Ancient Egypt

What is the contribution of Claudius Ptolemy?

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2010-12-16 06:36:56

Ptolemy developed several mathematical models to describe the

motions of the sun, the moon, the planets and the stars. The book

that describes these models is called "The Almagest" or "Ptolemy's

Almagest". The models all assume:

1. The earth is stationary.

2. All objects travel with uniform speed on a perfectly circular


However, a model that placed the earth in the centre of the

orbits of all other objects would not work because it does not

account for various anomalies, including the varying length of

seasons, the irregularity of eclipses of the moons and the

retrograde motion of the planets. Ptolemy used two mathematical

devices to deal with the anomalies: eccentricity (an offset from

the centre of a circle) and epicycles - circular orbits whose

centre (or eccentric) itself traveled in a uniform circular


Much of the material in The Almagest was already known to Greek

astronomers. Ptolemy's contribution was to consolidate all known

results, make large improvements to the models, and to write it all

down in a way that became a standard for the way to present the

results of scientific enquiry.

His book was the standard text book for astronomers until his

models were ultimately improved by Copernicus, Kepler and


Ptolemy also wrote a treatise on Astrology, based on his deep

knowledge of the motions of the heavenly bodies. This work is

called The Tetrabiblos.

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