Ptolemy's Almagest

The famous book after which Almagest the clock is named

His almageste, and bookes grete and smale,
His astrelabie, longynge for his art,
His augrym stones layen faire apart,
On shelves couched at his beddes heed...
from Chaucer's Canterbury Tales - The Miller's Tale

The miller is describing the living quarters of a 'poor scholar' in Oxford (Chaucer was writing in about 1380). He has already explained that the scholar is learned in astronomy, but with a leaning towards astrology. He likes to tell the future from the stars. Evidently, his 'Almagest' is one of the books that he needs to have to hand, along with his astrolabe.

What is this 'Almagest'? It seems that the word would have meant something to people in the 14th century. In those days when there were very few books at all, Almagest was well-known; today it has fallen into relative obscurity.

Almagest is a book with a fascinating history. Actually written in the 2nd century A.D., why was it so prized in Oxford (and elsewhere) 1200 years later?

The reason is, that Almagest was where you turned if you wanted to know how to calculate the movements of the planets. Many other books had been written on the subject, mostly in Arabic, but they were all based on this single masterwork, 'The Greatest'. For that is what 'Al-magest' means: a bizarre mixture of Greek and Arabic, the 'al' meaning 'the' in Arabic, the 'magest' being a version of 'megistos', the Greek for 'greatest'.

The author of Almagest was Claudius Ptolemaeus who lived in the city of Alexandria, Cleopatra's capital, at the mouth of the Nile. Again the author's name speaks of a fascinating mix of cultures. His first name 'Claudius' indicates that he was a Roman citizen, whereas 'Ptolemy' was the name of the Greek kings of Egypt, originally installed by Alexander the Great.

Ptolemy was a great collector and compiler of information. Besides astronomy he also wrote on geography and indeed astrology. But it seems that he made some original contributions to the science of mathematical astronomy, too. Almagest (originally called simply 'Mathematical Compilation') was an encyclopaedia of everything that was known at the time about that science. It is now the chief source of everything we know about the astronomy of the ancient world. Just how advanced this knowledge was, is shown by the fact that Almagest was only superseded as the cornerstone of astronomical calculation by Copernicus' momentous work De Revolutionibus Orbium Caelestium ('On the Orbits of the Celestial Bodies'), published in 1542.

What would Ptolemy make of Almagest the clock, if he could see it?

We believe he'd be impressed!

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