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How much of that energy comes from humans and how much of that energy comes from things other than humans. So just as an example, when we talk about human power, we're talking about someone literally hand plowing this field. So this woman right over here is literally she has this little cart that's digging up the soil ... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
When we talk about oxen power, we're talking about the oxen doing most of the work. They're the ones dragging this plow, which is digging up all of the soil. And this gentleman has to kind of be there to supervise. But this still is fairly intense labor that this gentleman is doing right over here. And then when we tal... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
But this still is fairly intense labor that this gentleman is doing right over here. And then when we talk about tractors, we're talking about a scenario like this, where the tractor is doing most of the work of actually digging up, dragging this plow behind it, and digging up all of the soil. And from this book right ... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
I'll tell you which numbers I got from them and which numbers I kind of reasoned through because I wasn't fully comfortable with the numbers that they had. But these are their numbers, that if you're of human power to till one hectare of soil, it'll take 400 hours. Oxen power, and this should be a pair, not a parry. Th... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
That should be a pair of oxen. 65 hours. A 6-horsepower tractor, 25 hours. A 50-horsepower tractor, 4 hours. And in case y'all are wondering, what is a hectare of soil? It is literally a plot of land, a hectare of soil. Let me write it. | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
A 50-horsepower tractor, 4 hours. And in case y'all are wondering, what is a hectare of soil? It is literally a plot of land, a hectare of soil. Let me write it. A hectare of land, I should say, is a plot of land that is 100 meters by 100 meters. And it's roughly equal to 2 and 1⁄2 acres. Not exactly 2 and 1⁄2 acres. | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
Let me write it. A hectare of land, I should say, is a plot of land that is 100 meters by 100 meters. And it's roughly equal to 2 and 1⁄2 acres. Not exactly 2 and 1⁄2 acres. It's like 2.4, I think 2.47 something, but roughly 2 and 1⁄2 acres. So we're just thinking about how many hours to essentially dig up all of the s... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
Not exactly 2 and 1⁄2 acres. It's like 2.4, I think 2.47 something, but roughly 2 and 1⁄2 acres. So we're just thinking about how many hours to essentially dig up all of the soil for a plot of land 100 by 100 meters. So what we have over here, so clearly human takes a lot longer. Oxen, they can do a little bit faster. ... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
So what we have over here, so clearly human takes a lot longer. Oxen, they can do a little bit faster. 6-horsepower tractor, even faster. 50-horsepower tractor, very powerful tractor, even faster than that. Now this column right over here is the amount of energy required to actually produce and maintain the machinery u... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
50-horsepower tractor, very powerful tractor, even faster than that. Now this column right over here is the amount of energy required to actually produce and maintain the machinery used. So this is a very unintuitive thing. Whenever you think about, like for example, whenever you think about the amount of energy to plo... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
Whenever you think about, like for example, whenever you think about the amount of energy to plow this land over here, you tend to think, OK, well this individual is going to have to expend a lot of her energy. You don't think about the amount of energy required to actually maintain the tool, to one, build the tool tha... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
Actually, one of them might be a gal. But it's about 6,000 kilocalories. And this is kilocalories for lowercase c. And one thing I want to emphasize here, 1 kcal is the same thing as 1 calorie with a capital C, which is the same thing as 1,000 calories with a lowercase c. And we talked about this in the last video. But... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
But these, when people talk about food calories, they're really talking about a calorie with a capital C. Or you could say they're talking about kilocalories. So your candy bar, 200 calories, they're talking about this right over here. In chemistry class, when you talk about the amount of energy to raise a gram of wate... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
So in all of these numbers in this chart right over here, you can either view them as this unit, kcals, kilocalories, or calories with a capital C. They're essentially the same units that we used in the last video. And these are the same numbers that you were used to from a dietary calorie point of view. So for example... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
So this is to maintain it over the course of these 400 hours in the case of the hand plow. And the total amount of calories that were needed to make the plow divided by the total number of hours. So whatever fraction of the plow's life is being used here, you use that fraction right over here to put this 6,000 calories... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
But needless to say, for at least the plow, for either the human or the oxen scenario, this isn't a significant amount of the total calories. So obviously, if you're doing it either with human power or oxen power, you're not using any gasoline. You're not using any petroleum. In all of these scenarios, we're going to a... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
In all of these scenarios, we're going to assume that someone has 10 working hours in the day. And that right over here, this is kind of a measure of how hard that person's work is. And I kind of estimated these numbers here. They're slightly different than what the original numbers were in this book right over here. B... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
