Ptolemy

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Claudius Ptolemaeus (c.85-c.165), or Ptolemy, was a Greek phiosopher who probably worked in Alexandria. He is best known for two works, Almagest which talks of the universe, and Geography, which talks of the shape of the world.

Ptolemaic Universe

Throughout almost all of our period of interest the accepted description of the universe was that of Ptolemy's model of c.150 AD as described in his Almagest. This placed a spherical Earth at the centre of the universe, orbited by the sun, planets and stars. This model was, in turn, based upon the cosmology of Aristotle. It was taught in universities and hence accepted by all educated men and women.

The Earth was spherical - partly because the sphere was the most perfect of all forms, and partly through logical reasoning. When you see a ship coming into the shore, you first see the mast, then the sails, and finally the hull. Aristotle reasoned that this could only be explained logically if the surface of the sea was curved. He also backed this up with observations of the moon and stars showing, for instance, that the pole star is higher in the sky the further north you travel, an observation that also supported the idea that people were travelling on the surface of a sphere.

Beyond the Earth was the moon, and the orbit of the moon divided the universe into the sub-lunar and trans-lunar spheres. The sub-lunar sphere, that below the orbit of the moon, was the realm of man, all living creatures and the laws of physics. The orbit of the moon marked the edge of that region in which man could live and breathe.

The trans-lunar sphere was the abode of the planets, where the sun and the five other wanderers - Mercury, Venus, Mars, Jupiter and Saturn in that order - orbited in perfect circular orbits with the sun orbiting between Venus and Mars. Orbits had to be circular, as the circle is the two-dimensional equivalent of the perfect sphere.

Ptolemy's universe This caused a problem, as it was obvious to anyone watching the paths of the planets across the night sky that they speeded up, slowed down and even moved backwards across at times. Fortunately there was a good circular work-around for this - Ptolemy's model had the planets performing small circular orbits around a point in space that itself orbited the Earth in a perfectly circular orbit. These epicycles, as they were known, made it possible to use the Ptolemeic model to predict the motions of the planets to a high degree of accuracy.

Beyond the planets were the fixed stars, attached to the firmament, the perfect sphere that contained the entire universe. This marked the boundary of what Ptolemy called the world on the far side of the firmament was heaven itself.

This world view had lasted for over a thousand years, but the next year, 1514, would mark a drastic change to the way scientists conceived the universe with the publication of Nicholas Copernicus' heliocentric model.


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