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Global winds and currents Middle school Earth and space science Khan Academy.mp3
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One of my favorite things to do is go camping. For me, there's nothing better than getting outside, breathing in some fresh air, and taking a swim in my favorite river. Have you ever jumped into a river and felt that the deeper, cooler water closer to your feet was moving faster than the shallow, warmer water at your knees? That's a current, which is the word we use to describe how water, or even air, flows within a larger body of water or air. But what causes a current? Well, let's start with the Sun. The Sun actually heats Earth unevenly.
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Global winds and currents Middle school Earth and space science Khan Academy.mp3
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That's a current, which is the word we use to describe how water, or even air, flows within a larger body of water or air. But what causes a current? Well, let's start with the Sun. The Sun actually heats Earth unevenly. We know that it's hotter near the equator, and it gets colder as you go towards the poles. Near the equator, the Sun's rays hit Earth's surface more directly, while near the poles, the Sun's rays hit Earth's surface less directly. In both regions, the same amount of solar energy is hitting Earth, but near the equator, this energy is concentrated into a smaller area, and near the poles, it's spread out over a larger area.
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Global winds and currents Middle school Earth and space science Khan Academy.mp3
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The Sun actually heats Earth unevenly. We know that it's hotter near the equator, and it gets colder as you go towards the poles. Near the equator, the Sun's rays hit Earth's surface more directly, while near the poles, the Sun's rays hit Earth's surface less directly. In both regions, the same amount of solar energy is hitting Earth, but near the equator, this energy is concentrated into a smaller area, and near the poles, it's spread out over a larger area. So the regions near the equator get more solar energy, which makes them warmer, and the regions near the poles get less solar energy, which makes them cooler. This uneven heating of Earth also affects air pressure. Where it's cooler, near the poles, cool air will sink, making the air pressure high.
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Global winds and currents Middle school Earth and space science Khan Academy.mp3
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In both regions, the same amount of solar energy is hitting Earth, but near the equator, this energy is concentrated into a smaller area, and near the poles, it's spread out over a larger area. So the regions near the equator get more solar energy, which makes them warmer, and the regions near the poles get less solar energy, which makes them cooler. This uneven heating of Earth also affects air pressure. Where it's cooler, near the poles, cool air will sink, making the air pressure high. But where it's warmer, near the equator, warm air will rise, resulting in low pressure. This is where the terms low pressure cells and high pressure cells come from. The low pressure, warmer air at the equator rises into the upper atmosphere, where it cools and flows away towards higher latitudes, away from the equator.
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Global winds and currents Middle school Earth and space science Khan Academy.mp3
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Where it's cooler, near the poles, cool air will sink, making the air pressure high. But where it's warmer, near the equator, warm air will rise, resulting in low pressure. This is where the terms low pressure cells and high pressure cells come from. The low pressure, warmer air at the equator rises into the upper atmosphere, where it cools and flows away towards higher latitudes, away from the equator. Because the air is now cooler, it starts to sink again and creates a high pressure band near these latitudes. This process repeats and creates a pattern of high and low pressure bands from the equator to the poles. We know that air flows from areas of high pressure to areas of low pressure.
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Global winds and currents Middle school Earth and space science Khan Academy.mp3
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The low pressure, warmer air at the equator rises into the upper atmosphere, where it cools and flows away towards higher latitudes, away from the equator. Because the air is now cooler, it starts to sink again and creates a high pressure band near these latitudes. This process repeats and creates a pattern of high and low pressure bands from the equator to the poles. We know that air flows from areas of high pressure to areas of low pressure. This creates air currents, or winds. Now, you might think that these winds would blow in straight lines from high to low pressure areas, but the global wind patterns, which we call prevailing winds, look like they curve to the right in the northern hemisphere and to the left in the southern hemisphere. This curving has to do with the rotation of Earth and is called the Coriolis effect.
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Global winds and currents Middle school Earth and space science Khan Academy.mp3
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We know that air flows from areas of high pressure to areas of low pressure. This creates air currents, or winds. Now, you might think that these winds would blow in straight lines from high to low pressure areas, but the global wind patterns, which we call prevailing winds, look like they curve to the right in the northern hemisphere and to the left in the southern hemisphere. This curving has to do with the rotation of Earth and is called the Coriolis effect. As these prevailing winds blow across the surface of the land and water, they also push against the surface of the ocean and produce wind-driven surface currents, which help to move ocean water. Here's what the global pattern of ocean surface currents looks like. Like wind currents, ocean surface currents are also curved due to the Coriolis effect.
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Global winds and currents Middle school Earth and space science Khan Academy.mp3
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This curving has to do with the rotation of Earth and is called the Coriolis effect. As these prevailing winds blow across the surface of the land and water, they also push against the surface of the ocean and produce wind-driven surface currents, which help to move ocean water. Here's what the global pattern of ocean surface currents looks like. Like wind currents, ocean surface currents are also curved due to the Coriolis effect. We can see that in these currents that are traveling north and south, which curve to the right in the northern hemisphere and to the left in the southern hemisphere. Together, as these surface currents of the ocean connect, they form giant rotating systems of ocean currents called gyres. The currents that drive these gyres extend from the surface to about 1 km down into the ocean and help to move water all around the globe.
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Global winds and currents Middle school Earth and space science Khan Academy.mp3
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Like wind currents, ocean surface currents are also curved due to the Coriolis effect. We can see that in these currents that are traveling north and south, which curve to the right in the northern hemisphere and to the left in the southern hemisphere. Together, as these surface currents of the ocean connect, they form giant rotating systems of ocean currents called gyres. The currents that drive these gyres extend from the surface to about 1 km down into the ocean and help to move water all around the globe. But these gyres aren't just moving water, they're moving heat energy as well. Water is pretty good at holding onto heat it absorbs from the sun. So, as the water in our oceans moves around the world through this gyre circulation, the water also carries heat.