They're slightly different than what the original numbers were in this book right over here. But we're saying, look, if you are actually walking along using this hand plow, that is actually very, very vigorous activity. So it's going to require about 400 calories per hour to do this type of activity. You do it over 10 ... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
You do it over 10 hours, it's going to require 4,000 calories just to do that over 10 hours. And then we're assuming that the rest of the day, you're going to walk around, and maybe you're going to cook dinner, eat breakfast, you're going to sleep, some of it. We're assuming that the other 14 hours of the day are going... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
And so this is the total. If someone were to, using this technique, work for a total of 10 hours, this is how many calories they would consume in the day. And you could see this is the most labor intensive. So it looks like that they would consume the most calories per day. These two are the least labor intensive. You'... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
So it looks like that they would consume the most calories per day. These two are the least labor intensive. You're sitting on a tractor, although that still requires more calories than sleeping or watching TV. And so that's the number of calories they would consume. Now this right over here, and this is the interestin... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
And so that's the number of calories they would consume. Now this right over here, and this is the interesting number, or one of the really interesting numbers. Based on all of these assumptions, this is the total human input in calories to do this task, to till this one hectare of soil. So over here, you're using 5,40... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
So over here, you're using 5,400 calories a day. If you're working 10 hours per day and it requires 400 total hours, you're going to be working 40 days, 400 divided by 10. 40 days times 5,400 calories per day, it's going to take a human, just the human part, not even thinking about the 6,000 calories necessary to maint... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
The human is going to spend 216,000 calories to till, to plow that one hectare of land. And if you add the other 6,000 in for the actual plow, and you could debate what this number should be, but it's not a significant number compared to this, you get about 222,000 total calories. When you go to the oxen situation, you... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
This is still labor intensive, but not as labor intensive as what this woman right over here is doing. So on a daily basis, you're using a little bit fewer calories, but since you're only doing 6 and 1 1 days of this, 65 hours divided by 10, you've significantly reduced the number of calories, the total number of calor... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
Now all of a sudden, the oxen have gotten involved. And if you assume that each oxen consumes about 20,000 calories a day, and you have two of them, so 40,000 calories per day just to feed the oxen, and you're going to do that for 6 and 1 1 days, 65 divided by 10, the oxen are going to consume 260,000 calories to do th... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
So this is an interesting phenomenon that is going on right over here. What the human is putting in as we get better and better technology goes down substantially, 216,000 to 33,000. And we'll see with the tractor it goes down even more. But the total energy, if you include the amount of energy that the oxen have to pu... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
But the total energy, if you include the amount of energy that the oxen have to put in, or if you include the amount of energy due to the gasoline that has to be used for the tractor, the total amount of energy is going up to plow that field. But the human energy goes down dramatically. Now, the last thing I want to hi... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
So if you look this up, and you can Google search it, they have much larger numbers here. But I did a little research, and it looks like for most petroleum-based, combustion-based engines or vehicles, roughly 20% of the total energy that's used in fuel, 20% of that energy is used for the actual production and maintenan... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
And then I just took 20% of that number for saying, well, how much energy had to be used to maintain that vehicle over that amount of time? And if you think about what fraction of this vehicle's life that 25 hours represents, that fraction times the total amount of energy required to produce that. So remember, these th... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
They had to be made in furnaces. So just producing a vehicle requires a lot of energy. And so this right over here is 20%. And I just use that rule of thumb for most petroleum-based or combustion-based vehicles. The 20% of the total energy expenditure over the course of that vehicle's life is roughly equal to the amoun... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
And I just use that rule of thumb for most petroleum-based or combustion-based vehicles. The 20% of the total energy expenditure over the course of that vehicle's life is roughly equal to the amount of energy used to produce that vehicle. But either way, you go all the way over here, the human has to spend less calorie... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
And then the total human input right over here for the 6-horsepower tractor is going to take them 2 and 1 half days, 25 hours at 10 hours a day, is going to be 8,500 calories. But of course, you have the petroleum used and then some estimate of the amount of energy used to produce that tractor. And you're just taking t... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
You're not taking the entire life of that 6-horsepower tractor. To produce a 6-horsepower tractor, this number would be much, much larger. If you talked about the total number of energy, we're just taking the small fraction of its life that we're using it right over here. Same thing for the 50-horsepower tractor. But e... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
