Some questions from Norway

I recently received an email from Per Ove Stige from Oslo who apparently had problems commenting on my Galileo’s great bluff post. As his mail contains some interesting questions and misunderstandings I thought I would deal with it here

“Fascinating. And a question could then be: Given the evidence in let’s say 1632, which astronomic system had the best arguments?

In 1632 the system with the best arguments would probably have been judged to be a helio-geocentric system with diurnal rotation and in fact this was certainly the leading candidate at around this time, however it was in the process of being overtaken by the Keplerian system which could deliver the most accurate prognoses of planetary positions.

I’m not sure if I’ve got it right, so please comment on my comments:

i)The systems of Ptolemaeus and Copernicus, when compared: On one hand, Copernicus thought he had an ellegant way of explaining the retrograde motions of the planets. But on the other hand Copernicus’ system needed other circles to explain the observations. In sum, the complexity of the two systems are practically just the same. Copernicus’ system was more exact, due to new and better parameters, but geometrically the systems were practically spoken equivalent. An important difference, though, was that with Copernicus’ system it was quicker to calculate the orbits of the planet. Did I get it right?

No! The Copernican system was not more exact because it was based on exactly the same data as the Ptolemaic system. This is why it was very necessary for new observational data to be obtained if astronomy were to progress, a need that was fulfilled by Tycho.

ii) The systems of Copernicus and Brahe, when compared: With Brahe’s system, the predictions of the orbits were better. Due to what? I’ve read that Kepler, with Brahes observations and Copernicus’ system, tried to explain the orbit of Mars, but found that Brahe’s system was more accurate, see here.

You have misunderstood what you have read. Kepler tried to calculate a circular orbit for Mars with Tycho’s data. He did this by calculating an obit using one set of planetary position observations and then comparing calculated results for other planetary positions from the obtained orbits with the observational data for those positions. The error was eight arc minutes, which was for the time incredibly accurate, however Kepler argued that Tycho’s observational data was more accurate so he rejected his own results and searched for a better fit, which led to his discovery of elliptical orbits.

iii) And so: Before, I thougt that with Kepler’s system, from an astronomical view there were no more questions. But you write that still in the early 1630s the Tychonic system with diurnal rotation was well ahead on points. I would very much appreciate if you could explain this more.

The problem with the acceptance of Kepler’s system was a physical rather than an astronomical one. Although people were prepared to accept diurnal rotation they still couldn’t accept that the Earth, a sphere of 40, 000 km circumference and immense mass, was hurtling through space at about 100, 000 km per hour. The physics necessary to convince people that this was possible was still being developed.

And finally, one thing that really puzzles me: That De Revolutionibus was on Index until as late as 1758, for more that 100 years, and even decades after the discovery of stellar aberration in 1725. As usual there are probably serveral reasons, but so far I find it difficult to explain without the traditional church-science view”

De revolutionibus remained on the index mainly as a result of inertia. The Catholic Church’s prohibition on the teaching of heliocentricity was never taken very seriously. Outside of Italy it was largely ignored even in Catholic countries and in Italy scientists just wrote something to the effect that heliocentricity contradicted Holy Scripture and so could not be true and then proceeded to teach or discuss it as if it were true. This being the situation there was no real reason to stir up old confrontations. Italy was in fact one of the first mainland European countries to accept and teach the new Newtonian physics despite the formal ban on all heliocentricity. Formally removing the ban was embarrassing for the Church so they just put it off as long as possible. The ban on Galileo’s Dialogo was only removed because some over correct Church censor in Italy at the end of the eighteenth century insisted on enforcing the letter of the law when censoring a science book.

1 Comment

Filed under History of Astronomy

One response to “Some questions from Norway

  1. Per Ove

    Thank you for explaining. Not very usual with post from Norway, if I read the header right 🙂
    For your last bit, about the church not taking the ban on heliocentrism very seriously, and not even in Italy: some good literature references here would be very helpful.

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