Dialogue Concerning the Two Chief World Systems – Galileo Galilei

20200825 Galileo

The final, and longest, book in my August reading marathon of important scientific works is also definitely the oldest and arguably the most significant in the leap of understanding passed on to those members of the public able to read a copy. Published originally in 1632 in Italian so that it was more accessible to the general public than it would have been if written in Latin it was immediately seen as an attack on the Catholic church as it presented as valid the then heretical Copernican system of the Sun at the centre of the Solar System rather than everything rotating around the Earth as taught by Aristotle and Ptolemy and adopted as clearly correct by the church as Earth should be the centre of Gods handiwork. Galileo was duly tried by the Inquisition and sentenced to life imprisonment at home and the book remained on the Catholic church’s list of banned works for over 200 years until 1835.

It is styled as a conversation over four days between three characters, Salviati is the instigator of the meetings and is clearly a Copernican, Simplicio is an adherent to Aristotelian and Ptolemaic systems and Sagredo is there initially to play devils advocate putting questions to both of the others but towards the end is obviously swayed by Salviati. Both Salviati and Sagredo are based on real people with those names who had been friends of Galileo but had both died many years before publication so could not be implicated by their names being used, whilst Simplicio appears to have got his name from the Italian word semplice which means simple minded. The choice of this name for the character backing the Ptolemaic system was also not lost on the Inquisition. Galileo was well aware that he was pushing at boundaries and originally got permission from the church to write a book about tides which grew into the final work covering far more than his original proposal, but even the characters acknowledge that this is potentially dangerous territory.

We are arguing for our own amusement, and are not obligated to any such strictness as one would be who was methodically treating a subject for professional reasons, with the intention of publishing it … it should be almost as if we had met to tell stories, so that it is permitted for me to relate anything which hearing yours may call to mind.

This edition was published by The Folio Society in 2013 using a translation originally done by Stillman Drake in 1953. It includes a modern introduction by Dava Sobel along with a foreword by Albert Einstein, which presumably dates back to the first publication of this translation. I did struggle a little with the verbose nature of the translation which whilst it may reflect Galileo’s original did also mean that I several times had to reread a sentence to make sure I followed the text correctly. This is not helpful when I was also trying to appreciate the leaps being made by Galileo whilst reminding myself that this was written decades before Newton formulated the Theory of Gravity so Galileo was truly groundbreaking in his explanations. His theoretically neutral but definitively pro-Copernican text starts from first principles with balls rolling down a slope to end up with not only the Earth rotating each 24 hours but also orbiting the Sun each year with the angle of the Earth’s axis also included to explain the equinoxes.

That is not to say that everything is correct as we would understand the cosmos now, Galileo has astronomical distances far too small, although much exceeding that of his contemporaries. A good example of this is the section of detailed calculations surrounding the two supernova that had been observed in the last few decades before he wrote the book. He is rightly dismissive of a book which aimed to prove that that these occurred within the orbit of the Moon so as to not disturb the changeless firmament which does so by carefully choosing between astronomical measurements of the period so as to find ones with sufficient error to support the authors position. However Galileo makes the same error in his selection by dismissing not only these examples but also any that would imply the nova occurred at an infinite distance from Earth which using the methods explained would actually have been the correct solution. Instead Galileo had decided that the stars were roughly six to eight times as far away as Saturn (then the furthest known planet) although some “could be two or three times further than that” to explain relative brightness and apparent size. He duly provides many pages of calculations regarding the sample set he has chosen, which are clearly there just to demolish the book and author he dislikes. Other ‘scientific’ books and papers from his time are likewise introduced and their methodology and reasoning torn apart. Galileo clearly wanted to leave no stone un-turned in his defence of Copernicus.

In the final section Galileo covers the subject that he originally stated was to be the main topic of the work, that is the tides and what causes them. Fortunately this makes up a tiny proportion of the whole book as sadly this is another area where he is in error by effectively ascribing them to the rotation of the Earth and the consequential ‘sloshing’ of the waters in the seas. The examples of mistakes given above are entirely understandable given the groundbreaking nature of the book and although I feel the translation could have been better this is still a book I thoroughly enjoyed as the insights presented by Galileo are not only good examples for today but give an understanding of the reasoning of the time and the turmoil between science and the Catholic church that would hold back scientific advances within its sphere of influence for decades if not centuries to come.

Galileo finally received an apology for his treatment at the hands of the church on the 31st October 1991 from Pope John Paul II over three hundred and fifty years too late.

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