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If you were to ask some people why we have seasons,

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they might say that maybe it's due to how far we are from the sun

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at different points in the year, different points in Earth's orbit.

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Now what I want to do in this video is to show you why that isn't the case.

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So, the line or reasoning would go something like this:

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This is the sun, at the center of our solar system,

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and roughly the center of Earth's orbit.

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And let me draw Earth's orbit over here.

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And so the line of reasoning is there's certain points in Earth's orbit

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where we're closer to the sun(and certainly we can draw a better job than that...)

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Let's say this is a point where we're closer to the sun

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we get a little further and we get a lot further

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then we get a little bit closer, and this is the closest point.

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So maybe Earth's orbit looks something like this:

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So the argument would go, look, there are points in Earth's orbit where we are closer to the sun and points that we are further from the sun

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and actually, that part of the argument is true.

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Earth's orbit is not a perfect circle,

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and there are points in Earth's orbit where we are closer to or further away from the sun.

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And actually when we are closest to the sun, so Earth is right over here, there's a word for that,

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it's called "perihelion," it just means, the closest point in orbit to the sun.

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And there is a furthest point from the sun, and this is called "aphelion," (a-feel-ion, ap-helion).

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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,

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it has a slighty elliptical shape, and because of that there are times of the years where we are closer to the sun,

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and there are times when we are furthest from the sun. And the difference is about 3%, it's not a huge difference in distance,

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I've really exaggerated the difference in this diagram right over here,

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but based on this reasoning people would say, and this is the flawed part, that when we're closer to the sun

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this must be the summer.

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And when we are furthest away from the sun, this must be the winter.

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And the most obvious point of evidence why this is not the case, is that it is not summer,

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when it is summer at one point on the planet, it is not summer throughout the planet at that time

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in particular when it is... (let me draw our planet) in particular, when it is summer in the northern hemisphere,

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(actually let me do summer in a more warm color) when it is summer in the northern hemisphere, so here it is summer,

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it is winter in the southern hemisphere.

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And when it is summer in the southern hemisphere, it is winter in the northern hemisphere

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so the entire planet does not experience the seasons at the same time.

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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.

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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.

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Perihelion right now, is during the northern hemisphere winter. And when we are furthest away from the sun, this is actually the northern hemisphere's summer.

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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,

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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.

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In particular, we don't have the same seasons in both the northern and southern hemispheres at the same time,

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and in particular, in the northern hemisphere, when we are closest to the sun, it's actually in January, it's actually in the middle of winter.

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So, I'll leave you there in this video... I've left you just saying, okay, so the closeness to the sun to the sun does not dictate what season we are in,

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it's just saying, what is the reason, 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.