Earth's rotation and tilt
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Seasons aren't dictated by closeness to sun
If you were to ask some people why we have seasons, they might say that maybe it's due to how far we are from the sun at different points in the year, different points in Earth's orbit. And what I want to do in this video is show you why that isn't the case. So the line of reasoning would go something like this. This is the sun at the center of our solar system and roughly at the center of Earth's orbit. And let me draw Earth's orbit over here. And so the line of reasoning is that there are certain points in Earth's orbit where we are closer to the sun and certain-- let me draw a better job than that. So let's say this the point where we're closer to the sun. We get a little further. Then we get a lot further. And then we get a little bit closer. And then a little bit closer. And then this is the closest point. So maybe Earth's orbit looks something like this. So the argument would go, look, there are points in Earth's orbit where we're closer to the sun and points where we are further from the sun. And actually that part of the argument is true. Earth's orbit is not a perfect circle. And there are points in Earth's orbit where we are closer or further away from the sun. And actually when we are closest to the sun, so if Earth is right over here, there's a word for that. It's called perihelion. It just means the closest point in orbit, perihelion. Closest point in orbit to the sun. And there is a furthest point from the sun, and this is called aphelion. Or aphelion. I've sometimes seen it called aphelion, pronounced ap-helion. So it is true that Earth's orbit is not a perfect circle around the sun. Although it's pretty close, but it's not a perfect circle. It has a slightly elliptical shape. And because of that, there are times in the year where we are closest to the sun, and there are times of the year where we are furthest to the sun. And the difference is about 3%. So it's not a huge difference in distance. I've really exaggerated the difference in this diagram right over here. But based on this reasoning, people would say-- and this is the flawed part-- that when we're closer to the sun, this must be the summer. And when we are furthest away from the sun, this must be the winter. And the most obvious point of evidence why this is not the case is that when it is summer at one point on the planet, it is not summer throughout the planet at that time. So 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 when it is summer in the Southern Hemisphere, it is winter in the Northern Hemisphere. So the entire planet does not experience the seasons at the same time. So that's probably, I guess you could say, the biggest point of data that we observe on our planet why this by itself cannot explain the change in seasons. 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. Perihelion in right now is during the Northern Hemisphere winter. And when we are furthest away from the sun, this is actually the Northern Hemisphere summer. So although it might seem like a fairly intuitive idea, hey, if we're closer to the sun, the whole planet's getting warmer, maybe that's summer. When we're further away, the whole planet's getting a little less energy, that's winter. The evidence we see on Earth goes directly against that. In particular, we don't have the same seasons in both the Northern and Southern Hemisphere at the same time. And in particular, in the Northern Hemisphere, when we're closest to the sun, it's actually in January. It's actually in the middle of winter. So I'll leave you there in this video, I've left you just saying, OK, so the closeness to the sun does not dictate what season we are in. And so you're saying, what is the reason? And what we'll see in the next video, the reason is the tilt of the axis of the earth, the rotational axis of the earth.