Philosophy 167: Class 5 - Part 6 - Letters on Sunspots: the Phases of Venus, and the Demise of Ptolemaic Astronomy.

Smith, George E. (George Edwin), 1938-


  • Synopsis: Discusses Galileo's Letters on Sunspots, in which he publishes his observations of sunspots and the phases of Venus, implying that Venus orbits the Sun.

    Opening line: "In the letters on sunspots, there are various points I'll come back to the others."

    Duration: 12:34 minutes.

    Segment: Class 5, Part 6.
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In the letters on sunspots, there are various points I'll come back to the others. This point I want to read out because it's so important historically. It's the part I've put the red lines next to. This is the fact that Venus changes shape precisely as does the moon.
And if Apelles, the person he's writing the letter to will now look through his telescope he will see Venus to be perfectly circular in shape. And very small though indeed it was smaller yet when it recently emerged as evening star. He may then go on observing it and he will see that as it reaches its maximum departure from the Sun it will be semi circular.
Vincent will pass into a horn shape gradually becoming thinner as it once more approaches the Sun. Jumping down now. With absolute necessity, we shall conclude in agreement with the theories of the Pythagorean's and of Copernicus, that Venus revolves about the Sun, just as do all the other planets.
Hence it is not necessary to wait for transits and occultations of Venus To make certain of so obvious a conclusion. Down at the bottom as a matter of fact nothing else could be said before the telescope came along to show us that Venus is naturally and actually dark like the moon, and like the moon has phases.
So this historically was the death nail of Ptolemaic astronomy. Once it was shown that Venus goes around the Sun, and of course it's an inference if Venus is casting off its own light, there's always the possibility that it does so in exactly the right tempo to be as if its reflected from the Sun, and then you say that some people said that.
But overwhelmingly within the astronomical community, the conclusion was Venus and therefore Mercury are definitely going around the Sun. Ptolemaic astronomy is wrong. Now, the conclusion that Mars, Jupiter, and Saturn are going around the Sun rather than the Earth, that was almost universally accepted too. But as you'll see in a few weeks when we come back to astronomy, we're gonna be away from astronomy for five weeks.
When we come back to it, there's a wonderful book by a priest working in Bologna named Riccioli, called the New Almagestine Translation. Where in the first edition, he has Jupiter and Saturn still going around the Earth, arguing the data do not clearly show otherwise, but in the second edition, he goes fully Tychonic.
So what happens now to people in the astronomical community who want to avoid problems with the church, or who may just believe this for the reason Tycho did. He wanted hard evidence that the Earth is moving in either rotation or orbit. They became Tychonist's, and to drive this point home, the leading observational astronomer of the entire century Cassini, for whom all sorts of things are named, he was a Tychonist at all times.
Now some people want to say that's just to avoid trouble with the church, but we don't have clear evidence of that and as all of you know who've written their first papers for me, for point of view of any observation made from Earth, it doesn't matter whether you're a Tychonist or a Copernican because all of the objects are in exactly the same position with relation to one another at all times.
So nothing's at stake. And the best way I can say that, if you open current Ephemerides or you open De Rev, what's the first part? Theory of the Sun. Okay, you're just relativizing to the Earth. But this was historically a major event. You will see a little later Galileo appealing to it.
You will see Newton appealing to it, you will see, you already saw Kepler appealing to it in the epitome, which of course comes after this. It really did prove just a decisive factor in making Ptolemy no longer taken seriously within the astronomical community. And that's a big deal.
Questions on that? I'll pause and then I'm gonna summarize what you got out of these observations. So that's the standard answer. I've just given you knee jerk history as you'll get in most books. What killed Ptolemaic astronomy the phases of Venus. Galileo claimed to have observed the phases of Mercury.
He didn't claim it at the time, but when somebody else with a far superior Keplerian telescope claimed it in the 1630's, he said he had already seen it. Now, I have no, I can't believe that for a moment. Since I have trouble even finding Mercury in the sky, I doubt very seriously he was able to see faces.
And it's very small compared to Venus. Venus is the size of the Earth. Mercury's substantially smaller. All right, so you get the picture. I can't figure out where this came from. So if you look at the notes in my credits, there's a question mark. If anybody sees where this came from I spent a God awful lot of time trying to find where I found this diagram years ago, and I can't figure out where it's from.
But the contrast between the two is pretty much as described. Looking at it from the Earth, you see what Venus is gonna look like versus Venus as seen on the Ptolemaic system. All right. Let me summarize the product of the telescope. By the way the most important, well, the most important thing.
