In the introduction you'll be reading next week, this is Astronomia Nova. This is in the complete works of Kepler, it's not a first edition. I had an opportunity when I was at Didner to just go down and stand the title page of the first edition but I didn't and now that book is in San Marino, California.

The second word in Greek is based on causes or celestial physics. I assume the word in Greek is a literary allusion, but I don't know to whom. You can immediately see, Aitio, is of course, Aitia, is clause in Greek, but the rest of it, I'm not willing to talk about.

Based on the observations of the gentleman Tycho Brahe, etc., done for Rudolph the second, knows how large that name is.

Only for the star Mars and it's published in 1609. He finished it in 1605. He worked on it from 1601 to 1605, but then the Tico heirs would not give him permission to publish because he was quoting data from observations from Tycho, in fact, they form the basis for a lot.

So there were an extended negotiation till he was allowed to publish the book. Sat on it for four years. In the introduction as you'll see it next week, he gives this figure. He refers to it as pretzel-like though that's a translation of the word he uses that Bill Donohue has adopted.

This is the motion relative to the Earth that Mars engages in either the Ptolemaic or the excuse me, Taconic system. And, his comment as you'll see when you read this next week, and I'm gonna do more with it on the next page, next slide is how could that possibly be physically realized?

Now of course we have an answer there's spheres on top of spheres etc., but as far as he's concerned at this point, spheres are gone. Tycho has shown, but with the comet, spheres are gone. So this motion's gonna have to be produced by something other than solids out there on which the planets sit, and he's just doing a plausibility argument.

You'll notice it starts in 1580 and he shows the pattern and this is properly to scale. It's fairly crazy if you think about it and it may not be totally properly to scale don't hold me to that. And that's immediately followed by a very nice basic history of how this problem got approached.

So I'm actually going to read a fair amount of this which I generally don't like doing. Reading a large amount but this is worth doing. Again, however, notice that these loops in each planets spirals are unequal in different signs of the Zodiac so that in some places the planet would retrogress through a longer arc of the Zodiac and other's through a shorter and now for a longer and now for a shorter time.

Nor is the increment of brightness of a retrograde planet always the same, also, if one were to compute the times and distances between the mid-points of the retrogressions neither times nor arcs would be equal nor would any of the times answer to its arc in the same proportion.

Nevertheless for each planet there was a certain sign of the Zodiac from which through the semi-circle to the opposite sign in either direction all those things excessively increased. Now that's essentially the summary of the first two weeks from which I hope you picked up on retrograde loops, etc.

Nice and clear statement. From these observations it came to be understood that for any planet, there are two inequalities mixed together into one. The first of which completes its cycle with the planet's return to the same sign of the Zodiac. The other with the sun's return to the planet.

Now, the causes and measures of these inequalities could not be investigated without separating the mixed inequalities and looking to each one by itself. They, therefore, thought they should begin with the first inequality, It being more nearly constant and simple since they saw an example of it in the Sun's motion without the interference of the other inequality.

Remember, the second inequality is a retrograde motion. But in order to separate the second inequality from this first one, they could perceive no otherwise than by considering the planets on those nights at who's beginning they rise while the sun is setting. Once they were called a chronici or night rising.

They're the ones where the planet goes overhead at roughly midnight that's the point. For since the presence and conjunction of the sun makes them go faster than usual, and the opposition of the sun has the opposite effect, before and after these points, they are surely much removed from the positions they were going to occupy through the action of the first inequality.

Therefore at the very moments of conjunction with an opposition to the sun, they are traversing their own true and proper positions. But since they cannot be seen when in conjunction with the sun, only the opposition to this sun remains as suitable for this purpose. Now the point being made there and it will be stressed In just a couple of minutes that Kepler's going to alter their whole tradition of Astronomy by saying this is unfortunate, by focusing on observations that opposition, you gain something.

You eliminate, you separate the two, you disentangle, to use a modern technical word, disentangle two effects. But it's going to turn out that that information is not nearly as representative as you want it to be. All right, all of that you're gonna read next week, I'm just giving it to you now.