Same thing for the 50-horsepower tractor. But either way you look at it, the human, and this is the really interesting thing, humans, by going from human power all the way to a 50-horsepower tractor, you're getting almost a factor of 200 improvement in terms of how little energy has to be put in by the human to till th... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
And the whole point of the simulation is to get an intuition for how galaxies form, why they have the structures they have, how solar systems form, how they have the structures they have, and how gravity alone can kind of define that structure. And what's really interesting about the simulation, besides the fact that i... | Accreting mass due to gravity simulation Cosmology & Astronomy Khan Academy.mp3 |
Fusion can now occur. And you can zoom in at different levels to really see how the different particles or the different masses are interacting. And then you can actually rotate that to see a little bit clearer. This is if I'm looking kind of right on top of it to see how they're interacting. And it's a three-dimension... | Accreting mass due to gravity simulation Cosmology & Astronomy Khan Academy.mp3 |
This is if I'm looking kind of right on top of it to see how they're interacting. And it's a three-dimensional simulation, so it's a very rich way of thinking about these. And what's exciting for me is it's highly dependent on what the initial conditions are. In an earlier version of Peter's simulation, he did not give... | Accreting mass due to gravity simulation Cosmology & Astronomy Khan Academy.mp3 |
In an earlier version of Peter's simulation, he did not give a net angular momentum to the system. And so you did not have as much of kind of the planet satellite or as much of the disk structures forming, although right here we don't have too much of a disk structure, although there does seem to be a dominant plane in... | Accreting mass due to gravity simulation Cosmology & Astronomy Khan Academy.mp3 |
Sometimes you restart it. You might not have a binary system. Depending on the initial conditions, you might have something that starts to look like the Milky Way. Sometimes you might have something that looks very different than the Milky Way. And it really gives us clues of why we see such diversity, especially when ... | Accreting mass due to gravity simulation Cosmology & Astronomy Khan Academy.mp3 |
Sometimes you might have something that looks very different than the Milky Way. And it really gives us clues of why we see such diversity, especially when we're looking at galaxies, the structure of galaxies, that it's highly dependent on initial conditions. One can argue that our own solar system did have some net in... | Accreting mass due to gravity simulation Cosmology & Astronomy Khan Academy.mp3 |
And so this isn't, at least in my mind, too unrealistic of a scenario. And it's really cool to look at. And it really gives you a sense of things. You already see you have a binary star. They're kind of orbiting around each other or orbiting around the center of mass, which kind of looks like around each other. And the... | Accreting mass due to gravity simulation Cosmology & Astronomy Khan Academy.mp3 |
You already see you have a binary star. They're kind of orbiting around each other or orbiting around the center of mass, which kind of looks like around each other. And then this star right over here has its own kind of captive planet that is just rotating around it. We can see it a little bit clearer. If we had a ver... | Accreting mass due to gravity simulation Cosmology & Astronomy Khan Academy.mp3 |
We can see it a little bit clearer. If we had a very, at least from this perspective, a very close range, we can zoom in a little bit more to see it a little bit better. This has a satellite, but then they're also kind of dancing around each other. So it's a really fascinating simulation. I could really stare at this a... | Accreting mass due to gravity simulation Cosmology & Astronomy Khan Academy.mp3 |
So it's a really fascinating simulation. I could really stare at this and play with it for days. And I encourage you to play with it, restart it, see how the initial conditions or what type of solar systems or galaxies you might end up with, whether they form disks, whether you have binary systems or not, whether you h... | Accreting mass due to gravity simulation Cosmology & Astronomy Khan Academy.mp3 |
And then if you are more advanced, actually play with the code and see if you can really change the initial conditions, the starting velocities of things, the number of particles of things, the distribution of mass that you start off with, the angular momentum that you start off with, and see how that might change the ... | Accreting mass due to gravity simulation Cosmology & Astronomy Khan Academy.mp3 |
And the answer there comes from the same technique that we saw Mohorovichik use in 1909 to essentially see how the behavior, or when you measure the seismic waves, or whether you can even measure the seismic waves, the different distances from an earthquake. So if there's an earthquake right here, and we're calling tha... | How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3 |
They can travel through solid or liquid, or air for that matter. So they can travel through anything. But S waves can only, S for secondary, these are the transverse waves, these can only travel through solids. So it turns out that if an earthquake happens at 0 degrees, and you have seismograph stations all over the wo... | How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3 |
So it turns out that if an earthquake happens at 0 degrees, and you have seismograph stations all over the world, and these are extremely sensitive in order to be able to measure earthquakes that are happening thousands of kilometers away, it turns out that there's something called an S shadow, an S wave shadow. You ca... | How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3 |
They can go all the way over here. They can go over here. They can go over there. You can measure them over here. So you can measure them at all of these points, but then all of a sudden, at 105 degrees, and so we're measuring 0 degrees here and we're going outwards like that, all of a sudden at 105 degrees and further... | How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3 |