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Global winds and currents Middle school Earth and space science Khan Academy.mp3
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The currents that drive these gyres extend from the surface to about 1 km down into the ocean and help to move water all around the globe. But these gyres aren't just moving water, they're moving heat energy as well. Water is pretty good at holding onto heat it absorbs from the sun. So, as the water in our oceans moves around the world through this gyre circulation, the water also carries heat. Here, warm water generally moves from the equator to the poles, and cold water moves from the poles to the equator. But the ocean has other, deeper currents that are affected by differences in temperature and density. Remember how we talked about areas heated directly and less directly by the sun, and how that results in low and high pressure areas?
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Global winds and currents Middle school Earth and space science Khan Academy.mp3
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So, as the water in our oceans moves around the world through this gyre circulation, the water also carries heat. Here, warm water generally moves from the equator to the poles, and cold water moves from the poles to the equator. But the ocean has other, deeper currents that are affected by differences in temperature and density. Remember how we talked about areas heated directly and less directly by the sun, and how that results in low and high pressure areas? Same thing with water, except that water density is affected by both temperature and salinity, which is a measure of how salty the ocean is. Cooler and saltier water is more dense, so it tends to sink, just like cool air, whereas warmer and less salty water is less dense, and tends to rise, just like warm air. So, with these deeper ocean currents, water actually moves vertically, or up and down.
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Global winds and currents Middle school Earth and space science Khan Academy.mp3
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Remember how we talked about areas heated directly and less directly by the sun, and how that results in low and high pressure areas? Same thing with water, except that water density is affected by both temperature and salinity, which is a measure of how salty the ocean is. Cooler and saltier water is more dense, so it tends to sink, just like cool air, whereas warmer and less salty water is less dense, and tends to rise, just like warm air. So, with these deeper ocean currents, water actually moves vertically, or up and down. For example, water near the poles gets very cold. It also gets very salty because when sea ice is formed, the salt can't go into the ice. Instead, the salt stays in the water, and the water is more dense.
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Global winds and currents Middle school Earth and space science Khan Academy.mp3
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So, with these deeper ocean currents, water actually moves vertically, or up and down. For example, water near the poles gets very cold. It also gets very salty because when sea ice is formed, the salt can't go into the ice. Instead, the salt stays in the water, and the water is more dense. Instead, the salt stays behind in the water, and so the water gets saltier, or more saline. Together, the coldness and salinity makes the water very dense, causing it to sink deep into the ocean. In other parts of the ocean, wind drags deep water up to the surface in a process called upwelling.
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Global winds and currents Middle school Earth and space science Khan Academy.mp3
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Instead, the salt stays in the water, and the water is more dense. Instead, the salt stays behind in the water, and so the water gets saltier, or more saline. Together, the coldness and salinity makes the water very dense, causing it to sink deep into the ocean. In other parts of the ocean, wind drags deep water up to the surface in a process called upwelling. These vertical currents are connected by horizontal currents at the surface and in the deep ocean. Collectively, this system of currents is known as the overturning circulation. You might also hear it called the global ocean conveyor belt.
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Global winds and currents Middle school Earth and space science Khan Academy.mp3
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In other parts of the ocean, wind drags deep water up to the surface in a process called upwelling. These vertical currents are connected by horizontal currents at the surface and in the deep ocean. Collectively, this system of currents is known as the overturning circulation. You might also hear it called the global ocean conveyor belt. So here is a map showing the overturning circulation. This map might look a little bit strange, but here we're looking at Earth from the South Pole. So Australia and the southern tips of Africa and South America are closest to the center of the map, while Europe, most of Asia, and North America are at the edges.
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Global winds and currents Middle school Earth and space science Khan Academy.mp3
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You might also hear it called the global ocean conveyor belt. So here is a map showing the overturning circulation. This map might look a little bit strange, but here we're looking at Earth from the South Pole. So Australia and the southern tips of Africa and South America are closest to the center of the map, while Europe, most of Asia, and North America are at the edges. Now, if you follow the currents in the overturning circulation, you can see that they flow all over the world's oceans, from the Southern Ocean around the South Pole, to the Pacific, the Indian, and all the way into the North Atlantic. Like the currents in the gyre circulation, the currents in the overturning circulation also carry and disperse heat energy all around the world. Here I am, back in my favorite river, wading around and enjoying currents of cool, water flowing around my feet.
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Global winds and currents Middle school Earth and space science Khan Academy.mp3
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So Australia and the southern tips of Africa and South America are closest to the center of the map, while Europe, most of Asia, and North America are at the edges. Now, if you follow the currents in the overturning circulation, you can see that they flow all over the world's oceans, from the Southern Ocean around the South Pole, to the Pacific, the Indian, and all the way into the North Atlantic. Like the currents in the gyre circulation, the currents in the overturning circulation also carry and disperse heat energy all around the world. Here I am, back in my favorite river, wading around and enjoying currents of cool, water flowing around my feet. And even though this river is small, the currents that flow through it are similar to the global wind and ocean currents that flow all around the world. So these currents connect in our atmosphere and oceans, which means that we are all connected. So, currents connect us all.
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The rock cycle The geosphere Middle school Earth and space science Khan Academy.mp3
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No, rocks don't shapeshift into unicorns, but they do change shape and composition. Sometimes they change quickly, like when an erupting volcano launches molten rock into the air. And sometimes they change slowly, like when a rock is heated and compressed in the earth for thousands to millions of years. When geologists stare at a rock, they can figure out the rock's composition and tell the story of how it was created. And it turns out rocks can be made in many different ways. When put at the right combination of heat and pressure, rocks can become molten and liquid. When this molten rock cools and becomes solid, we call the rock it makes igneous rock.
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The rock cycle The geosphere Middle school Earth and space science Khan Academy.mp3
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When geologists stare at a rock, they can figure out the rock's composition and tell the story of how it was created. And it turns out rocks can be made in many different ways. When put at the right combination of heat and pressure, rocks can become molten and liquid. When this molten rock cools and becomes solid, we call the rock it makes igneous rock. This term comes from the Latin word igneous, which means fiery or burning hot. Igneous rocks make up more than 90% of the Earth's crust. One common kind of igneous rock is granite, which you've probably seen in bridges, buildings, and countertops.