All of this is very important. Understand what effect the telescope had on astronomy. The astronomy we've been talking about in the four weeks so far is mathematical astronomy requiring real capacity in working within spherical trigonometry. And therefore it was not for the faint of heart, so to speak.
Very, very few even highly educated people were in a position to do the mathematics of mathematical astronomy. That was part of the reason those who could do it always presented themselves as so superior to those who could not do it etc. The way our friends in physics departments tend to do today.
I just got through with that with an audience of physicists in Texas, so I once again appreciated that. But now the telescope's different. Anybody that can construct one or buy one can go out and look at the sky. You don't need to know any mathematics at all to go look at the sky.
So all of a sudden, astronomy is opened up to a huge audience of participants that were never able to do anything before. They were in effect at the mercy of those who could do the mathematics, what they would tell them. So that's a big change. Lots and lots of people want to look through telescopes, get their own, etc.
One thing that I skipped over that I shouldn't have, I'll mention it again later. When people started challenging whether what Galileo was seeing was correct or not, in his first trip to Rome, Clavius, Father Clavius, SJ. Anybody know what SJ means, after people's name? Anybody not know? I'm sorry, Society of Jesus, Jesuit.
And if you didn't know this, the Jesuits as an Order often rivaled in power the Pope and the Vatican sufficiently that occasionally they tried to cut them back or forbid the Order. So having not all priests would give their order after their names, all Jesuits always still do SJ.
So Clavius was the leading Ptolemaic astronomer of the time. He had written texts on it. He was teaching on it in the college of Rome. And he looked through the telescope and said these things are real. He gave a different interpretation than them, but that validated what Galileo was seeing in the eyes of the church, that their foremost expert had in effect, had validated.
So after that it became much, much harder to simply dismiss what was being seen in the telescope. Okay, this is just a list of the things discovered. I'll run through it one by one. First, from Sidereus Nuncius, surface of the moon is highly irregular including mountains four miles high.
Earthshine, reflected light from the Earth lights the part of the moon that is not reached by the Sun. The fixed stars do not appear as disc's when viewed in a telescope, but the planets all appear as disc's. So the planets are near enough that the magnification chose them as disc's.
You saw how small the disc's were, but they're disc's, whereas the stars remain just points. Of course one of the things that means is the stars really are an awful lot further away, or at least it's consistent with the stars being an awful lot further away. The fixed stars are quote, so numerous as almost to surpass belief.
Remember that Ptolemy had 1,000, Galileo seeing of the order of 10,000. And they just weren't visible before. Nobody knew they were there. The most spectacular one is the Milky Way. This is again very close to a translation of what he says, conjuries of innumerable stars grouped in clusters.
And then the little clouds away from the Milky Way that have been dubbed nebulae, they too turned out to be collections of stars. So all of that disappeared. And then of course the four quote planets are in orbit around Jupiter lending support to Copernicus. I hope everybody understands why that lends support to Copernicus.
Copernicanism. Because the objection was, how can the moon be going around the Earth and the earth going around the Sun? Well here's Jupiter now going around something with four things going around it. So the idea that you have systems of orbiting bodies within systems of orbiting bodies. That as an objection simply disappeared.
And at every turn through Sidereus Nuncius, every time he announced anything, he says, this is evidence for Copernicus. And often saying the Pythagorean's, as I told you before, saying you were Pythagorean was a very bad thing to do in the church. That was one of the accusations made against Bruno, that he was a Pythagorean.
He was burned at the stake, for which of his various offenses remains to be seen. Then, letters is controversial among people who look at it. Venus exhibits phases, I just showed you that, akin to those exhibited by the moon. The Sun's surface displays spots persisting in relation to one another, and as inferred from the spots, the Sun rotates on its axis roughly 25 days.
That was very controversial. Another Jesuit priest named Scheiner in Germany, said those aren't spots on the Sun. Those are atmospheric phenomenon. So now you have a conflict. Just the sort of thing Galileo loved to do cuz he could make a fool of somebody. At least rhetorically. So you have these exchanges with him, there was a big controversy, etc.
And then finally I showed you, Saturn exhibit's two small stars tied to it on opposite sides. He could not make out what was going on there. Nor could anybody else for a very long time figure out what was going on with Saturn. And if I showed you Saturn, it does not appear to be, nice rings around Saturn.
That's much, much harder to fathom and to see, than you may think.