Copernicus Suggests a Heliocentric Cosmology: The Historical Context. We have considered the Ptolemaic model, and recognized its shortcomings. It is perhaps remarkable that this model held sway for about 1500 years, until the Renaissance -- at least, this was the case in Europe, although we must recognize that there were developments and other lines of thought in other parts of the world (see for instance the article ``Islamic Astronomy'' in the April 1986 edition of the magazine Scientific American). This lack of progress reflects the stagnation which prevailed in Europe through the Dark Ages. What a contrast, then, to consider the varied developments and the social context which prevailed at and just after 1500 AD, a point which was beautifully made by a diagram which I showed in class (and which you will find in PowerPoint Presentation number 9). Those years saw: the development of the printing press (allowing new ideas to be widely promulgated) Columbus's discovery of the New World, and the inauguration of a new age of exploration the revolutionary thinking of Martin Luther (considered heretical by the Catholic church, of course) the artistic creations of Michaelangelo, Leonardo da Vinci, Shakespeare, and others new technological breakthroughs, such as the development of the telescope In short, the time was right for free-thinking scientists to reject the old hypotheses which had long been accepted merely because some earlier authority had espoused them. This encouraged speculation about new models of the universe (which, in those days, meant the Solar System: the stars were seen as merely the background realm against which everything else happened). On the other hand, this was the time of the Spanish Inquisition and general religious intolerance, especially of scientific theories which sought to repudiate the doctrine that we were in a divinely-chosen place at the centre of the universe. This threatening atmosphere certainly had an effect on the pace and style of astronomical development, as we will see. Suggested reading : ``The Sleepwalkers'' by Arthur Koestler.

Meet the Principal Players.

In what follows, we will recognize several important steps: suggested the idea of a heliocentric solar system - not, admittedly, for the first time in history; but the time was right for it to be given serious thought. made the positional measurements which would allow certain aspects of the idea to be tested. carried out the analysis of Tycho's data, and developed a few empirical laws which described the behaviour of the planets in their orbits. using the newly-invented telescope, made critical observations which demonstrated once and for all the correctness of the heliocentric model. considerably later, was able to deduce the physical laws which explain why the planets move as they do. In this, he was encouraged by Halley, who brought his work to a wider audience. We now begin a closer scrutiny of the lives and contributions of these scientists.

Copernicus Provides the Idea.

The contributions of Copernicus are briefly summarized in the text, on page 70. One important point is that Copernicus was the first to say unequivocally that the daily motions we see - the rising and setting of the sun and stars - is simply a result of the Earth's spinning on its axis. Moreover, in anticipation of the objection that his model should lead to the discovery of stellar parallax, he simply stated that the stars are enormously far away (which would render the parallactic shift too small to detect). One noteworthy point made in the text is the philosophically important one that Copernicus rejected the Ptolemaic theory basically because he found it too contrived, reasoning that there had to be a simpler hypothesis which could explain everything to his satisfaction. This sentiment had already been expressed in a remark attributed to Alfonso X, the King of Castille and Leon, who lived from 1221 until 1284. He is said to have remarked, on having the Ptolemaic system explained to him: ``If the Lord Almighty had consulted me before embarking upon Creation, I should have recommended something simpler.'' As we noted in an earlier lecture, this is not uncommon in science: there is an immediate appeal to a theory which is elegant, simple, and - yes! - beautiful. The danger, of course, is that Nature may not be so obliging: maybe the correct theories are those that are very complex indeed, so we always have to be alert to any evidence which suggests that we are wrong. Copernicus was essentially right, however, and it is interesting to speculate about what his excited reaction must have been when he realized that the retrograde loops of the planets had a simple and natural interpretation in a sun-centred model. Copernicus did not get it completely right, however - he still imagined the planets moving in perfect circles, so in the end he had to use epicycles anyway, to explain some of the details of the motions. And compared to modern scientists he had a cavalier attitude towards data: he scarcely ever made observations himself, so there were almost no data to allow for the testing of his hypothesis. Indeed, in his book "The Sleepwalkers," Arthur Koestler records that Copernicus made something like twenty-seven serious observations spread over the best part of thirty-two years! Copernicus's book, in Latin, was published almost literally as he lay on his deathbed; he hesitated for years because he believed it would be considered heretical. A man named Osiander, who had been supportive of Copernicus, encouraged the publication but prepared an anonymous prologue which was meant to defuse criticism or prosecution by explaining that the ideas contained in the book were clearly nonsensical and not to be taken seriously! This ruse fooled no one, however, and his revolutionary thoughts described found a quick acceptance in an scientific society ready to question theories of long standing, although it had little impact on the general populace. (Koestler describes it as one of the worst-selling books of all time.) Sadly, Copernicus himself did not live to see the enormous influence his new thoughts were to have. Previous chapter:Next chapter

0: Physics 015: The Course Notes, Fall 2004 1: Opening Remarks: Setting the Scene. 2: The Science of Astronomy: 3: The Importance of Scale: A First Conservation Law. 4: The Dominance of Gravity. 5: Looking Up: 6: The Seasons: 7: The Spin of the Earth: Another Conservation Law. 8: The Earth: Shape, Size, and State of Rotation. 9: The Moon: Shape, Size, Nature. 10: The Relative Distances and Sizes of the Sun and Moon: 11: Further Considerations: Planets and Stars. 12: The Moving Earth: 13: Stellar Parallax: The Astronomical Chicken 14: Greek Cosmology: 15: Stonehenge: 16: The Pyramids: 17: Copernicus Suggests a Heliocentric Cosmology: 18: Tycho Brahe, the Master Observer: 19: Kepler the Mystic. 20: Galileo Provides the Proof: 21: Light: Introductory Remarks. 22: Light as a Wave: 23: Light as Particles. 24: Full Spectrum of Light: 25: Interpreting the Emitted Light: 26: Kirchhoff's Laws and Stellar Spectra. 27: Understanding Kirchhoff's Laws. 28: The Doppler Effect: 29: Astronomical Telescopes: 30: The Great Observatories: 31: Making the Most of Optical Astronomy: 32: Adaptive Optics: Beating the Sky. 33: Radio Astronomy: 34: Observing at Other Wavelengths: 35: Isaac Newton's Physics: 36: Newtonian Gravity Explains It All: 37: Weight: 38: The Success of Newtonian Gravity: 39: The Ultimate Failure of Newtonian Gravity: 40: Tsunamis and Tides: 41: The Organization of the Solar System: 42: Solar System Formation: 43: The Age of the Solar System: 44: Planetary Structure: The Earth. 45: Solar System Leftovers: 46: The Vulnerability of the Earth: 47: Venus: 48: Mars: 49: The Search for Martian Life: 50: Physics 015 - Parallel Readings.

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