes. Here, in 129 BC, he completes the first scientific star catalogue. He lists about 850 stars, placing each in terms of its celestial latitude and longitude and recording its relative brightness on a scale of six.

He measures the altitude of a star by means of an astrolabe, a revolving calibrated disc which will be used for this purpose for nearly two millennia. It is invented either by Hipparchus himself or by his 3rd-century predecessor, Apollonius of Perga. Hipparchus also imagines another use for his astronomical instruments, to create maps of the earth's surface. But this is a task even more demanding than his charting of the heavens.


Hipparchus is so accurate in his placing of the stars that he becomes the first scientist to observe an important phenomenon. Although almost fixed in relation to the sun, the stars move gradually over a long period. This means that at any repeated and identifiable moment in the sun's year, such as the equinox (when day and night are of equal length), the star positions will be seen to have shifted very slightly.

Hipparchus observes this effect in relation to the equinox, and calculates that there is a shift each year of about 45 seconds of arc. It is a phenomenon known now as precession, or the precession of the equinoxes.


Hipparchus has no way of explaining this phenomenon (which is due to a slow wobble of the earth's axis, completing one cycle every 26,000 years), but his accuracy is astonishing. Modern measurements give a figure close to 50 seconds of arc. His 45 seconds are only about 10% out.

The works of Hipparchus are lost. They are known only through the use made of them by Ptolemy, a much less scientific astronomer whose influence derives from the encyclopedic nature of his work. Ptolemy acknowledges the greatness of Hipparchus, and fails lamentably when he tries to improve on his predecessor. Attempting to make the figure for precession more accurate, he moves in the wrong direction - and comes up with 36 seconds of arc.


Greek atmospheric devices: 1st century AD

Hero, a mathematician in Alexandria in about AD 75, enjoys inventing mechanical gadgets, which he describes in his work Pneumatica. Whether he has the technology to make them we do not know, but his scientific principles are correct.

One such gadget is a primitive version of a steam turbine. Hero says steam should be directed into a hollow globe with outlets through nozzles on opposite sides of the circumference. The nozzles are directed round the rim of the globe. As the steam rushes out, like sparks from a catherine wheel, the globe spins.


Hero makes another significant use of atmospheric pressure in a magic altar, putting to work the expansion and contraction of air. A fire heats the air in a container, causing it to expand and force water up a tube into a bucket. The increased weight of the bucket opens the doors of an altar. When the fire is extinguished, the air contracts, the water in the bucket is sucked out and the doors close.

Any temple managing to work this trick is certain to attract more pilgrims, and more money, than its rivals.


The influential errors of Ptolemy: 2nd century AD

Ptolemy, working in Alexandria in the 2nd century AD, is one of the great synthesizers of history. In several important fields (cosmology, astronomy, geography) he brings together in encyclopedic form an account of the received wisdom of his time.

His influence derives from the accident that his predecessors' works are lost while his have survived. Their achievements are known only through him, and when he disagrees with them it is usually he who is wrong. Just as in astronomy he wrongly adjusts the degree of precession of Hipparchus, so in geography he rejects Eratosthenes, whose calculation of the circumference of the earth is very close, and prefers instead another estimate which is 30% too small.


Ptolemy's astronomical work is divided into thirteen books. The first proves that the earth is the immovable centre of the universe; the last five describe the movement of the sun, moon and five planets, each attached to its own crystal sphere. By adding adjustments to reflect the erratic behaviour seen in the sky, Ptolemy achieves a system capable of satisfying scientific enquiry in the unscientific centuries of the Middle Ages.

His book becomes known as Ho megiste astronomas (Greek for 'the greatest astronomer'), or Megiste for short. The Arabs call it Al Megiste (the Megiste). Reaching northern Europe through the Arab civilization in Spain, it acquires its eventual title - as Ptolemy's Almagest.


In geography Ptolemy seems to offer what Hipparchus had proposed - the location of the world's natural and man-made features on a grid of 360° of latitude and longitude. He lists and places some 8000 towns, islands, rivers and mountains. But he is no more capable of providing accurate data, astronomically based, than Hipparchus was. The relative positions of his named features are calculated by collating travellers' accounts of the number of days taken on their journeys.

The results are wildly inaccurate. But the great prestige of Ptolemy means that with the revival of classical learning, in the Renaissance, his errors become enshrined in the earliest printed maps.


The influential errors of Galen: 2nd century AD

The newly appointed chief physician to the gladiators in Pergamum, in AD 158, is a native of the city. He is a Greek doctor by the name of Galen. The appointment gives him the opportunity to study wounds of all kinds. His knowledge of muscles enables him to warn his patients of the likely outcome of certain operations - a wise precaution recommended in Galen's Advice to doctors.

But it is Galen's dissection of apes and pigs which give him the detailed information for his medical tracts on the organs of the body. Nearly 100 of these tracts survive. They become the basis of Galen's great reputation in medieval medicine, unchallenged until the anatomical work of Vesalius.


Through his experiments Galen is able to overturn many long-held beliefs, such as the theory (first proposed by the Hippocratic school in about 400 BC, and maintained even by the physicians of Alexandria) that the arteries contain air - carrying it to all parts of the body from the heart and the lungs. This belief is based originally on the arteries of dead animals, which appear to be empty.

Galen is able to demonstrate that living arteries contain blood. His error, which will become the established medical orthodoxy for centuries, is to assume that the blood goes back and forth from the heart in an ebb-and-flow motion. This theory holds sway in medical circles until the time of Harvey.


The Greek legacy

By the time Ptolemy and Galen are putting into lasting form the fruits of Greek science in two important fields, astronomy and medicine, Rome has long displaced Greece as the dominant power in the Mediterranean and Middle East.

The relative scientific record of these two ancient civilizations is one of the amazing contrasts of history. From Miletus in the 6th century BC to Alexandria in the 2nd century AD, the Greeks produce a glittering stream of scientific experiment and speculation. In Rome's equivalently long period of wealth and power, there is political and military genius in abundance but not a scientist to be seen.


After the official establishment of Christianity in the Byzantine empire, in the 4th century, the Greeks themselves become more interested in theological than scientific speculation. And with the fall of classical civilization to German tribes in the west and to Arabs in the east, it seems at first that the Greek scientific legacy may be lost in the widespread destruction of the 5th to 7th centuries.

But the Arab conquerors, establishing their own civilization in previously Byzantine lands, develop an interest in the old Greek texts. In Arabic translation, Greek manuscripts find their way through Spain to a western Europe ready, by about 1200, for a renewed interest in scientific theory (see Greek texts and the Arabs).


When the texts begin to circulate among the learned in medieval monasteries, it is soon clear how broad a basis has been provided by the Greeks. In fields capable of proof by theorem, such as geometry and mathematics, answers are available in surviving texts of Euclid and Archimedes. In areas where accurate observation is required, Aristotle's work in natural history offers a model of the appropriate method. And in two subjects of absorbing interest, astronomy and medicine, Ptolemy and Galen will stimulate Copernicus and Vesalius to fruitful disagreement.

Medieval science, recovering the Greek texts, is not inclined to experiment. But the springboard is in place for a new attitude to science in the Renaissance.


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