All right. Now, I'm not going to put up a slide immediately because I don't have an appropriate slide to discuss with this right away. From Ptolemy down to Copernicus, Copernicus published De Revolutionibus. Sorry, that, reflex action, they call it De Rev. In 1543, Ptolemy, we don't know the date of the publication of Almagest, but we think it was around mid century, of the 2nd century.

So, that's 1400 years, separating the Almagest from De Rev., and until fairly recently, the thought was, those were the two great works with nothing intervening. It's probably best not to think of the pure intervening period from 150 AD to 15 1540 AD. It's probably better to set it at 1450 AD, because two things happen around 1450 AD.

One of them much more important than the other. One of them, of course is the fall of Constantinople, which ended the any tie Hellenistic period in Through the Byzantium the Byzantine or eastern Christian empire. That's important but that's not really important. It's about 1450 that the printing press gets introduced.

And that changed everything so dramatically that when, a few years ago they were saying, what's the most important event in the millennium? And I actually thought that was a silly question. Everybody should be immediately aware, it was the printing press that changed everything more radically than anything else that happened in the entire millennium.

For one thing, it changed the kind of person who got into universities. Before that you needed to have a voluminous memory. Cuz you couldn't carry books around, okay? So, you memorized manuscripts and went away with, you may make copies of them, but you better have a gigantic memory.

Kepler had a lousy memory. He would never have gotten into college before the printing press. It changed the kind of person and the kind of intelligence required. It changed it to one of problem solving, not memory. But that's minor. It's also the dissemination of information and the access to information.

So let's do it at 1450 and ask what happened. Now I've let up that's prefatory to saying there are two historiographies about what happened between Ptolemy and Copernicus, or Ptolemy and 1450. There's an old historiography that's been around for quite sometime, that even when I started teaching this course,I was blind enough to think it was the only historiography.

The story goes as follows, Alexandria finally fell in 1641 A.D. to the Islamic invaders. That's remarkable to a great extent because Mohammed had died. I have to look at the date. Nine years earlier. That tells you how quickly Islam moved into northern part of Africa and came to dominate it.

At that point, the great library at Alexandria was sapped so all that learning somewhat disappeared, except it didn't disappear because people had started the Islamic world started, which by the way at that time had a very high toleration for other religions. Tolerance is the word I want for other religions.

Islamic people picked it up and put it into Arabic. And the way the standard story went is, they preserved in Arabic, the great Hellenic and Hellenistic learning. That Zen came down to Europe, when the Moors, by this point the Islamic world stretched from well east of Iran, all the way to what's now Morocco, with a total dominance and a very rich intellectual culture.

And, I guess it's around 1085 that the Moors managed to attack Spain to overtake Toledo and now all that learning in Arabic moves into Spain, and we get a period that's actually called the Period of Translation. Where all sorts of people started translating works in Arabic from Aristotle from Ptolemy etc., into Latin.

And that was the supposed root the standard root by which the ancient learning other than Plato managed to get into western Europe. Shortly after that, 1200 roughly, the great universities were formed Bologna and Oxford, argue with one another about which one was first. But, they both are formed right around 1200.

Right after that, University of Paris, and then you start getting other universities emulating these, and Ptolemaic astronomy was the standard of the curriculum. It was the most advance part of the curriculum because Plato said it should be the most advance. But, one worked one's way up to studying astronomy.

And so, there were textbooks. One of the early textbooks is by somebody names John of Sacrobosco, circa 1250. You'll see as you do reading around Galileo, there are still commentaries at the time of Galileo and John of Sacrobosco's Sphaera and what he did was of course a watered down version of the Almagest.

The Almagest was translated in Latin in a very bad translation for what its worth, virtually useless translation, but King Alfonse is staying the middle of the 13th century, decided that they needed new astronomical cables because of course, the possession of the equinox was slipping all over the place, et cetera, versus the Julian calendar.

So we get a set of tables based on Ptolemy called the Alfonse scene tables. And the presence of textbooks about Ptolemy, are what fed learning in astronomy from the time of 1250 until Copernicus goes to college in 1490. And on this picture, what the Islamic world did was to preserve the Greek learning, okay?

That's the standard picture of what happened.