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The rock cycle The geosphere Middle school Earth and space science Khan Academy.mp3
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When this molten rock cools and becomes solid, we call the rock it makes igneous rock. This term comes from the Latin word igneous, which means fiery or burning hot. Igneous rocks make up more than 90% of the Earth's crust. One common kind of igneous rock is granite, which you've probably seen in bridges, buildings, and countertops. Another kind of igneous rock is obsidian, which is smooth and glassy and is sometimes used to make knife blades. Both of these rocks were made of molten rock. So why did it look so different?
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The rock cycle The geosphere Middle school Earth and space science Khan Academy.mp3
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One common kind of igneous rock is granite, which you've probably seen in bridges, buildings, and countertops. Another kind of igneous rock is obsidian, which is smooth and glassy and is sometimes used to make knife blades. Both of these rocks were made of molten rock. So why did it look so different? Well, granite was made from magma, which is molten rock that exists below the surface of the Earth. Magma tends to cool slowly underground, which gives time for the elements in it to form large crystals. And this obsidian rock was made from lava, which is molten rock that flows above the ground.
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The rock cycle The geosphere Middle school Earth and space science Khan Academy.mp3
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So why did it look so different? Well, granite was made from magma, which is molten rock that exists below the surface of the Earth. Magma tends to cool slowly underground, which gives time for the elements in it to form large crystals. And this obsidian rock was made from lava, which is molten rock that flows above the ground. Lava tends to cool quickly, which causes the rocks it forms, like obsidian, to have smaller crystals. I always remember the difference between magma and lava like this. I keep my lava lamp above the ground, just like lava is above the ground.
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The rock cycle The geosphere Middle school Earth and space science Khan Academy.mp3
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And this obsidian rock was made from lava, which is molten rock that flows above the ground. Lava tends to cool quickly, which causes the rocks it forms, like obsidian, to have smaller crystals. I always remember the difference between magma and lava like this. I keep my lava lamp above the ground, just like lava is above the ground. If I buried my lava lamp in the ground, then I can call it a magma lamp. Once an igneous rock is created, its journey isn't over. The rock might begin to change through a process called weathering.
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The rock cycle The geosphere Middle school Earth and space science Khan Academy.mp3
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I keep my lava lamp above the ground, just like lava is above the ground. If I buried my lava lamp in the ground, then I can call it a magma lamp. Once an igneous rock is created, its journey isn't over. The rock might begin to change through a process called weathering. This is when water, weather, wind, and other physical forces chip away at a rock and cause little pieces to break off. These little particles are called sediments. The sediment from our igneous rock can be moved around by wind and water.
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The rock cycle The geosphere Middle school Earth and space science Khan Academy.mp3
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The rock might begin to change through a process called weathering. This is when water, weather, wind, and other physical forces chip away at a rock and cause little pieces to break off. These little particles are called sediments. The sediment from our igneous rock can be moved around by wind and water. This is called erosion. Eventually, the sediment will settle down somewhere, maybe at the bottom of an ocean or a lake. And it might join other sediment from other rocks, crushed up shells, and plant matter.
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The rock cycle The geosphere Middle school Earth and space science Khan Academy.mp3
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The sediment from our igneous rock can be moved around by wind and water. This is called erosion. Eventually, the sediment will settle down somewhere, maybe at the bottom of an ocean or a lake. And it might join other sediment from other rocks, crushed up shells, and plant matter. And over time, this sediment will become a rock. Rocks that are formed from sediments are called, you guessed it, sedimentary rocks. But how does sedimentary goop at the bottom of a lake become a rock?
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The rock cycle The geosphere Middle school Earth and space science Khan Academy.mp3
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And it might join other sediment from other rocks, crushed up shells, and plant matter. And over time, this sediment will become a rock. Rocks that are formed from sediments are called, you guessed it, sedimentary rocks. But how does sedimentary goop at the bottom of a lake become a rock? Well, it goes through a process called lithification. Lith comes from the Greek word for stone. So you can think of this process as stonification.
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The rock cycle The geosphere Middle school Earth and space science Khan Academy.mp3
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But how does sedimentary goop at the bottom of a lake become a rock? Well, it goes through a process called lithification. Lith comes from the Greek word for stone. So you can think of this process as stonification. The first step of lithification is called deposition. This is when sediment is deposited in a new location, and it spreads out to form a layer. As more and more layers are created, the layers underneath them become squished together.
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The rock cycle The geosphere Middle school Earth and space science Khan Academy.mp3
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So you can think of this process as stonification. The first step of lithification is called deposition. This is when sediment is deposited in a new location, and it spreads out to form a layer. As more and more layers are created, the layers underneath them become squished together. This is called compaction. When water moves through the layers of sediment, it can carry dissolved minerals with it. The water can leave these minerals in between the sediment particles, which makes everything stick together.
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The rock cycle The geosphere Middle school Earth and space science Khan Academy.mp3
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As more and more layers are created, the layers underneath them become squished together. This is called compaction. When water moves through the layers of sediment, it can carry dissolved minerals with it. The water can leave these minerals in between the sediment particles, which makes everything stick together. This cementing of sediment is called cementation. Limestone is a sedimentary rock that is made when sediment that contains a lot of calcium carbonate in it goes through lithification. But once a sedimentary rock has been created, it doesn't mean that it is done changing.
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The rock cycle The geosphere Middle school Earth and space science Khan Academy.mp3
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The water can leave these minerals in between the sediment particles, which makes everything stick together. This cementing of sediment is called cementation. Limestone is a sedimentary rock that is made when sediment that contains a lot of calcium carbonate in it goes through lithification. But once a sedimentary rock has been created, it doesn't mean that it is done changing. Sometimes a rock is squeezed or heated so much that the minerals inside it actually change composition. It is now a metamorphic rock. You might have heard of the word metamorphosis, which describes when something changes from one form and structure to another.
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The rock cycle The geosphere Middle school Earth and space science Khan Academy.mp3
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But once a sedimentary rock has been created, it doesn't mean that it is done changing. Sometimes a rock is squeezed or heated so much that the minerals inside it actually change composition. It is now a metamorphic rock. You might have heard of the word metamorphosis, which describes when something changes from one form and structure to another. Like when a caterpillar turns into a butterfly. Metamorphic rocks are made when igneous, sedimentary, or even other metamorphic rocks change form and structure because of heat and pressure. For example, when the sedimentary rock limestone gets put under a lot of heat and pressure, it can turn into marble.
