Dava Sobel, the biographer of Copernicus and Galileo, tells us about the men whose painstaking work changed our understanding of Earth’s place in the universe.
Our theme today is the early history of astronomy. Could you tell us what century we are talking about here and what were the commonly held beliefs at the time?
We begin in the mid-16th century and go to the early 17th century. This was a time when the common belief held was that the earth was the centre of the universe and did not move. So it was very difficult to disabuse people of that notion. Today every child knows that the earth rotates on an axis and revolves around the sun once a year. But we know those things because we have been told them. If you had to figure it out for yourself it would be extremely difficult. There aren’t a lot of clues. The earth doesn’t feel as though it is moving.
So this was very much the explosive century where lots of big minds came along to prove otherwise. One of the people at the forefront of that is Nicolaus Copernicus who is at the centre of Owen Gingerich’s book, "The Book Nobody Read." Before we discuss the book can you explain who Copernicus was.
This is someone who really changed the world because he turned our perception of the universe inside out. He dared to suggest that the earth was in motion – that it rotated and revolved – which was an insane idea at the time.
How did people react to his ideas?
He put his ideas in a book called "On the Revolution." But he didn’t want to publish it for a long time because he was worried he would be laughed at. So by the time his book came out he was dying and he never really heard any of the reaction to it. And for a while there was no explosive reaction because the book is a great big dense difficult mathematical text written in Latin! So it was only the educated few who could really appreciate its intent.
And that is why the 20th century author and journalist Arthur Koestler dismissed it as “the book that nobody read,” which is something that Owen Gingerich is at pains to correct with this book.
Yes, he is referring to Koestler’s comment with his title. This was the insult hurled at Copernicus’s book because it is so long and mathematical. What is interesting about Gingerich’s book is that he writes about his own study of "On the Revolution." Over a period of 30 years, wherever he travelled he sought out all the extant copies of it, examining more than 600 surviving copies from the 16th century, and while he was doing that he noticed that the book hadn’t only been read but properly studied. He knew this because the margins of the copies were full of notes. So he was able to prove that it was, in fact, an extremely important book. It was also a very expensive book. It cost about the equivalent of a term’s tuition at university. So people who bought it were likely to take very good care of it.
What kind of people were buying it?
Galileo owned a copy. [Johannes] Kepler owned a copy. In fact any astronomer or mathematician from that period would have owned that book.
With your next book we are continuing with Copernicus. This time you have chosen "The Copernican Question" by Robert Westman.
Robert Westman was interested in exploring the idea that Copernicus was also an astrologer as well as an astronomer. For many years people thought that Copernicus paid no attention to astrology. But in his day, if you were an astronomer you were also an astrologer. Why else would you care about the positions of the planets? But he never wrote about astrology.
Why do you think Copernicus never mentioned the topic of astrology in his writings?
Well, this is what Westman tries to figure out and one of the explanations he gives is that he was planning to write a second book about astrology in addition to his book on astronomy. Copernicus in many ways modelled "On the Revolution" on a very famous Greek text by Ptolemy. Ptolemy wrote several books – one was just about the planets, where he tried to work out their position, and then he wrote a separate book on astrology. Westman thinks that perhaps Copernicus had that same intention but he never got around to writing the other book. There is a lot of evidence that ties him to other astrologers which is all believable and convincing. So I personally find this idea fascinating. Westman is so good on the detail he goes into – to work out whether his theory is plausible or not. He looked at all the people that Copernicus might have known and explored their ties to astrology. And I thought he made a convincing argument.
You have also written about Copernicus. Why, for you, is he such an important person in the history of astronomy?
Because he had the courage to go against common wisdom and common sense. It is hard today to imagine what a strange idea he entertained. I also like the fact that instead of just imagining it he also spent years making observations in the night sky to try to back up his theory. Remember, he didn’t have a telescope or any instrument with lenses; he was just using the naked eye. All of this was a question of mapping the positions of the planets. I think it must have seemed crazy to him to expect the stars to spin all the way around the earth every 24 hours. It makes much more intuitive sense in terms of size and distance to have the earth move. But then you have the problem of the earth moving, which everyone thought was ridiculous.
Your next choice is written by another one of your heroes. You have chosen Galileo’s personal account, "Sidereus Nuncius," which translates as "The Sidereal Messenger" or "Starry Messenger."
This is a thrilling book. It is the moment that astronomy became an observational science. Until Galileo’s time, the most that anyone could know about a planet was where it was. With his telescope Galileo was able to determine the composition of the moon. He was also able to solve various longstanding mysteries, such as what is the Milky Way – by looking through the telescope he could see that it was made up of myriad stars. Perhaps his most exciting discovery was finding the moons of Jupiter. Because of that he could say to the world: “Earth is not the only centre of revolution; there are these bodies going around Jupiter.” In Galileo’s time another objection to the earth’s motion was, what about the moon? How could the earth travel around the sun and carry the moon along with it? Well here was Jupiter with four moons and so, obviously, having moons was no impediment to orbiting the sun. Whether you thought Jupiter orbited the earth or orbited the sun it was moving along with all those moons. And so he removed an obstacle to accepting the idea.
How did he manage to get hold of the telescope?
