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Earth's place in the universe

Early astronomers developed the geocentric model to describe Earth's place in the universe. In this model, Earth is at the center of the universe. Later, the geocentric model was replaced by the heliocentric model. In this model, the sun is at the center of the universe. Created by Khan Academy.

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Video transcript

- [Narrator] Hello, everyone. Today, we are going to be talking about Earth's place in space. So for as long as there have been humans, we've been looking up at the stars, and wondering about our place in the universe. Our understanding about this has improved over time, and we have developed models that can help us understand something as vast as space. Now, models are just a way of representing a system of ideas that help us identify and understand patterns that we see. Models are a tool that can explain, and predict what we see in the world and space around us. And as we gain new information, we can update our models. So what kind of model might we construct about how Earth interacts with other bodies in the universe? To begin with, every day, we can see the sun rise in the east, and set in the west, and other objects in the sky follow a similar pattern. It was logical then for early models to consider the Earth as a stationary object, around which all of the universe revolves. This is called a geocentric model, geo for Earth, and centric for center. And this geocentric model was quite good at explaining a lot of the things that we see in the sky, like the apparent motion of the sun and the stars. But there were issues with this geocentric model. For example, the motions of other planets in the sky. A planet like Jupiter might sometimes move backwards in the opposite direction, which isn't really very well explained by this geocentric model. Now, when a model doesn't match our observations like this, we can add new pieces to improve the model. So astronomers working with the geocentric model did this by adding smaller cycles within the planets' orbits around the Earth. Adding these cycles did help match the geocentric model to the observations, but there were many of these cycles required, and there wasn't a good explanation for why these cycles existed. And there wasn't very much reasoning for them, and they made the model very complicated, and not particularly useful. Let's pause and talk a little bit about motion, specifically apparent versus actual. Imagine you're sitting in a car, and driving through a forest. To your perspective, as you look out the window, the trees seem to be whizzing by, but trees, of course, are rooted to the ground. It's actually you that are moving relative to the surface of the Earth. This is the difference between apparent and actual motion. So back to our geocentric model. Another way to explain some of the issues with a model like the geocentric model, besides adding evermore complicated features to it, is to in fact, just come up with a new model. And this is what astronomers did. The new model that was proposed to replace the geocentric model was the heliocentric model. In this model, the Earth and all the other planets actually revolve around the sun. Helio means sun. This heliocentric model didn't need complicated and unexplained cycles to explain the way the other planets moved across the sky. It was an expected outcome of the planets' orbits. Now you may be wondering, in this model, the sun is stationary. So why does it seem to be moving across the Earth's sky each day? This is actually also easily explained, because the motion of the sun in the sky is not actual motion, it is apparent motion. Not only is the Earth revolving around the sun, revolving means orbiting, but the Earth is also spinning on its own axis. And so as the surface of the Earth moves with its spinning motion, it makes it appear as if the sun is moving across the sky. The spinning of the Earth actually explains why celestial objects appear to rise and set over the course of a day, while the revolution, or orbit, around the sun explains the motion of those objects over the course of a year. It's important to remember that while the heliocentric model does a better job of explaining and predicting our solar system, which is what we want models to do, it is still a model, and not a perfect recreation of reality. For example, while the heliocentric model considers the sun as stationary, we know now that the sun is actually just one star of many billions orbiting the center of our own galaxy. And so the sun, along with the rest of the solar system, is in fact, moving. To summarize, in this video, we talked about how we can use models to help us understand what we see in the sky, and Earth's place in space. As we learn more, we improve our models, and we can develop new ones that do a better job of explaining our observations, and predicting the world around us, such as the change from a geocentric model to a heliocentric model. The Earth then is a planet that orbits the sun, like the other planets of the solar system. And the sun appears to move across the sky, because the Earth spins on its axis. Thanks for watching, and I hope you learned a little bit of something.