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The rock cycle The geosphere Middle school Earth and space science Khan Academy.mp3
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You might have heard of the word metamorphosis, which describes when something changes from one form and structure to another. Like when a caterpillar turns into a butterfly. Metamorphic rocks are made when igneous, sedimentary, or even other metamorphic rocks change form and structure because of heat and pressure. For example, when the sedimentary rock limestone gets put under a lot of heat and pressure, it can turn into marble. But metamorphic rocks still might not be done changing. Rocks can actually keep on changing forever. Igneous and metamorphic rocks can become sedimentary rocks if they are broken apart into sediments and go through lithification.
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The rock cycle The geosphere Middle school Earth and space science Khan Academy.mp3
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For example, when the sedimentary rock limestone gets put under a lot of heat and pressure, it can turn into marble. But metamorphic rocks still might not be done changing. Rocks can actually keep on changing forever. Igneous and metamorphic rocks can become sedimentary rocks if they are broken apart into sediments and go through lithification. And metamorphic rocks and sedimentary rocks can become igneous rocks when they are melted and then cools. And igneous rocks and sedimentary rocks can become metamorphic rocks when they're exposed to high heat and pressure. This is called the rock cycle.
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Solar eclipses The Earth-sun-moon system Middle school Earth and space science Khan Academy.mp3
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Have you ever been minding your own business, enjoying the sun? When someone steps in front of you and blocks your sunlight? This is pretty much what happens during a solar eclipse, except on a planetary scale. As Earth revolves around the sun, the moon revolves around Earth, too. Once in a while, the sun, the moon, and Earth all line up so that the moon's shadow falls onto Earth, causing a solar eclipse. During a solar eclipse, the moon blocks the sun's light and energy from reaching part of Earth. So how does a solar eclipse appear from Earth?
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Solar eclipses The Earth-sun-moon system Middle school Earth and space science Khan Academy.mp3
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As Earth revolves around the sun, the moon revolves around Earth, too. Once in a while, the sun, the moon, and Earth all line up so that the moon's shadow falls onto Earth, causing a solar eclipse. During a solar eclipse, the moon blocks the sun's light and energy from reaching part of Earth. So how does a solar eclipse appear from Earth? Well, if you're standing in the center of the moon's shadow, you'd see the moon appear to completely cover the sun's disk. This is called a total solar eclipse. There are other types of solar eclipses, but we're going to focus on total solar eclipses in this video.
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Solar eclipses The Earth-sun-moon system Middle school Earth and space science Khan Academy.mp3
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So how does a solar eclipse appear from Earth? Well, if you're standing in the center of the moon's shadow, you'd see the moon appear to completely cover the sun's disk. This is called a total solar eclipse. There are other types of solar eclipses, but we're going to focus on total solar eclipses in this video. During a total solar eclipse, the outermost layer of the sun's atmosphere, called its corona, is visible. On a non-solar eclipse day, the corona is invisible to us because we can't see past the bright light from the sun's surface. Remember to never stare or look at the sun with your naked eye, binoculars, or a telescope.
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Solar eclipses The Earth-sun-moon system Middle school Earth and space science Khan Academy.mp3
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There are other types of solar eclipses, but we're going to focus on total solar eclipses in this video. During a total solar eclipse, the outermost layer of the sun's atmosphere, called its corona, is visible. On a non-solar eclipse day, the corona is invisible to us because we can't see past the bright light from the sun's surface. Remember to never stare or look at the sun with your naked eye, binoculars, or a telescope. Looking even for a few seconds can cause severe and permanent eye damage. Just because you can't look directly at the sun doesn't mean that you can't enjoy watching a solar eclipse, though. You can watch the eclipse in action through a pinhole viewer or ultraviolet ray-blocking solar eclipse glasses.
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Solar eclipses The Earth-sun-moon system Middle school Earth and space science Khan Academy.mp3
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Remember to never stare or look at the sun with your naked eye, binoculars, or a telescope. Looking even for a few seconds can cause severe and permanent eye damage. Just because you can't look directly at the sun doesn't mean that you can't enjoy watching a solar eclipse, though. You can watch the eclipse in action through a pinhole viewer or ultraviolet ray-blocking solar eclipse glasses. If you're in the moon's shadow during a total solar eclipse, you might also notice a drop in temperature. On average, the temperature drops about 5.5 degrees Celsius during a total solar eclipse. This is because the moon blocks solar energy from reaching Earth.
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Solar eclipses The Earth-sun-moon system Middle school Earth and space science Khan Academy.mp3
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You can watch the eclipse in action through a pinhole viewer or ultraviolet ray-blocking solar eclipse glasses. If you're in the moon's shadow during a total solar eclipse, you might also notice a drop in temperature. On average, the temperature drops about 5.5 degrees Celsius during a total solar eclipse. This is because the moon blocks solar energy from reaching Earth. Total solar eclipses last only a few minutes. This is because the moon is always revolving around Earth. As the moon keeps on moving, the sun starts to reappear around the moon.
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Solar eclipses The Earth-sun-moon system Middle school Earth and space science Khan Academy.mp3
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This is because the moon blocks solar energy from reaching Earth. Total solar eclipses last only a few minutes. This is because the moon is always revolving around Earth. As the moon keeps on moving, the sun starts to reappear around the moon. And eventually, the sun returns to its usual round and sunny self. But how is it even possible for the moon to appear to block the sun in the first place? The sun and the moon aren't the same size.
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Solar eclipses The Earth-sun-moon system Middle school Earth and space science Khan Academy.mp3
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As the moon keeps on moving, the sun starts to reappear around the moon. And eventually, the sun returns to its usual round and sunny self. But how is it even possible for the moon to appear to block the sun in the first place? The sun and the moon aren't the same size. In fact, the sun is so big that 64.3 million moons could fit inside of it. However, coincidentally, the sun is far enough away that the moon and the sun appear to us Earthlings to be the same size in the sky. So when the moon moves between the sun and Earth during a total solar eclipse, it appears from Earth to cover the disk of the sun almost exactly.
