Cosmology and astronomy
Scale of Earth and Sun. Created by Sal Khan.
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- Around7:40you were talking about the size of an AU, is that from the corona of the sun to the atmosphere of earth, from the surface of each, or from the center of earth and the sun?(74 votes)
- Until 1976 the AU was defined as the semi-major axis of earth's orbit: 149,598,261 km. Where the center of mass of the solar system is at one focus, and the center of mass of the earth is on the path of the ellipse. See the video "Conic Sections: Intro to Ellipses" for more information on what the semi-major axis and the focus is.
In the 1970's there was a major push to define all of our constants against natural laws of the universe, in ways that could be reproduced in laboratories. As a result, the AU was given an even more complex definition.
In 2012, the IAU re-defined it to be 149,597,870,700 m. Independent of the exact orbital parameters of earth.(73 votes)
- Is the Earth orbit elliptical or cicular? and if elliptical how is the AU compared to the minimum and the maximum distances?(18 votes)
- It is elliptical but its almost circular. At farthest, Earth is 1.01671388 AU, at closest, Earth is 0.98329134 AU.(38 votes)
- Does the Sun have an orbit?(9 votes)
- What is dark energy and dark matter?(4 votes)
- Dark energy a proposed force to explain why the universe is expanding at an accelerated rate. Its an energy that seems to come from space itself. Dark matter is a strange type of matter that does not interact with light and would undetectable if not for its gravity.(13 votes)
- why do we not go to mars now ?(5 votes)
- We still don't have the technology ready to go to Mars. One of the biggest concerns is radiation shielding.
The Earth's magnetic field protects us from a lot of solar radiation. However, a trip to Mars would involve astronauts being exposed directly to this radiation for long periods of time, posing a significant health risk. Shielding this amount of radiation using current technology would require significant weight, and therefore significant cost.
So, we must figure out a better way to shield without increasing the weight, to keep an already expensive endeavor more manageable.(8 votes)
- how fast is the speed of light in hours(4 votes)
- why doesn't the sun suck in any of the planets?(1 vote)
- An object orbiting the sun is falling towards the sun but the orbital velocity is enough that it keeps missing the sun.(7 votes)
- I dont really know if this would belong here but, if Mars hit the Earth how big would the crater be?
I mean i spent dozens of hours doing numbers and math but i get a hole about 60% of the planet, is that right?(1 vote)
- You would not get a crater from that type of event. What will happen is that the entire crust of the Earth will be vaporized an launched into space. The entire surface will now consist of magma and any crater would disappear because it would be liquid.(7 votes)
- how do you now how many miles the earth is(2 votes)
- If you are asking how they came up with the number, I am sure there are many methods to it. It was actually first done over 2200 years ago by a guy named Eratosthenes.
"Eratosthenes' method for determining the size of the Earth was an elegant application of simple geometry to an otherwise very difficult problem. By using the difference in the elevation of the noontime sun at two different locations, he was able to measure the angular difference betwen the vertical directions at those two locations. This angular difference told him what fraction of the way around the earth separated the two locations. He then used this fraction and the measured distance between the two locations to estimate the distance around the earth (a.k.a. the circumference)."
- 0:16. Most of us? ?(3 votes)
- Is it likely that everyone who is watching the video would recognize that image as the earth but you can't assume that everyone will.(3 votes)
My goal in this video and the next video is to start giving a sense of the scale of the earth and the solar system. And as we see, as we start getting into to the galaxy and the universe, it just becomes almost impossible to imagine. But we'll at least give our best shot. So I think most of us watching this video know that this right here is earth. And just to get a sense of scale here, I think probably the largest distance that we can somehow relate to is about 100 miles. You can get into a car for an hour, hour and a half, and go about 100 miles. And on the earth that would be about this far. It would be a speck that would look something like that. That is 100 miles. And also to get us a bit of scale, let's think about a speed that at least we can kind of comprehend. And that would be, maybe, the speed of a bullet. Maybe we can't comprehend it, but I'll say this is the fastest thing that we could maybe comprehend. It goes about-- and there are different types of bullets depending on the type of gun and all of that-- about 280 meters per second, which is about 1,000 kilometers per hour. And this is also roughly the speed of a jet. So just to give a sense of scale here, the earth's circumference-- so if you were to go around the planet-- is about 40,000 kilometers. So if you were to travel at the speed of a bullet or the speed of a jetliner, at 1,000 kilometers an hour, it would take you 40 hours to circumnavigate the earth. And I think none of this information is too surprising. You might have taken a 12- or 15-hour flight that gets you-- not all the way around the earth-- but gets you pretty far. San Francisco to Australia, or something like that. So right now these aren't scales that are too