It was great fun presenting a talk last night to the History of Astronomy group on the idea that Johannes Kepler was so nearly Isaac Newton!
My main argument was that he came excruciating close to making the discoveries that Newton did many decades later – in particular the fact that gravity works by the inverse square law and that that then dictates that planetary orbits are elliptical.
Kepler argued that the Sun had a ‘motive force’ that moved the planets, and that that force reduced as distance increased, hence planets move more slowly when further away from the Sun, more quickly when closer. He said that this force from the Sun reduced just as the intensity of sunlight decreases.
But he had worked out that the intensity of the light from the Sun diminishes by the inverse square law. He just never quite connected the two to argue that the ‘motive force’ also acted by the inverse square law.
It was fun explaining why Kepler’s Second law was discovered by the First law.
If anyone is interested in learning more about Kepler, the main books I used were these:
Also very good is The Invention of Science: A new history of the scientific revolution, by David Wooton.
And if you want Einstein’s own explanation of Kepler’s work, have a look here: http://www.larouchepub.com/eiw/public/2015/eirv42n20-20150515/31-32_4220.pdf
I absolutely enjoyed your lecture, Andy. You delivered it with great passion and sprinkled with your usual wry humour that always have me chuckling. By the end I was certainly convinced in your argument of Kepler being almost Newton. I laughed when you said, at the end that was all you had on Kepler so you might not be able to answer the questions yet we still bombarded you with questions and you almost answered it all, convincingly too!
I was going to ask a couple of more questions. One was a little confusion I had on your explanation of Einstein’s explanation of Kepler’s explanation of how he calculated the orbit of the planets, using a constant static positional reference (ie the lantern). But thank you for clarifying that during the after-lecture drinks. Ie in that Kepler used Mars as the static lantern reference when it reaches the same point every orbital cycle. I get it now.
EDIT: Just read your reference pdf of Einstein’s article on Kepler. Wonderful article. Thanks for that. And also thanks for those other reference links. Look forward to reading through them.
My other question I didnt get to ask (due to everyone’s desperate desire to get a pint in before the bar closed!) was about the thirty year war that you mentioned. I have heard of a an eighty year war and a thirty year war which I think overlapped. I am not very knowledge about these wars but I know the eighty year war primarily involved the Netherlands and the thirty year war (a more broader European religious war) came later. I think that Thirty year war started when Kepler was in his fifties…I think. Might it be the eighty year war that Kepler was trying to avoid, rather than the thirty year one? It was a Netherlands war after all?
I also have one request for you. Would you mind sharing that excellent timeline diagram of the lives of Kepler, Brahe, Netwon, Copernicus, and Gallileo? I think that is so useful to have as a look up reference card!
You’re quite right about the 30-years war, Tej. It didn’t actually break out until around 1618, when he was in his late-40s.
There was, however, a very uncomfortable religious environment in the decades before then, with Lutherans, Calvinists and Catholics all arguing nonsensically. In 1555 agreement was reached between the Lutherans and Catholics that meant that every ruler in each of the states within the Holy Roman Empire could decide whether that state was Catholic or Lutheran. And so the rulers’ subjects could be forced to switch to the religion of that ruler or go into exile.
In 1598, twenty years before the outbreak of war, Ferdinand, the Catholic ruler of Graz, issued a decree that all Lutheran preachers and teachers had to leave. When it was pointed out to him that this would cause considerable economic disruption, he replied, “I would rather rule a country ruined than a country damned.” Kepler had to leave – though for reasons that aren’t quite clear, he was allowed back a little while later. That’s when Tycho Brahe popped up in Prague and invited Kepler to join him.
The ‘Five lives’ chart is attached. Glad you liked it!
When we discussed the lantern/Mars problem, did we also talk about how they actually knew what Mars’s orbital period was?
Thanks Andy, a very informative and enjoyable talk.
(Alas, your secret plan to have no questions so we could get to the bar early sadly failed.)
Hi, Tej, hope this helps:
Eighty Years’ War, (1568–1648), the war of Netherlands independence from Spain.
Belgium and Holland were once ruled by Spain. After, the mainly Catholic part became Belgium and the maily Protestent part became Holland.
Thirty Years’ War, (1618–48), in European history, a series of wars fought by various nations for various reasons, including religious, dynastic, territorial, and commercial rivalries.
It ended with the Treaty of Westphalia in 1648.
Ferdinand the second battled for five years to impose his Roman Catholism on Bavaria and Austria.
King Christian the fourth of Danemark fought against Germany and lost.
Sweden’s Gustav the second Adolf, having ended a four-year war with Poland, invaded Germany.
Poland attacked Russia and established a dictatorship in Moscow under Władysław, Poland’s future king.And later attacked Germany.
Also, there were problems between France and Spain.
At the same time three denominations vied for dominance: Roman Catholicism, Lutheranism, and Calvinism.
Brexit will be a piece of cake!
Hi, Tej, hope this helps: !
Thanks for that summation of the wars, Bill! The Eighty Years wars is clearcut. But that thirty years war….wow, tad complex, eh?!! Dont think I can ever get my head around it that one. Why can’t wars be simple, “oy get get of me land or I’ll punch your teeth”, “nope, we’ll just run through you with our swords” “not before we shoot you down with our guns”, “Not behind our canons, you wont which btw we just fired at ya”, “We’ve built a wall”, “Then we’ll nuke you”. “Oh…”.
….There was, however, a very uncomfortable religious environment in the decades before then…..The ‘Five lives’ chart is attached…..When we discussed the lantern/Mars problem, did we also talk about how they actually knew what Mars’s orbital period was?
Thanks for that background, Andy, that makes a lot of sense in Kepler’s movement. And thanks for the chart. Fab! You did infer that we knew the Lantern position but not quite specifically connecting to knowing Mars orbital period already…how did Kepler know Mars orbital period in the first place, would that have been due to measuring its velocity and waiting for it to reach that specific velocity again?
how did Kepler know Mars orbital period in the first place
A bit of brilliance on Copernicus’s part, I believe.
Imagine that a planet is at opposition – so it culminates/appears due south at midnight. Then wait for the next time it’s at opposition. Suppose that’s 456 days (1.25 years) later. So in the time it has taken the Earth to travel one-and-a-quarter orbits, the other planet has done only a quarter of an orbit (you know it’s a quarter of an orbit because Earth and the other planet are in alignment with the Sun again). Earth has caught up with and aligned with that other planet.
So if a quarter of that other planet’s orbital period is 1.25 years, then its actual orbital period is 5 years.
Here’s the figures for Mars:
Now go back to that figure 1.82, the ratio of Mars’s orbital period (T) to the Earth’s. Square that number and take the cube root to get 1.52 – Mars’s orbital radius (R) in AU!
Well, there’s a coincidence!! Two of the contestants on tonight’s Mastermind had as their specialist subjects (i) Johannes Kepler and (ii) Isaac Newton!!
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