You can measure them over here. So you can measure them at all of these points, but then all of a sudden, at 105 degrees, and so we're measuring 0 degrees here and we're going outwards like that, all of a sudden at 105 degrees and further, you stop measuring S waves. They don't get, for some reason, you would think tha... | How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3 |
Maybe they would be a little bit weaker, but they would be able to get all the way over here. But they just abruptly stop. No more S waves. So in this whole area right over here, you get no S waves. And obviously, I could flip this picture over and you'd see a symmetric thing on the other side of the globe that all of ... | How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3 |
So in this whole area right over here, you get no S waves. And obviously, I could flip this picture over and you'd see a symmetric thing on the other side of the globe that all of this area over here, you also would not see. You would also not see S waves. You'd only see them from 105 degrees in this direction and 105 ... | How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3 |
You'd only see them from 105 degrees in this direction and 105 degrees in that direction. And the only reasonable explanation that we can give is that there must be some material that an S wave cannot travel through, that it would have to travel through to get to these points beyond 105 degrees. And we know that S wave... | How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3 |
So the assumption there is that at some point beyond 105 degrees, it's hitting liquid. So that's what tells us that this right here is probably a liquid. So it's hitting some layer that is liquid. So that tells us that there's a core, and at least the outer part of that core is liquid, enough to stop S waves. So the S ... | How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3 |
So that tells us that there's a core, and at least the outer part of that core is liquid, enough to stop S waves. So the S waves, because it only travels in solids, it leads to this S wave shadow. And this tells us that we have a core. And that core, at least the outer part, is liquid. We don't know yet whether the inn... | How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3 |
And that core, at least the outer part, is liquid. We don't know yet whether the inner part is liquid or solid. Now the next point of evidence is how do we know that there's an inner core? And we can use P waves for that. The P wave can travel through anything. But remember, as you get denser material, in general, for ... | How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3 |
And we can use P waves for that. The P wave can travel through anything. But remember, as you get denser material, in general, for the same type of material, if you get denser material, it's going to move faster. So it's going to refract outwards like we've seen over here. But if it goes into a liquid, in general, soun... | How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3 |
So it's going to refract outwards like we've seen over here. But if it goes into a liquid, in general, sound waves, or I should say P waves, seismic waves, move slower in liquids. And so the refraction patterns we get when we do measure from seismograph stations around the world is that it looks like the P waves are do... | How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3 |
But then they're getting refracted as if they're going into a slower medium as they go through the outer core. And we see that right over here. And then they get refracted again to get to some point on the other side. Now that is just what you would expect if it was all liquid. But if you go to stations that are even f... | How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3 |
Now that is just what you would expect if it was all liquid. But if you go to stations that are even further out, it looks like, if you just look at the refraction patterns, and you can now model this with fancy computers and get all the data points, but you could say, well, the only way that reality can fit the data t... | How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3 |
And then they're just continuing to refract the way you would expect. So it's really the refraction pattern of the P waves. And frankly, the fact that there's this what you call a P wave shadow. The P wave shadow by itself, all that tells you is that kind of roughly crazy things are happening someplace in the core. But... | How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3 |
The P wave shadow by itself, all that tells you is that kind of roughly crazy things are happening someplace in the core. But the real way to know that we have an inner core that's solid, as opposed to the whole thing being liquid, is that the P waves is the pattern of how the P waves reach essentially the other side o... | How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3 |
And he was a meteorologist and seismologist, and he was the first one to notice in 1909 when there was an earthquake in Croatia, a little bit southeast of Zagreb. So the earthquake was roughly over here, and lucky for him and lucky for us, before that earthquake there was actually a bunch of seismographic stations alre... | The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3 |
And what was interesting about this, Andrija realized that if the entire Earth was just kind of a uniform material, so let's draw that scenario, it would get denser as you go down, and so you would have kind of this refraction, this continuous refraction or these curved paths happening. But he realized that, let's say ... | The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3 |
But in general, the further you are away from the earthquake, or the time it takes for the waves to get to a point, is going to be proportional to the distance that point is away from the earthquake. So you would expect to see something like this. So if you were to plot on the horizontal axis, if you were to plot dista... | The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3 |
You should see a straight line. And that's just because it's traveling roughly the same velocity along any of these arcs. It's maybe getting a little bit faster as it's getting deeper, but roughly the same velocity is traveling along these arcs, and the distance of these arcs are proportional to the distance along the ... | The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3 |