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Solar eclipses The Earth-sun-moon system Middle school Earth and space science Khan Academy.mp3
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The sun and the moon aren't the same size. In fact, the sun is so big that 64.3 million moons could fit inside of it. However, coincidentally, the sun is far enough away that the moon and the sun appear to us Earthlings to be the same size in the sky. So when the moon moves between the sun and Earth during a total solar eclipse, it appears from Earth to cover the disk of the sun almost exactly. Total solar eclipses, like all solar eclipses, happen only during a new moon, which is the only time in the moon's orbit where the moon is located between the sun and Earth. Solar eclipses don't happen every new moon, though. Most of the time, the moon, Earth, and the sun are not lined up.
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Solar eclipses The Earth-sun-moon system Middle school Earth and space science Khan Academy.mp3
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So when the moon moves between the sun and Earth during a total solar eclipse, it appears from Earth to cover the disk of the sun almost exactly. Total solar eclipses, like all solar eclipses, happen only during a new moon, which is the only time in the moon's orbit where the moon is located between the sun and Earth. Solar eclipses don't happen every new moon, though. Most of the time, the moon, Earth, and the sun are not lined up. This is because the plane of the moon's orbit around Earth is at a different angle than the plane of Earth's orbit around the sun. This means that the moon's shadow is usually too high or too low to hit Earth and cause an eclipse. Total solar eclipses won't be visible from Earth in about 600 million years.
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Solar eclipses The Earth-sun-moon system Middle school Earth and space science Khan Academy.mp3
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Most of the time, the moon, Earth, and the sun are not lined up. This is because the plane of the moon's orbit around Earth is at a different angle than the plane of Earth's orbit around the sun. This means that the moon's shadow is usually too high or too low to hit Earth and cause an eclipse. Total solar eclipses won't be visible from Earth in about 600 million years. This is because the moon's orbit is gradually getting bigger, with the moon moving about 4 centimeters away from Earth each year. So, one day, the moon will appear to be too small in the sky to entirely block the sun's light. Total solar eclipses take place on Earth every 18 months or so, but sometimes they're only visible from the middle of an ocean or a desert.
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Solar eclipses The Earth-sun-moon system Middle school Earth and space science Khan Academy.mp3
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Total solar eclipses won't be visible from Earth in about 600 million years. This is because the moon's orbit is gradually getting bigger, with the moon moving about 4 centimeters away from Earth each year. So, one day, the moon will appear to be too small in the sky to entirely block the sun's light. Total solar eclipses take place on Earth every 18 months or so, but sometimes they're only visible from the middle of an ocean or a desert. If you're curious about when the next total solar eclipse will happen near you, look it up online. Scientists are able to calculate when solar eclipses will happen centuries in advance, so you can start planning far ahead. Personally, I'll need to travel if I want to see a total solar eclipse anytime soon, because the next one visible in my hometown is on September 14th, 2099.
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Weather Weather and climate Middle school Earth and space science Khan Academy.mp3
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Unfortunately, we can't control the weather. Weather changes from day to day and from place to place, so scientists have to consider a lot of different factors to understand it. Weather can refer to humidity, or amount of water vapor in the air, temperature, air pressure, wind, and of course, precipitation, like rain or snow. When heat from the sun enters Earth's lower atmosphere, it affects the temperature and humidity of the air. As this National Aeronautics and Space Administration map shows, air masses with certain characteristics tend to form in similar places. There's many ways to classify these air masses, but to keep it simple, these blue blobs represent colder air masses, which tend to be closer to the poles, and the red blobs represent warm air masses, which are often found near the equator, where the sun's energy is strongest. The qualities of an air mass may also depend on where it forms.
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Weather Weather and climate Middle school Earth and space science Khan Academy.mp3
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When heat from the sun enters Earth's lower atmosphere, it affects the temperature and humidity of the air. As this National Aeronautics and Space Administration map shows, air masses with certain characteristics tend to form in similar places. There's many ways to classify these air masses, but to keep it simple, these blue blobs represent colder air masses, which tend to be closer to the poles, and the red blobs represent warm air masses, which are often found near the equator, where the sun's energy is strongest. The qualities of an air mass may also depend on where it forms. Air masses that form over land tend to have less humidity than those that form over an ocean. Air masses can even change characteristics as they move. For example, when a very cold and dry air mass moves over the ocean, it will likely pick up warmth and moisture, transforming it into a slightly warmer and more humid air mass.
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Weather Weather and climate Middle school Earth and space science Khan Academy.mp3
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The qualities of an air mass may also depend on where it forms. Air masses that form over land tend to have less humidity than those that form over an ocean. Air masses can even change characteristics as they move. For example, when a very cold and dry air mass moves over the ocean, it will likely pick up warmth and moisture, transforming it into a slightly warmer and more humid air mass. Weather results from these air masses moving from areas with high air pressure to areas with low air pressure. When this happens, a high or low pressure system can form. A high pressure system has higher pressure at its center than the areas around it, so air moves out from the center.
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Weather Weather and climate Middle school Earth and space science Khan Academy.mp3
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For example, when a very cold and dry air mass moves over the ocean, it will likely pick up warmth and moisture, transforming it into a slightly warmer and more humid air mass. Weather results from these air masses moving from areas with high air pressure to areas with low air pressure. When this happens, a high or low pressure system can form. A high pressure system has higher pressure at its center than the areas around it, so air moves out from the center. High pressure systems typically bring clear skies. A low pressure system has lower pressure at its center than the areas around it, so air is sucked into the center of the system. Low pressure systems typically bring more intense weather.
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Weather Weather and climate Middle school Earth and space science Khan Academy.mp3
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A high pressure system has higher pressure at its center than the areas around it, so air moves out from the center. High pressure systems typically bring clear skies. A low pressure system has lower pressure at its center than the areas around it, so air is sucked into the center of the system. Low pressure systems typically bring more intense weather. Why is this? Well, low pressure systems are often associated with what we call fronts. A front is a boundary that forms when air masses of different temperatures collide near Earth's surface.
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Weather Weather and climate Middle school Earth and space science Khan Academy.mp3
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Low pressure systems typically bring more intense weather. Why is this? Well, low pressure systems are often associated with what we call fronts. A front is a boundary that forms when air masses of different temperatures collide near Earth's surface. Different types of fronts can form depending on how those air masses meet. A warm front occurs when a warm air mass moves in to replace a cold air mass. Since warm air is less dense than cold air, it rises above the colder air, causing a lot of overcast skies, moderate rain or snow, and warm temperatures.
