Philosophy 167: Class 11 - Part 4 - Observational Astronomy in Bologna: Cassini's Observations at San Petronio.

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


  • Synopsis: Cassini executed high-precision observations of the sun and planets, producing very accurate measurements of their motions and features.

    Opening line: "The next few slides are going to be focused on Bologna and I will confess to everybody there is nothing I am more vain about. Than my own photographs."

    Duration: 11:29 minutes.

    Segment: Class 11, Part 4.
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The next few slides are going to be focused on Bologna and I will confess to everybody there is nothing I am more vain about. Than my own photographs.
And it's funny because I have no real talent as a photographer. But once I won two or three legal cases with my own photographs I started thinking at least I know what I'm trying to capture when I make a photograph.
Whether it's good or not. And thanks to digital cameras I can make 50 or 60 shots tried to get one that's good knowing what I want. So I gave a week long course on the state of history as science in Bologna somewhere around 2005. My motel was right next door.
This is San Petronio it was supposed to have been had it been allowed to be finished. The largest cathedral in all of Italy. But these fools down in Rome started complaining about something being larger than St. Peter's. So San Petronio was never quite finished, okay? But it is a huge, huge basilica.
This was taken early on a Sunday morning and I then went inside with my camera, which I wasn't allowed to do, and took as many photographs inside as I could get away with. They never caught me. But of course a service was going on and the first thing I was stunned by is the total Religious service had maybe seven or eight rows of seats, way up in the front.
And the whole rest of the thing was completely empty. You know? So much for a turnout on the average summer Sunday morning. What's important about this, I'm gonna pass this book around. This is one of the more beautiful books anybody's ever written. John is a personal friend, but he's Tompkins, Tompkins had three PhD students.
John was the first, and the one who was most happy with Coon, and Coon was most happy with him. He's done a lot of different things, John has done. But in recent years, while living just outside of Oxford, he spends as much time in Florence as he possibly can which is a nice place to spend your time if you've never been there.
So he did this book in 1997 and then he did the biography of Galileo that I recommended to you very strongly as the best biography. What this is about is the sun the cathedrals being used as observatories? And there are several of them. The whole book's filled with case after case.
One of their preoccupations was getting the calendar right, because Easter's a big deal. But there were other preoccupations, too. Remember a couple weeks ago, I said Things changed at this point. Everybody started taking empirical considerations seriously. The Church is among them. The Church suddenly started making sure that the way they built cathedrals allowed them to use the cathedral as an observational device.
So I'll pass this book around, it's quite stunning and beautiful. I think that's in paperback, it's Harvard University Press, but I'm not sure. This is a diagram from that book. What happened here, young Cassini joins the faculty of Bolonia in 1650. Now, remember, that's the year before Richie Olly's new appears, and he and Grimaldi have done all this vertical fall experiments at this point.
They're working, Grimaldi's working on refraction and Riccioli's finishing the book. But young Cassini joins them and he's very deeply committed to observation. So gets the San Petronio to give him a nomen, I'll show you the nomen in just a second, it's nothing more than a very very small aperture in the roof that allows him to determine, he had to fix this and that's what the upper diagram is about is how he did it.
He got a Meridional line on the floor of the church, a perfect, as perfect as he could get it, north south line, you notice it barely gets through two of those columns, but it gets through. That means that he can every day observe the sun. Both as it crosses the meridian, getting a very precise time for it.
But also, how it moves north and south while doing so, over the year, okay? So, over a ten year period, he did extraordinarily careful observations of the sun. There were four consequences of it and I'm gonna then show you a bunch of pictures from it. The first consequence is he decided that Tico's Obliquity of the Ecliptic, the angle between our equator and ecliptic was not quite right.
Off by a few minutes of arc, and once he decided that, he further concluded that Tycho's parallax corrections were way too large. Okay, purely from looking at the sun, decided they had to be way too large. But that also meant the refraction corrections weren't right. Because they presupposed the parallax corrections and looked for secondary irregularities after you do the parallax.
So all of the Tyco fundamentals of Tyconic astronomy Cassendi during the 1650s decided had to be redone. The other beautiful thing though is he wanted to see if the earth's sun orbit was truly odd bisected eccentricity as Kepler had proposed. And the way he can do that, first, that's my photograph of the Norman.
You see in the ceiling there a little tiny hole. This is John's from John's book Photograph. This is the Meridian barely missing the two pillars. I would like to have my own photograph of that but i would've been pretty glaring what I was doing at that juncture. And this is the sun crossing the Meridional line on a given day.
And you can see on either side of it slightly discolored, that's a path the sun follows during the course of the year. I'll show you that in a moment. But now from the shape of that circle as it crosses. Changing during the year in the size of it.
He can get the relative distances of the sun from the earth. And he concludes it actually is bisected eccentricity. And he then convinces Riciloli to shift to shift to bisected eccentricity from then on, because here was a solid empirical argument, not the indirect argument that had been offered by Kepler.
Kepler's argument was a very good argument, but it was based on triangulation, and the triangulations were at least open to challenge. This was a very direct so what you're seeing here is very very high quality work to say the least. I'll show you a couple more pictures and then I'll come back.
This is actually, and you can't quite see it I'm sorry, there's a line here, see it crossing? Showing the noon time sun during the course of a year. Thanks to latitude and ovariations, et cetera. On either side of the north south line. So you're actually seeing the sun move slightly.
This is on the top floor of the university's observatory, which is now a museum. The observatory that they actually used at the time. I think when Cassini was there. Next shot are Componi, Componi lenses in the top floor of this museum. One after another I would love to have taken them out and had them and the way Paolo Galluzzi did.
Had them checked with lasers to see just how good they were but they're universally described as the best lenses anybody was producing for like a 15 year period. And as I say, Campagne used them and he allowed Cassini to use them but not many others had the opportunity.
This is a telescope, Campagne telescope that Cassini himself used it's around 40 feet long. My photographs will show you. The bar there is to prevent swaying. That is the support structure. You can see where it would point out of a window. The window of course probably wasn't there as glass at that time.
This shows you the eyepiece using a Huygen's type eyepiece at the end. But it is a quite long substantially long telescope, would not remotely fit in this room that he is using with Compani lenses, and I'll give you a list of some of the things he discovered. For 15 years, he put effort into getting the motion of the four satellites of Jupiter, using this telescope primarily most of the time.
The tables were finally published in 1668. And they were very, very accurate tables for the motions of the four satellites. They'll become important later precisely because they were so important. He established that Jupiter rotates. He established that Mars rotates on its axis and Venus rotates on its axis.
With Mars and Venus being more or less 24 hour, I didn't dig out what he did on the time, of Venus's rotation. Excuse me, Jupiter's rotation. He did pick up the non sphericity of Jupiter. Hooke had already proposed it, but Hook didn't have good enough telescopes to pick it up.
Cassini does a very good job showing that it's fatter at it's rotational equator than it is at it's poles. I think it's 15 over 17 is what he concluded. You can look in the notes and see the actual value. There a couple of very beautiful things just to mention it.
His telescopes were good enough that he could see the shadow of Jupiter's moons as they crossed in front of Jupiter. And, that's quite stunning. Just a few years later, of course, he discovered that the rings of Saturn have a divide. It's called the Cassini divide, because of course, he found it, again, with a Campani telescope.
So what we're looking at here, and I'm not going to go on at length about it. I put the chapter from book on Caccini and Valonia so if you get interested you can read all he did there. But he's sitting in Valonia not as his clergyman. Making by far the most rapid process in observational astronomy of anybody in the world.
Working several hours a day with the very best equipment and the very best facilities. It's a major, major advance going on sitting in Bologna.