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Difficulties for Copernicus

Copernicus had a number of obstacles to overcome. The first was that he wanted to present an idea in which the sun was the central point around which the planets all move in roughly circular orbits (or at least elliptical ones). The selling point was that such paths were much simpler to understand than the complex epicyclical tracks one would plot if the sun and planets circled Earth. In fact, this was not a totally novel idea, as a Greek (Aristarchus) in the third century BC had considered a heliocentric set of motions. Unfortunately, Copernicus was a priest, and the Church believed that if the earth was not the centre of the universe, it undermined the central importance of mankind (and especially the Church) as the epitome of the universe. Consequently, Copernicus presented his ideas with some caveats several decades after he had considered them, and his book only appeared in the year of his death. He also dedicated it to the Pope as a wise political move.

The second problem in accepting his theory was a perfectly valid scientific one at the time. There were excellent astronomical measurements by Tycho Brahe, who realized that a heliocentric model was very successful for the planets orbiting the sun. However, the model predicted that because the stars appear to be fixed, they must be at immense distances from the solar system (i.e. much farther away than any of the planets). Also, it was unlikely that all stars were at the same distance.

Nevertheless, the sizes of the stellar images that we see are all very similar, and not much smaller than those of the planets. Effectively, we see star images that are far too large for really distant sources.

Brahe was correct in his comment and criticism. It was only two centuries later that our understanding of light had advanced and we realized that it has properties of waves. This means that our knowledge had improved to the level where we understood that for tiny images, the ‘point size’ that we see is set by the optics of telescopes and our eyes (not the distance to the original star). Whilst distant objects we see on the surface of the earth look smaller than similar ones that are close to us, if the object is extremely far away and the image size is minute, then we need to add in more complex physics. It is now well understood and involves the wave-like properties of the light.

Overall, this is a nice example of how we can reject ideas because we lack all the necessary information, rather than from prejudice.

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