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Weather Weather and climate Middle school Earth and space science Khan Academy.mp3
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A front is a boundary that forms when air masses of different temperatures collide near Earth's surface. Different types of fronts can form depending on how those air masses meet. A warm front occurs when a warm air mass moves in to replace a cold air mass. Since warm air is less dense than cold air, it rises above the colder air, causing a lot of overcast skies, moderate rain or snow, and warm temperatures. A cold front occurs when a cold air mass moves in to replace a warm air mass. The dense cold air pushes the warmer air mass upward, causing larger clouds to billow overhead and bring heavy rain or thunderstorms, strong winds, and cool temperatures with them. Weather is so complex that it can be difficult to predict what will happen next.
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Weather Weather and climate Middle school Earth and space science Khan Academy.mp3
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Since warm air is less dense than cold air, it rises above the colder air, causing a lot of overcast skies, moderate rain or snow, and warm temperatures. A cold front occurs when a cold air mass moves in to replace a warm air mass. The dense cold air pushes the warmer air mass upward, causing larger clouds to billow overhead and bring heavy rain or thunderstorms, strong winds, and cool temperatures with them. Weather is so complex that it can be difficult to predict what will happen next. Luckily, scientists have developed technologies that help model weather patterns, so we can all make more informed choices about what to expect when we go outside. To do that, they set up tools like satellites and Doppler radar to observe atmospheric conditions, such as wind speed, temperature, air pressure, and nearby geographic features. They can input the data they record into computer models, which can use that information to predict future weather.
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Weather Weather and climate Middle school Earth and space science Khan Academy.mp3
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Weather is so complex that it can be difficult to predict what will happen next. Luckily, scientists have developed technologies that help model weather patterns, so we can all make more informed choices about what to expect when we go outside. To do that, they set up tools like satellites and Doppler radar to observe atmospheric conditions, such as wind speed, temperature, air pressure, and nearby geographic features. They can input the data they record into computer models, which can use that information to predict future weather. Have you ever seen a map like this used in a weather forecast? This National Oceanic and Atmospheric Administration map uses data from a Doppler radar station that measured the progression of a severe storm over the northeastern United States. The most intense central parts of the storm are shown in red, with colors down the rainbow representing progressively less intense parts of the storm.
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Weather Weather and climate Middle school Earth and space science Khan Academy.mp3
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They can input the data they record into computer models, which can use that information to predict future weather. Have you ever seen a map like this used in a weather forecast? This National Oceanic and Atmospheric Administration map uses data from a Doppler radar station that measured the progression of a severe storm over the northeastern United States. The most intense central parts of the storm are shown in red, with colors down the rainbow representing progressively less intense parts of the storm. As technology has advanced, these weather reports have gotten more accurate, but with so many factors to consider, weather is still only somewhat predictable. That's why you'll always hear probabilistic weather predictions, providing a percentage that refers to how confident the model is that the prediction will happen. If your local weather forecast says there's a 30% chance of rain, it's less likely, but still possible that you're going to get rain.
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Weather Weather and climate Middle school Earth and space science Khan Academy.mp3
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The most intense central parts of the storm are shown in red, with colors down the rainbow representing progressively less intense parts of the storm. As technology has advanced, these weather reports have gotten more accurate, but with so many factors to consider, weather is still only somewhat predictable. That's why you'll always hear probabilistic weather predictions, providing a percentage that refers to how confident the model is that the prediction will happen. If your local weather forecast says there's a 30% chance of rain, it's less likely, but still possible that you're going to get rain. If it says there's a 90% chance of snow on another day, you're most likely going to get snow. Even if weather predictions aren't perfect, it's amazing that scientists figured out how to understand such a complicated phenomenon. Without modern tools and models, it would be much more difficult for us to plan ahead and be prepared for possible weather hazards.
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Fossils and rock layers The geosphere Middle school Earth and space science Khan Academy.mp3
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Have you ever wanted to travel back in time? Would you go meet your younger self? Would you go and ride a dinosaur? Or would you meticulously create a timeline of the Earth's 4.6 billion year long history based on major geological events? Even though geologists can't go back in time, they've been able to do just that. Their timeline is called the geologic timescale, and it breaks up all of Earth's history into units called eons and eras. And you might be wondering, how can we map out what happened on Earth billions of years ago?
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Fossils and rock layers The geosphere Middle school Earth and space science Khan Academy.mp3
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Or would you meticulously create a timeline of the Earth's 4.6 billion year long history based on major geological events? Even though geologists can't go back in time, they've been able to do just that. Their timeline is called the geologic timescale, and it breaks up all of Earth's history into units called eons and eras. And you might be wondering, how can we map out what happened on Earth billions of years ago? The answer is rocks. Geologists look for clues in sedimentary rock layers, which we call strata. The fossil record, which are the remains of organisms that have been preserved in rock layers, can give us glimpses of creatures that breathed long ago.
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Fossils and rock layers The geosphere Middle school Earth and space science Khan Academy.mp3
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And you might be wondering, how can we map out what happened on Earth billions of years ago? The answer is rocks. Geologists look for clues in sedimentary rock layers, which we call strata. The fossil record, which are the remains of organisms that have been preserved in rock layers, can give us glimpses of creatures that breathed long ago. Also, the difference in mineral composition in different rock layers can tell geologists when a volcano erupted or an asteroid hit. But how do geologists know when one rock layer is older than another? Well, when sedimentary rocks form, they are deposited in layers, one on top of the other.
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Fossils and rock layers The geosphere Middle school Earth and space science Khan Academy.mp3
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The fossil record, which are the remains of organisms that have been preserved in rock layers, can give us glimpses of creatures that breathed long ago. Also, the difference in mineral composition in different rock layers can tell geologists when a volcano erupted or an asteroid hit. But how do geologists know when one rock layer is older than another? Well, when sedimentary rocks form, they are deposited in layers, one on top of the other. So, unless the layers are disturbed or turned over, the layers at the bottom are always older than the layers at the top. Geologists call this rule the law of superposition. So, if a geologist finds a fossil of an ammonite in a layer of rock above a fossil of a Dunkleosteus, the scientist would know that the Dunkleosteus is older than the ammonite.
