But now on top on that, the planets do something that the sun and moon do not do at all. As they move from west to east every once in a while, in the case of Mars, it's roughly every 780 days. The planet comes to a stop among the stars, sits there for two or three days, and then starts going in the opposite direction for a period of time.

I'll show you how long in just a moment, until it then stops again, goes back to resuming its normal motion from west to east and continues. That's called Retrograde Motion. And this is an example of a retrograde loop. This is a NASA photograph. You can see the dates being taken.

So we start here with October 19th followed by 24th etc. And we come down there to December 14th, 22nd, and 28th. So in a 14-day period, Mars moves almost none at all. Then, it starts moving in the wrong direction, thus so from 12 28 til February 28th, that's two months, moving in that direction.

At which point, it comes to a nether stop, it's called a station or a Stationary Point where just sort of sits, it actually comes to what appears to be a full stop, and then resumes motion from west to east. Okay? So the planets are different from the Sun and the moon.

They have a funny characteristic retrograde motion. The two inner planets, Mercury and Venus, they do loops like this but we can't see them because they're going in front of the sun or they're on the other side of the sun. But what the two inner planets do is to reach a maximum elongation, a maximum distance from the sun.

Let's do it with Venus. So in the evening, when Venus is highest in the sky, it's as far away from the sun as it gets and it's at a stationary point. And then it resumes its motion in the opposite direction til it becomes a morning star and stops again.

So, that loop is quite substantial. That's how the Babylonians discovered that the morning star is the evening star, because they figured this all out, okay, and got the timing on Venus and Mercury etc. The Babylonians had one break. They started about 800 BC and collected data for over 500 years.

It's the way to do this right.

Okay. But it took a good deal of time. So that's a typical retrograde loop of Mars. The real problem you're about to see, these loops don't repeat in the same form. So, here is another NASA photograph, also of Mars, a different retrograde loop.

They didn't stick dates on this one, but you see the shape is quite different. Here's another one. Shape is completely different, it doesn't even form a loop. It just does a zigzag. And here's another one where it does a loop, but it does a really nice almost circular loop.

The line in the background there, interestingly enough, is Uranus, moving from night to night. Uranus wasn't discovered til 1781. So, period we're looking at, nobody would've realized, of course, they didn't have photography to capture this. But they would not have picked up the line of Uranus moving from night to night.

But it shows you how little the outer planets move in any one day. Cuz these are, again, something like six or eight day segments among them, we're getting quite a bit of spacing. All right, so that's the problem of the planets. The problem of the planets, fundamental problem, is simply to predict when the stationary points are going to occur and what the shape of the loops are going to be for each occasion, when it's gonna stop again and resume.

I've shown you for Mars. This is for Jupiter starting in 310 BC and going down to 298 BC. Jupiter repeats a substantially more often 399 days is the time from one of these loops to another, versus Mars which is 780 days. So you can see the difference. But once again, look at the variability of the loops.

They're nearer the same size than the Mars one is, but they're all over the place in terms of shape. Okay? The same thing is true for all five planets. They do not repeat the same shape when they go retrograde, nor is the timing the same. When I say 780 degrees, that's an average.

That's a typical time. It's plus or minus two or three days, etc. And now, the problem was to, number one, just be able to predict that. Okay, the problem of the planets. When you read Kuhn next week, he will talk about the problem of the planets, where planet means wandering star.

Obviously, the sun and moon are wandering stars too. But I repeat, the sun and moon do not have this retrograde pattern. And it's this retrograde pattern that's the challenge that one faces, that everybody faced from time in memorial. I'm gonna stop there for any questions before starting to go into what people did with this.

I hope you're impressed that the Babylonians could pick this all up, and you must. I should be quite frank with you, they were picking it up for astrological reasons. They thought this was really important to pick up. They weren't doing science. They were really stationary points or possible omens.

No questions? You're all melting. Believe me, I'm in worse shape than you're in.

All right.