Class is in session.

Now, all cultures have known about five of the planets since basically forever.

And it's easy to spot a planet on the sky.

It's the things that don't twinkle. It's the stars that aren't quite behaving exactly as stars should. Now, the reason the stars twinkle is because there is a thin beam of lights passing through our atmosphere, and our atmosphere is a very complicated place with lots of hot air layers and cold layers and all

shifting around and that bends the path of light. So if a light is passing through a cold

layer and then hits a warm layer, it's going to shift its path and then back to cold,

it's going to wiggle around. It's going to twinkle. And planets don't twinkle, because they're close enough that they don't appear as a tiny dot

of light, they appear as actually a small disk of light.

So all that light is still getting twinkled up, but when you take a disk of light and just

mix it up on the inside, it still looks like a disc.

And so the planets don't twinkle.

So immediately, you start to notice some weird things on the sky.

And then planets do this other weird thing, which has changed their position relative to the stars.

As you track them night by night by night, they'll

be close to another star, and then they'll get really close to it, and then they'll move far away

from it, and then they'll be in between stars, and they'll approach another star, and on and on

and on. And then sometimes they even go backwards. That's right. Like, over night to night, you can see a steady progression of the movement of the planets.

And then sometimes they retrace their steps like they drop something and then they continue

on their way.

We call this retrograde motion.

The normal direction of motion is called prograde motion, by the way.

But this is called retrograde motion.

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