Cosmology and astronomy
- Seasons aren't dictated by closeness to sun
- Season simulator
- How Earth's tilt causes seasons
- Are southern hemisphere seasons more severe?
- Milankovitch cycles precession and obliquity
- Precession causing perihelion to happen later
- What causes precession and other orbital changes
- Apsidal precession (perihelion precession) and Milankovitch cycles
Why our closeness to the sun does not dictate the seasons. Created by Sal Khan.
Want to join the conversation?
- When sal talks about perihelion, is that a way of saying, 'hey! were closer to the sun' or something else?(20 votes)
- The prefix "peri-" denotes to the point nearest to a specified celestial body. If you read anything in Astronomy/Geography with the prefix "peri-" assume that it means the closest to that body.(8 votes)
- why is earth's orbit not a perfect circle?(19 votes)
- A perfectly circular orbit is unstable, any disturbance will cause the orbit to become an elipse.(36 votes)
- 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)
- What if the earth stops turning?:0(5 votes)
- If the earth stops spinning, the air would not, at least not immediately. The equator spins at around 1000 miles per hour so if the earth stopped spinning, there would be 1000 mile per hour winds at the equator.(13 votes)
- What about winter and summer on the equator? Are there no seasons on the equator?(4 votes)
- 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)
- 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)
- 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)
- 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)
- 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?(3 votes)
- 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).(4 votes)
- what is the difference between perihelion and opposition(3 votes)
- Perihelion and Aphelion (using the root from helios meaning sun) simply refer to when the Earth is closest or furthest to the sun. Opposition refers to our relationship to another body (like a planet or our Moon), specifically when it is "on the other side of the Earth" to the sun. It's frequently closest to us at that moment, and also it's presenting it's fully-lit side to our night-time side, thus it may be brightest then. Full Moon is when the moon is in opposition.(2 votes)
- 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)
- The Earth (compared to us Humans) is really, really big. The curvature of the Earth is only 8 inches per mile, completely imperceptible to us here on the ground (without measuring instruments).(2 votes)
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.