Wednesday, June 9, 2010

A Brief History of Cosmological Ideas

A Brief History of Cosmological Ideas


The Greek philospher Aristotle proposed that the heavens were literally composed of 55 concentric, crystalline spheres to which the celestial objects were attached and which rotated at different velocities (but the angular velocity was constant for a given sphere), with the Earth at the center.

The Ptolemaic System

The prevailing theory in Europe as Copernicus was writing was that created by Ptolemy in his Almagest, dating from about 150 A.D. The Ptolemaic system drew on many previous theories that viewed Earth as a stationary center of the universe. Stars were embedded in a large outer sphere which rotated relatively rapidly, while the planets inhabited smaller spheres as . The idea that the Earth was at the centre of the universe with everything revolving around it was one that fitted with religious beleifs. Afterall, man is the most important of God's creations and so it was proper that the Earth should be at the center of a perfect and uniform universe.

Retrograde motion of Mars

The Ptolemaic model of the universe, had the Earth at the center of the universe with the Sun and the planets travelling around in circular orbits. To accurately describe the observed data, the planets travelled in smaller orbits known as epicycles as they orbited around the Earth. This reproduced the motion of the planets as the travelled across the sky. In particular, the phenomena of retrograde motion, in which the planet sometimes appears to travel backwards or loop the loop. The illustration shows the orbit of Mars over a period of several month.

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