Aristotle, Ptolemy and Copernicus

Aristotle was a famous Greek philosopher who contributed, through his book On The Heavens, in proving that the Earth was round rather than flat. His first argument was the lunar eclipse. During the lunar eclipse, the Earth comes in between the Sun and the moon and blocks the sunlight from reaching the surface of the moon by casting a shadow on it. This shadow, being circular, confirms that the Earth is spherical. If the Earth was in fact flat, the shadow would have been long and ovular. His second argument was the change in the position of the North Star which would not occur had the Earth been a flat disc. The apparent position of the North Star is much lower in the sky when viewed from the south than when viewed from the north. For an observer in the northern hemisphere, the North Star is directly above whereas for an observer at the equator, the North Star is at the horizon. Furthermore, owing to their travel in the sea, the Greeks had observed that the sail of a ship is made visible before the hull yet again showing that the Earth is round.

Aristotle believed in the geocentric theory, i.e., that the Earth was stationary at the center of the universe and the planets and the Sun orbited it in circular orbits. He believed this picture of the universe to be the most perfect due to “mystical reasons”. Claudius Ptolemy, in support of Aristotle, devised a complete cosmological model having the Earth at the center of the orbit of the moon, the Sun and the five known planets of the time – Mercury, Venus, Mars, Jupiter and Saturn. The last orbit was of fixed stars. The definition of fixed stars was vague but it meant that beyond the orbit of Saturn the universe was not observable. Ptolemy was aware that this model had a flaw, yet he continued to advocate about accuracy, owing to the simplicity of the model. For the purpose of predicting the position of the celestial bodies in the universe, Ptolemy was forced to make the supposition that the orbit of the moon was such that sometimes the moon was twice as close to the Earth as at other times. This meant that the moon would appear twice as big on some nights while not on other nights which was in conflict with observation. Although the model was not universally accepted, the Churches were foremost in adopting it because they believed it represented what the Bible wrote; the model left ample space to allow heaven and hell to exist.

Finally, the heliocentric theory was proposed by a Polish priest, Nicholas Copernicus. According the Copernican model, the Sun was stationary at the center of the universe and the planets orbited it in circular orbits. Although this did not match observation, it was publicly supported by Johannes Kepler and Galileo Galilei. With Kepler’s hypothesis that the planets orbited the Sun in elliptical rather than circular orbits, the picture of the universe matched with observation. The modified heliocentric theory is now universally followed.

Sources: A Brief History of Time by Stephen Hawking