crazy. Although, even for me, the earth itself is a pretty mind-blowingly large object. Now, with that out of the way let's think about the sun. Because the sun starts to approach something far huger. So this obviously here is the sun. And I think most people appreciate that the sun is much larger than the earth, and that it's pretty far away from the earth. But I don't think most people, including myself, fully appreciate how large the sun is or how far it is away from the earth. So just to give you a sense, the sun is 109 times the circumference of the earth. So if we do that same thought exercise there-- if we said, OK, if I'm traveling at the speed of a bullet or the speed of a jetliner, it would take me 40 hours to go around the earth. Well, how long would it take to go around the sun? So if you were to get on a jet plane and try to go around the sun, or if you were to somehow ride a bullet and try to go around the sun-- do a complete circumnavigation of the sun-- it's going to take you 109 times as long as it would have taken you to do the earth. So it would be 100 times-- I could do 109, but just for approximate-- it's roughly 100 times the circumference of the earth. So 109 times 40 is equal to 4,000 hours. And just to get a sense of what 4,000 is-- actually, since I have the calculator out, let's do the exact calculation. It's 109 times the circumference of the earth times 40 hours. That's what it would take to do the circumference of the Earth. So it's 4,360 hours to circumnavigate the sun, going at the speed of a bullet or a jetliner. And so that is-- 24 hours in the day-- that is 181 days. It would take you roughly half a year to go around the sun at the speed of a jetliner. Let me write this down. Half a year. The sun is huge. Now, that by itself may or may not be surprising--and actually let me give you a sense of scale here, because I have this other diagram of a sun. And we'll talk more about the rest of the solar system in the next video. But over here, at this scale, the sun, at least on my screen-- if I were to complete it, it would probably be about 20 inches in diameter. The earth is just this little thing over here, smaller than a raindrop. If I were to draw it on this scale, where the sun is even smaller, the earth would be about that big. Now, what isn't obvious, because we've all done our science projects in third and fourth grade--or we always see these diagrams of the solar system that look something like this-- is that these planets are way further away. Even though these are depicted to scale, they're way further away from the sun than this makes it look. So the earth is 150 million kilometers from the sun. So if this is the sun right here, at this scale you wouldn't even be able to see the earth. It wouldn't even be a pixel. But it would be 150 million kilometers from the earth. And this distance right here is called an astronomical unit-- and we'll be using that term in the next few videos just because it's an easier way to think about distance-- sometimes abbreviated AU, astronomical unit. And just to give a sense of how far this is, light, which is something that we think is almost infinitely fast and that is something that looks instantaneous, that takes eight minutes to travel from the sun to the earth. If the sun were to disappear, it would take eight minutes for us to know that it disappeared on earth. Or another way, just to put it in the sense of this jet airplane-- let's get the calculator back out. So we're talking about 150 million kilometers. So if we're going at 1,000 kilometers an hour, it would take us 150,000 hours at the speed of a bullet or at the speed of a jet plane to get to the sun. And just to put that in perspective, if we want it in days, there's 24 hours per day. So this would be 6,250 days. Or, if we divided by 365, roughly 17 years. If you were to shoot a bullet straight at the sun it would take 17 years to get there, if it could maintain its velocity somehow. So this would take a bullet or a jet plane 17 years to get to the sun. Or another way to visualize it-- this sun right over here, on my screen it has about a five- or six-inch diameter. If I were to actually do it at scale, this little dot right here, which is the earth, this speck-- I would have to put this back about 50 feet away from the sun. 50 or 60 feet away from the sun. If you were to look at the solar system-- and obviously there's other things in the solar system, and we'll talk more about them in the next video-- you wouldn't even notice this speck. This is a little dust thing flying around this sun. And as we go further and further out of this solar system, you're going to see even this distance starts to become ridiculously small. Or another way to think about it-- if the sun was about this size, then the earth at this scale would be about 200 feet away from it. So you could imagine, if you had a football field-- let me draw a football field. These are the end zones-- one end zone, another end zone. And if you were to stick something-- maybe the size of a medicine ball, a little bit bigger than a basketball, at one end zone-- this little speck would be about 60 yards away, roughly 60 meters away. So it's this little speck. You wouldn't even notice it on the scale of a football field, something this size. Anyway, I'm going to leave you there. Hopefully that just starts to blow your mind when you think about the scale of the sun, the earth, and how far the earth is away from the sun. And then we're going to see even those distances, even those scales, are super small when you start thinking about the rest of the solar system. And especially when we start going beyond the solar system.