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Hypatia of Alexandria – Part 2

Posted November 08, 2016 12:00 AM by amichelen

(This is the second part of Hypatia of Alexandria)

The ideas developed by the Pythagoreans represent a solid contribution to the scientific knowledge of the times. The most important contribution to cosmology was the idea that the Earth and the other planets move around the Sun (Sun-centered system, or heliocentric), making the Earth simply another planet. This is an important contribution of 4th-century science at a time when biblical ideas put the Earth at the center of the solar system, following the Ptolemaic Earth-centered principle, as indicated in the following figure.

This vision of the universe was not new. Seven centuries before – 3rd century B.C. – the ancient Greek astronomer Aristarchus of Samos was the first to develop a heliocentric model of the solar system. Unfortunately his original work was lost and we only know about his work is through references to it by Archimedes and Plutarco.

In the Ptolemaic system (geocentric) each planet moves in a small circle called the epicycle that moves in a large sphere called a deferent. The stars moved in an outer celestial sphere around the planetary system. In order to explain the actual observation of the movement of the planets, Ptolomy used three tricks: (1) locate the Earth out of the center of the large sphere, putting us in an “eccentric” position; (2) then create the epicycle; (3) then define a point – not in the center of the large sphere either– called the “equant”, as illustrated. These three tricks – the epicycle, the “eccentric” Earth location, and the equant – allowed Ptolomy to mathematically explain the movement of the Sun and the rest of the planets around the Earth in a very accurate manner. Just as a curiosity here, the Ptolemaic model was so mathematically accurate that today planetariums are built based on this principle. A planetarium consists of a projector, one motor or gear to move the projector around in a big circle (the deferent), and a gear or motor moving in a small circle, following the large circle, to mimic the epicycle. In this way the sky is viewed from a stationary Earth. It is certainly amazing to build our planetariums using a model developed almost 2,000 years ago. It is even more impressive to know that this fake model was accepted and not seriously challenged by science and society for over 1,300 years!

When in 1543 Nicolaus Copernicus’ treatise De revolutionibus orbium coelestium was published posthumously, a new conception of the universe was brought back. Copernicus was a student of Domenico Maria de Novara, an astronomy professor in Bologna, who had a critical position of the Ptolemaic system. He taught Copernicus about the Pythagorean ideas of a Sun-centered model, where there are no epicycles.

Hypatia never accepted the Earth-centered model. All her life she supported the heliocentric model, and her observations in the commentaries of Theon Ptolemy's Almagest prove that she clearly rejects the geocentric Ptolemaic theory in favor of the heliocentric model. It is very possible that Copernicus read the Almagest with Theon and Hypatia’s comments when he was in Florence studying the Ptolomy works, given that the only copy of the Almagest was in the Medici library of that city, a library that Copernicus visited for his research. If this is the case, this would imply that Hypatia may have had a direct influence in the development of the Copernican Revolution.

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Re: Hypatia of Alexandria – Part 2

11/10/2016 2:28 PM

I was surprised when reading your post because I didn't realize the Geocentric model, as described by Ptolemy was so complicated, nor did I realize how accurate it was. Some research on my part indeed confirms it was a very accurate model, only having difficulty with the slightly more eccentric orbit of our neighbor Mars.

I've also found, though I had to dig deep because this seems to be rarely mentioned, that the Copernican model was also more complicated then we are led to believe and not really much more accurate than the Ptolemaic system. That's because of the use of circular orbits, which cannot account for the speeding up and slowing down of planets in their orbits. Thus the Copernican model had to use epicycles and offsets just like the geocentric model.

In fact, it really wasn't till Kepler introduced elliptical orbits and Galileo observed the phases of Venus in the early 17th century that a Sun centered model was better at modeling astronomical observations. Furthermore, the Copernican model was far more complicated to calculate the positions of the planets since it involved a moving reference frame (Earth).

So again, thanks for the post, it really showed me once again that history is a bit more complicated than what we're taught in school.

Ptolemaic Model:

Copernican Model:

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