'OK, I understand what the tilt is, but how does that change the seasons, the .. of the Northern or that Southern Hemisphere.

Notice all of our rovers and all of our landers have gone to the northern hemisphere.

Northern hemisphere

Southern hemisphere

Let me draw the Equator which separates our Northern and Southern hemispheres.

I'll draw the Equator in the same color actually, in that green color.

Hemisphere

Where r is the radius of the sphere.

So this is the total amount of sunlight that's reaching the Earth at any point time.

There's some missing puzzle pieces.

The Sun is 93 million miles away, and we're tilting like this, right?

So the South will have summer.

Well, let's think about it.

The equator divides the globe into two hemispheres.

The equator divides the earth into two hemispheres.

But look in the southern hemisphere.

When you think about it, the oceans are 75 percent of the planet.

Finally, the Greeks also knew that the sun appears lower in the sky as you travel north.

The United States is in the Northern Hemisphere.

And then down here, the Southern Hemisphere is pointed away from the Sun, so, this is winter in the Southern Hemisphere.

I've pedaled this damn thing halfway around the world.

Japan is located in the Northern Hemisphere.

Roughly, at the same time, Carnegie

No, no, that's the only place a ship's been.

And in particular, it really goes against what we experience in the northern hemisphere, because our perihelion right now is occurring in January. It is occurring during the winter, the northern hemisphere winter.

So that's probably (I guess you could say) the biggest point of data that we observe on our planet why this by itself can not explain the change in seasons.

The acoustic transit time for each of these paths is about three hours.

So, this is the Equator.

And yes, the red planet is red.

So, this is summer in the South, the Southern Hemisphere, and it's winter in the North.

This is a sculpture of the Sun and the Earth, in proportion.

We've got 33 cameras out this moment.

And the most obvious point of evidence why this is not the case, is that it is not summer, when it is summer at one point on the planet, it is not summer throughout the planet at that time in particular when it is... (let me draw our planet) in particular, when it is summer in the northern hemisphere, (actually let me do summer in a more warm color) when it is summer in the northern hemisphere, so here it is summer, it is winter in the southern hemisphere.

And this is what the system involves, a number of sites in the southern hemisphere, shown in those circles.

Light travels around the earth seven and a half times a second.

And that's why when you get something declassified, that's what it looks like.

First, let's imagine this guy.

And what I want to think about is how much sunlight will different parts of the planet receive, and I'll focus on the Northern Hemisphere, but you can make a similiar argument for the Southern Hemisphere.

In Jared Diamond's wonderful book, Guns, Germs and Steel, he talks about how it was germs, more than guns and steel, that conquered the new hemisphere the Western hemisphere that conquered the rest of the world.

It's exactly the opposite I'm against legalization.

I saw birds that breed in Greenland, in the Gulf.

All the stuff from Jurassic Park that you know of all those small animals they all come from northern continents.

So majority of the rays are hitting the Southern Hemisphere and much fewer are hitting the Northern Hemisphere.

But, because the Earth spins, the moving air curves to the right in the Northern Hemisphere because of the coriolis effect.

Now, our current explanation of seasons is that the Earth's axis is tilted like that, so each hemisphere tilts toward the sun for half the year, and away for the other half.