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Seasons aren't dictated by closeness to sun

Why our closeness to the sun does not dictate the seasons. Created by Sal Khan.

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  • piceratops ultimate style avatar for user Spaceboyjonah_SRG
    When sal talks about perihelion, is that a way of saying, 'hey! were closer to the sun' or something else?
    (20 votes)
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  • leafers tree style avatar for user PantheraTigrisAltaica
    why is earth's orbit not a perfect circle?
    (19 votes)
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  • mr pink red style avatar for user 20ViteS
    What if the earth stops turning?:0
    (5 votes)
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  • blobby green style avatar for user Salvatore Dragone
    So are southern hemisphere summers warmer than nothern hemisphere summers due to to the fact that the earth is closer to the sun when the southern hemisphere is in summer mode? Visa versa, does the southern hemisphere experiance a colder winter than the north for the oposite reason, it is further away from the sun than the north is during its winter?
    (10 votes)
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  • purple pi purple style avatar for user nikhilgupta1997
    What about winter and summer on the equator? Are there no seasons on the equator?
    (4 votes)
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  • mr pants teal style avatar for user Kate
    The Earth obits the sun and is kept in its orbit by the sun's gravity. But why does the Earth rotate around the sun? Why isn't it stuck at a fixed point a certain distance away from the sun?
    (3 votes)
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    • piceratops ultimate style avatar for user AegonTargaryen
      An orbit is a balance between the Earth's velocity and the Sun's gravity. As the Earth moves through space, the Sun's gravity attracts Earth bending its path. This means that the Earth is constantly "falling" towards the Sun but because it is moving, it keeps missing. The Earth must move around the Sun or fall into it.
      (6 votes)
  • leaf green style avatar for user shreyansh
    earth REVOLVES because of its attraction to the sun and other heavenly bodies. but plz tell me why does it ROTATE on its axis ?
    (3 votes)
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    • male robot hal style avatar for user Andrew M
      It rotates because it was formed from the same stuff that was spinning in a big disc that eventually became the solar system. It's conservation of angular momentum, similar to the way when a twirling ice skater pulls in her arms, she rotates faster.
      (5 votes)
  • male robot johnny style avatar for user Jacksen
    Also I have another question... Why does the earth look flat even though all the mountains and biomes... I know about the far away perspective but, even when I look and try to find a rugged angle but can't!
    (3 votes)
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  • piceratops ultimate style avatar for user April
    If the sun is pulling on the Earth, such that gravity keeps it in place, why doesn't the same point on the earth always face toward the sun? I'm thinking like a large ball connected by a string to a smaller ball?
    (2 votes)
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    • male robot hal style avatar for user Andrew M
      because gravity does not just act on one point of the earth like a string would act on one point of the ball. It pulls on the entire earth. You can think of it's pull as being centered on the center of the earth. So a better analogy might be the way a yo-yo keeps spinning even when you do a trick like "around the world".

      (Note: because of something called "tidal friction", planets and moons do sometimes end up in the situation you described, which is called "tidal lock". For example, the moon always presents the same face toward the earth. Mercury always presents the same face to the sun. The earth's rotation is in fact slowing down, very very gradually).
      (3 votes)
  • male robot hal style avatar for user Jason Michael McCann
    Hmm. Not sure about this. Yes what you have said is true, but when one or other of the hemispheres is tilted toward the sun that is also a distance relationship. So - no doubt as you will instruct us in the next video - it is still a matter of proximity to our sun which determines the Seasons. Am I just being pedantic?
    (3 votes)
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    • blobby green style avatar for user wyo.master.mason
      the overall change in distance from earth to the sun is only about 3 million miles (extremely small in solar system terms). however, the earth is tilted only about 23 degrees from vertical. Because earth is a sphere the degree to which one hemisphere or the other is closer or farther to the sun is essentially zero when compared to the overall distance to the sun. Insolation angle has a much larger influence.
      (2 votes)

Video transcript

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.