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Fossils and rock layers The geosphere Middle school Earth and space science Khan Academy.mp3
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Well, when sedimentary rocks form, they are deposited in layers, one on top of the other. So, unless the layers are disturbed or turned over, the layers at the bottom are always older than the layers at the top. Geologists call this rule the law of superposition. So, if a geologist finds a fossil of an ammonite in a layer of rock above a fossil of a Dunkleosteus, the scientist would know that the Dunkleosteus is older than the ammonite. This process is called relative dating, because it helps scientists determine the relative ages of rock layers and fossils. But what does relative ages mean? Let's say that I told you my friend Ava is younger than her sister, but older than her cat.
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Fossils and rock layers The geosphere Middle school Earth and space science Khan Academy.mp3
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So, if a geologist finds a fossil of an ammonite in a layer of rock above a fossil of a Dunkleosteus, the scientist would know that the Dunkleosteus is older than the ammonite. This process is called relative dating, because it helps scientists determine the relative ages of rock layers and fossils. But what does relative ages mean? Let's say that I told you my friend Ava is younger than her sister, but older than her cat. Then I wouldn't be telling you Ava's exact age, but her relative age in relation to her sister and her cat. Let's take a look at another example of relative ages, this time with rocks. Imagine that one day you found a big rock with many layers like this.
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Fossils and rock layers The geosphere Middle school Earth and space science Khan Academy.mp3
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Let's say that I told you my friend Ava is younger than her sister, but older than her cat. Then I wouldn't be telling you Ava's exact age, but her relative age in relation to her sister and her cat. Let's take a look at another example of relative ages, this time with rocks. Imagine that one day you found a big rock with many layers like this. Because the rock is a sedimentary rock, and you remember the law of superposition, you know that the layers at the bottom are older, and you know that the layers up here are younger. But what about this section of rock, right here, that slices through all the other layers? Geologists would call this section an igneous intrusion, since it was formed from molten rock that has cooled.
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Fossils and rock layers The geosphere Middle school Earth and space science Khan Academy.mp3
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Imagine that one day you found a big rock with many layers like this. Because the rock is a sedimentary rock, and you remember the law of superposition, you know that the layers at the bottom are older, and you know that the layers up here are younger. But what about this section of rock, right here, that slices through all the other layers? Geologists would call this section an igneous intrusion, since it was formed from molten rock that has cooled. This particular igneous intrusion isn't clearly on top of or below any of the sedimentary rock layers, so we can't use the law of superposition here. Luckily, geologists have figured out another rule called the law of cross-cutting relationships. This rule says that when two geologic features cross or intersect, the feature that cuts through the other is younger.
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Fossils and rock layers The geosphere Middle school Earth and space science Khan Academy.mp3
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Geologists would call this section an igneous intrusion, since it was formed from molten rock that has cooled. This particular igneous intrusion isn't clearly on top of or below any of the sedimentary rock layers, so we can't use the law of superposition here. Luckily, geologists have figured out another rule called the law of cross-cutting relationships. This rule says that when two geologic features cross or intersect, the feature that cuts through the other is younger. And this makes sense, because you can't break something before it even exists. So since our igneous intrusion cuts through the layers of the sedimentary rock, we know that it must be even younger than the rock layers. And what about this fault or break over here?
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Fossils and rock layers The geosphere Middle school Earth and space science Khan Academy.mp3
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This rule says that when two geologic features cross or intersect, the feature that cuts through the other is younger. And this makes sense, because you can't break something before it even exists. So since our igneous intrusion cuts through the layers of the sedimentary rock, we know that it must be even younger than the rock layers. And what about this fault or break over here? Well, since it slices through all the layers, as well as the igneous rock, the law of cross-cutting relationships tells us that the faults must be the very youngest geologic feature here. Other pieces of evidence, like fossils, can be used for relative dating. For example, suppose a geologist found a rock containing fossils of extinct organisms, and another rock that contained fossils of modern organisms.
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Fossils and rock layers The geosphere Middle school Earth and space science Khan Academy.mp3
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And what about this fault or break over here? Well, since it slices through all the layers, as well as the igneous rock, the law of cross-cutting relationships tells us that the faults must be the very youngest geologic feature here. Other pieces of evidence, like fossils, can be used for relative dating. For example, suppose a geologist found a rock containing fossils of extinct organisms, and another rock that contained fossils of modern organisms. The geologist could then deduce that the rock with the extinct organisms is older. Certain kinds of fossils, called index fossils, are especially helpful when determining the relative age of a rock. Index fossils are fossils that are only found on a specific section of the geologic timescale.
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Fossils and rock layers The geosphere Middle school Earth and space science Khan Academy.mp3
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For example, suppose a geologist found a rock containing fossils of extinct organisms, and another rock that contained fossils of modern organisms. The geologist could then deduce that the rock with the extinct organisms is older. Certain kinds of fossils, called index fossils, are especially helpful when determining the relative age of a rock. Index fossils are fossils that are only found on a specific section of the geologic timescale. I always remember what an index fossil is like this. Just like I use my index finger to point to give directions, index fossils can point to when on the geologic timescale a rock was formed. So if a geologist finds the same index fossil in rock layers in two different rocks, they know that those rock layers must have been formed around the same time period.
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Fossils and rock layers The geosphere Middle school Earth and space science Khan Academy.mp3
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Index fossils are fossils that are only found on a specific section of the geologic timescale. I always remember what an index fossil is like this. Just like I use my index finger to point to give directions, index fossils can point to when on the geologic timescale a rock was formed. So if a geologist finds the same index fossil in rock layers in two different rocks, they know that those rock layers must have been formed around the same time period. But fossils aren't the only thing that can help a geologist figure out a rock's relative age. For example, if a geologist found two rock layers that contained evidence of the same geologic event, like a layer of ash from a major volcanic eruption, the geologist could deduce that those rock layers were made at the same time. Relative dating is how geologists figured out that stegosauruses were extinct long before T. rexes even existed.