He had heard of such a thing being invented as a novelty and so he figured out how to build one. And although at first he considered it a military tool, which was passed to the navy in Italy to keep watch on the horizon for enemy ships, he very soon realized he could turn it skywards. So he made these amazing discoveries and published them.
What kind of person does he come across as in "The Sidereal Messenger"?
A brilliant, inventive, religious and excited individual who was open to new things. He was an outsized character.
Stillman Drake’s book, "Galileo at Work" sounds like a good accompaniment to "The Sidereal Messenger."
Yes, this book really examines Galileo’s day-to-day life. It looks at what kind of correspondence he maintained with scientists all over Europe. It also shows what his thinking was on various problems. It is a close-up look at how he performed his experiments and how he worked with others to shape his ideas.
Can you give me an example of one of the experiments he performed?
He devised an ingenious apparatus for rolling balls down an inclined plane and timing their descent. In this fashion he discovered how bodies accelerate during free fall. Of course he had no clock to time such small intervals, but he counted the moments by letting water drip from a vessel, and then weighed the drops of water.
Did he get any help from other scientists?
Yes, and sometimes he got criticism from them as well. He didn’t take criticism well. He was a very potent opponent in a debate or even in correspondence because he was so clever. You really didn’t want to get into an argument with him.
You mentioned that he was religious – how did that tally with his scientific work?
Galileo lived in Italy in the late 16th and early 17th century and just about everybody was Catholic. His own daughters were nuns and his sisters, for at least part of their lives, had been in the convent world, and so this was very much his upbringing and his outlook. When he writes about his discoveries he thanks God for being so kind to make him the first person to see marvels that have been hidden in the dark for all previous centuries.
And the church didn’t have an issue with what he was doing?
Not at that point. The minute he started agreeing out loud with Copernicus and writing about it in Italian and not Latin then he became more controversial. "The Sidereal Messenger" is written in Latin but soon after that he switched to Italian and that is when it became an issue. His controversial views were investigated by the Roman Inquisition which concluded that his ideas could only be supported as a possibility and not an established fact, and he spent the rest of his life under house arrest.
One aspect of him that isn’t so widely known, which gives a good insight into his character, is to be found in the letters that he wrote to his daughter, which I know you have written about.
He had a very close relationship with one of his daughters and a correspondence which was life-long for her. When I found out about her and the letters they wrote it surprised me. Given the relationship he had with the church in later life, I had always assumed he was not religious and that he was a modern scientist with no belief. But when I found that he had daughters who were nuns it made me think that many of the things I had learnt about Galileo were wrong. So that was the impetus to find her letters and translate them into English and see what they said about who he was.
What kind of a relationship did this very clever man have with his daughter?
It was quite close and loving. In fact he outlived her. She died quite young. He didn’t really discuss his work with her. There are just a couple of references to it in his letters but she did a lot of copy work for him. And she may have helped him copy over the manuscript for his big book. The letters are not about science, they are about things that are going on in the convent and help that she needs from him. So from them you get more of a sense of what he was like as a man.
Lastly, you have chosen "The Sleepwalkers" by Arthur Koestler, which is an overview of that period, though he is not quite so complimentary about Copernicus and Galileo as the other authors you have chosen.
Arthur Koestler was a journalist with an interest in science. He really got fascinated by this subject. So this book traces the early history of astronomy because he too found it fascinating. Unfortunately, as you say, he didn’t like Copernicus, or Galileo for that matter. The only one he seems to really have liked was Kepler. So one reads his book sceptically. But it is a book that was widely read and it had a tremendous influence on people. Even though it came out in the 1950s you still meet people who will talk about that book. And for many it was the book that got them interested in astronomy. I read it years ago as well and it has stayed with me.
Why do you think he was so scathing of Copernicus and Galileo?
It is hard to say. He found Copernicus dull, and I admit that his book "On the Revolution" makes dull reading for a person who is not capable of understanding the math. But Copernicus is far from dull. From my own research I can tell you that he was incomparable. He did all this work for his own interest – he was not part of a university but working completely alone. He was also a medical doctor and had a job with the church that actually involved him in active warfare. So his life was rich and colourful, although admittedly his book would probably put you to sleep.
So what was the next stage of observation after Copernicus and Galileo?
What was needed was someone to come along and make physical and mathematical sense of it all. And that person was Johannes Kepler and later Isaac Newton. Kepler was the one who had access to truly exceptional data gathered by Tycho Brahe. The two scientists made an interesting pair. They had no personality traits in common, but their work united them. They began their collaboration in the early 17th century. Tycho was Danish and Kepler was German but they worked together in Prague for the Emperor Rudolf II. Tycho died soon after their collaboration began. And Kepler won a fight to have access to the data. Tycho’s family were a bit difficult on that score but he managed to win the right to use the data and he was able to create tables that were the best anyone had ever seen on the positions of the planets. Kepler could show that the orbits were not circular, as Copernicus believed, but were elliptical. With that breakthrough he went on to achieve the greatest, up until that point, understanding of the motion of the planets.
What do you think is the big question in astronomy at the moment that is comparable to the things they were discovering?
We are all wondering what is the nature of dark matter and dark energy. These mysterious entities constitute the bulk of the universe and no one knows what they are. When they do we will have another huge, inside-out turnabout.
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