So essentially, they're all traveling roughly at the same velocity, and they're just traveling different distances, so the time it takes is just going to be proportional to the distance. But he noticed something interesting. When he actually measured when the waves from that earthquake reached different seismographic s... | The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3 |
So this is the theoretical, if we had this uniform one-layered Earth. But he saw something interesting. So once again, this is distance, and this right over here is time. And at 200 kilometers away from the earthquake, so until 200 kilometers, he saw exactly what you would expect from a uniform Earth. It was just the t... | The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3 |
And at 200 kilometers away from the earthquake, so until 200 kilometers, he saw exactly what you would expect from a uniform Earth. It was just the time was proportional to the distance. But at 200 kilometers, he saw something interesting. All of a sudden, the waves were reaching there faster. The slope of this line ch... | The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3 |
All of a sudden, the waves were reaching there faster. The slope of this line changed. It took less time for each incremental distance. So for some reason, the waves that were going at these farther stations, the stations that were more than 200 kilometers away, the stations, somehow they were accelerated. Somehow they... | The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3 |
So for some reason, the waves that were going at these farther stations, the stations that were more than 200 kilometers away, the stations, somehow they were accelerated. Somehow they were able to move faster. And he's the one that realized that this was because the waves that were getting to these further stations mu... | The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3 |
So let's just think about it. So if we have a more dense layer, it will fit this information right over here. So if we have a layer like this, which we now know to be the crust, and then you have a denser layer, which we now know to be the mantle, then what you would have is, so you have your earthquake right over here... | The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3 |
Closer by, while you're still within the crust, it would be proportional. It would be proportional. And then let's say that this is exactly, this right here is 200 kilometers away. But then if you go any further, the waves would have to travel. So they would travel, so they would go like this. And then they would get r... | The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3 |
But then if you go any further, the waves would have to travel. So they would travel, so they would go like this. And then they would get refracted even harder. So they would get refracted even, so they would be a little bit curved ahead of time. But then they're going into a much denser material, or it's not gradually... | The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3 |
So they would get refracted even, so they would be a little bit curved ahead of time. But then they're going into a much denser material, or it's not gradually dense. It's actually kind of a, all of a sudden, a considerably more dense material. So it would get refracted even more. And then it'll go over here. And since... | The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3 |
So it would get refracted even more. And then it'll go over here. And since it was able to travel all of this distance in a denser material, it would have traveled faster along this path. And so it would get to this distance on the surface that's more than 200 kilometers away. It would get there faster. And so he said ... | The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3 |
And so it would get to this distance on the surface that's more than 200 kilometers away. It would get there faster. And so he said that there must be a denser layer that those waves are traveling through, which we now know to be the mantle. And the boundary between what we now know to be the crust and this denser laye... | The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3 |
And the boundary between what we now know to be the crust and this denser layer, which we now know to be the mantle, is actually named after him. It's called the Mohorovicic discontinuity. And sometimes it's just called the Moho for short. So that boundary between the crust and the mantle is now named for him. But this... | The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3 |
So that boundary between the crust and the mantle is now named for him. But this was a huge discovery, because not only was he able to tell us, based on the data, based on kind of indirect data, just based on earthquakes happening and measuring when the earthquakes reach different points of the Earth, that there probab... | The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3 |
This right here is a fresco by Raphael of Plato hanging out with his best student Aristotle. And you may or may not know that these two fellows, along with Plato's teacher Socrates, are considered kind of the fathers of Western philosophy. But that's not what this video is about. I'm just, this is actually just a small... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
I'm just, this is actually just a small little video about different dates. Or maybe a better way to think about it, different ways to specify dates or dating mechanisms. And so if you were to look up Plato's birth, well you might get either 428 or 427, but we'll go with 428. If you were to look up Plato's birth, you m... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
If you were to look up Plato's birth, you might see it written as 428 BC, or you might see it written as 428 BCE. And the natural question is, well what's the difference here, they both have a BC, but this one has an E, it's the same year right now. And the answer is, is that these are referring to the exact same year ... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
BC literally stands for before Christ. So if the date is written 428 BC, the implication is that this is 428 years before the birth of Christ. We'll see in a second that that's not exactly right, but that's what the implication is. If someone writes BCE, they're saying something very different. The B still stands for b... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
If someone writes BCE, they're saying something very different. The B still stands for before, but the C in CE does not stand for Christ anymore. It now stands for common. And so the CE part is common era. Even though it's not referring to Christ anymore, and this kind of the intention here is so that it's I guess less... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
And so the CE part is common era. Even though it's not referring to Christ anymore, and this kind of the intention here is so that it's I guess less religious than the term before Christ, it's still kind of putting an importance on Christ's birth. Because it's saying that the common era is the time period after the bir... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
But there's essentially the same exact dating scheme, one not directly referring to Christ, one that is directly referring to Christ. Similarly, this right here is a painting of Christopher Columbus. And if you were to look up in history books, when was his first voyage and when did he first show up in the New World, f... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
And once again, these are all referring to the same year, just using different acronyms. One of them's a little bit more religious or more directly refers to Christ, and one is a little less religious. So AD, some people think it refers to after death. It does not refer to after death. Because if you think about it, if... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
It does not refer to after death. Because if you think about it, if you have years before the birth of Christ, and if you started numbering after his death, how would you number the years during his life? So AD does not stand for after death. It stands for Anno Domini, which literally means year, and domini means Lord ... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
It stands for Anno Domini, which literally means year, and domini means Lord or the Lord. So it's the year of the Lord or the year of our Lord. So it's years since. And one Anno Domini would be the year of Jesus Christ's birth. So not after death. It stands for Anno Domini, but literally year of our Lord, so years sinc... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
And one Anno Domini would be the year of Jesus Christ's birth. So not after death. It stands for Anno Domini, but literally year of our Lord, so years since that Jesus was born, with year one being implicitly starting with his birth. And we'll see in a second that's not exactly right. CE stands for Common Era. Once aga... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
And we'll see in a second that's not exactly right. CE stands for Common Era. Once again, 1 CE is the same thing as AD 1. Sometimes we now write, instead of writing AD 1492, we'll write 1492 AD, all referring to the exact same thing. Now, all of these things refer to, when we say 428 BC, it implies 428 years before the... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
Sometimes we now write, instead of writing AD 1492, we'll write 1492 AD, all referring to the exact same thing. Now, all of these things refer to, when we say 428 BC, it implies 428 years before the birth of Christ. 1492 AD, that's in the year of our Lord, 1492. It implies 1,492 years since the birth of Christ. But the... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
It implies 1,492 years since the birth of Christ. But the reality is that we're not really quite sure when Christ was born. And so these aren't exactly. So Columbus didn't sail across the Atlantic exactly 1492 years after the birth of Christ. Most historians put the birth of Christ at 7 to 2 BC or BCE, depending on how... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
So Columbus didn't sail across the Atlantic exactly 1492 years after the birth of Christ. Most historians put the birth of Christ at 7 to 2 BC or BCE, depending on how you want to view it. Remember, BC is before Christ, which is a little ironic because we're talking about the actual birth of Christ. BCE is before the C... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
BCE is before the Common Era. And they put his death at 30 to 36 AD, which is 30 to 36 in the year of our Lord. Or, that's what this stands for, Anno Domini, or in the Common Era, CE. Now, some people, they obviously don't like BC. They don't like the BC AD naming mechanism because it's explicitly referring to Christ. ... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
Now, some people, they obviously don't like BC. They don't like the BC AD naming mechanism because it's explicitly referring to Christ. And every year, it makes Christ the central figure in all of history. So they'll say that this is clearly too Christian. And they would prefer the situation, they would prefer the less... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
So they'll say that this is clearly too Christian. And they would prefer the situation, they would prefer the less Christian naming scheme, where you use BCE and CE. But a lot of people would still say, hey, look, OK, you changed, well, first of all, some Christians wouldn't like this, that you removed the direct refer... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
But even here, and they'll say, hey, you've removed it. But even here, some people would complain that although you've made the direct reference, that this is saying common before the Common Era and the Common Era, even though you've removed the direct reference, it still makes Christ's birth the central thing in all o... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
In order to have the same reference point, and it would be too logistically difficult to switch at this time, everyone has essentially settled on this. And it's really just a matter of letters of which naming scheme you pick. But the whole point of this video is that you don't get confused between BC and BCE. You don't... | Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3 |
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