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Fossils and rock layers The geosphere Middle school Earth and space science Khan Academy.mp3
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So if a geologist finds the same index fossil in rock layers in two different rocks, they know that those rock layers must have been formed around the same time period. But fossils aren't the only thing that can help a geologist figure out a rock's relative age. For example, if a geologist found two rock layers that contained evidence of the same geologic event, like a layer of ash from a major volcanic eruption, the geologist could deduce that those rock layers were made at the same time. Relative dating is how geologists figured out that stegosauruses were extinct long before T. rexes even existed. It also helped them figure out the first single cellular life, formed about 3.8 billion years ago, and the first multicellular life, formed about 600 million years ago. And they figured out that modern humans didn't even exist until about 200,000 years ago, which is right at the very end of the geologic timescale. In other words, it took a really, really long time for life to evolve into what it looks like today.
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Fossils and rock layers The geosphere Middle school Earth and space science Khan Academy.mp3
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Relative dating is how geologists figured out that stegosauruses were extinct long before T. rexes even existed. It also helped them figure out the first single cellular life, formed about 3.8 billion years ago, and the first multicellular life, formed about 600 million years ago. And they figured out that modern humans didn't even exist until about 200,000 years ago, which is right at the very end of the geologic timescale. In other words, it took a really, really long time for life to evolve into what it looks like today. We humans are very young in the grand scheme of the Earth's history. Now, if you'll excuse me, I'm going to go saddle and ride this brontosaurus. Let's go, gingersnap!
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Galaxies and gravity Earth in space Middle school Earth and space science Khan Academy.mp3
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Today we're going to be talking about galaxies and gravity. We know the Earth is a planet that is in orbit around the Sun. This is called the heliocentric model. And the solar system is an enormous space for us, encompassing every place that humans or our robots have ever been. But it's actually quite small, cosmically speaking. Even our furthest spacecraft, Voyager 1, has only traveled less than 0.1% of the distance to the nearest star. Our solar system is just a small component of a much, much larger set of structures.
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Galaxies and gravity Earth in space Middle school Earth and space science Khan Academy.mp3
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And the solar system is an enormous space for us, encompassing every place that humans or our robots have ever been. But it's actually quite small, cosmically speaking. Even our furthest spacecraft, Voyager 1, has only traveled less than 0.1% of the distance to the nearest star. Our solar system is just a small component of a much, much larger set of structures. The Sun is just one star of hundreds of billions of stars in our galaxy, which is called the Milky Way. And our galaxy is one of several dozen in our galaxy group, which is called the local group. Other galaxies might belong to larger collections that are called galaxy clusters.
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Galaxies and gravity Earth in space Middle school Earth and space science Khan Academy.mp3
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Our solar system is just a small component of a much, much larger set of structures. The Sun is just one star of hundreds of billions of stars in our galaxy, which is called the Milky Way. And our galaxy is one of several dozen in our galaxy group, which is called the local group. Other galaxies might belong to larger collections that are called galaxy clusters. Now, the same thing that holds Earth in its orbit around the Sun is what holds together massive groups and clusters of galaxies. Gravity. This is the same as the gravity that makes a ball fall back to the ground when you toss it up into the air.
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Galaxies and gravity Earth in space Middle school Earth and space science Khan Academy.mp3
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Other galaxies might belong to larger collections that are called galaxy clusters. Now, the same thing that holds Earth in its orbit around the Sun is what holds together massive groups and clusters of galaxies. Gravity. This is the same as the gravity that makes a ball fall back to the ground when you toss it up into the air. Gravity is a force that operates between two objects that have mass and tries to pull them closer together, whether those objects are a ball in the Earth or two galaxies. Now, the strength of this pull depends on two things. First is the mass of both objects.
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Galaxies and gravity Earth in space Middle school Earth and space science Khan Academy.mp3
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This is the same as the gravity that makes a ball fall back to the ground when you toss it up into the air. Gravity is a force that operates between two objects that have mass and tries to pull them closer together, whether those objects are a ball in the Earth or two galaxies. Now, the strength of this pull depends on two things. First is the mass of both objects. So as either object gets more massive, the strength of gravity increases. For example, you have a lot more mass than a ball. So the force of gravity between you and the Earth is pulling harder than the force of gravity between a ball and the Earth, and that's why it's a lot harder to throw you into the air than a ball.
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Galaxies and gravity Earth in space Middle school Earth and space science Khan Academy.mp3
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First is the mass of both objects. So as either object gets more massive, the strength of gravity increases. For example, you have a lot more mass than a ball. So the force of gravity between you and the Earth is pulling harder than the force of gravity between a ball and the Earth, and that's why it's a lot harder to throw you into the air than a ball. Gravity also depends on the distance between the centers of the objects. As the objects get farther apart, the strength of gravity decreases. For example, if there is a ball on top of Mount Everest, the pull of Earth's gravity on it is actually slightly less than the pull of gravity on that same ball at sea level, because the top of the mountain is farther away from the center of the Earth.
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Galaxies and gravity Earth in space Middle school Earth and space science Khan Academy.mp3
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So the force of gravity between you and the Earth is pulling harder than the force of gravity between a ball and the Earth, and that's why it's a lot harder to throw you into the air than a ball. Gravity also depends on the distance between the centers of the objects. As the objects get farther apart, the strength of gravity decreases. For example, if there is a ball on top of Mount Everest, the pull of Earth's gravity on it is actually slightly less than the pull of gravity on that same ball at sea level, because the top of the mountain is farther away from the center of the Earth. Now, this is a very small effect, less than a percent difference, because the difference between sea level and mountaintop is still very small, compared to the distance to the center of the Earth. But that's just on Earth. We know that things in space are really far apart.
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Galaxies and gravity Earth in space Middle school Earth and space science Khan Academy.mp3
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For example, if there is a ball on top of Mount Everest, the pull of Earth's gravity on it is actually slightly less than the pull of gravity on that same ball at sea level, because the top of the mountain is farther away from the center of the Earth. Now, this is a very small effect, less than a percent difference, because the difference between sea level and mountaintop is still very small, compared to the distance to the center of the Earth. But that's just on Earth. We know that things in space are really far apart. The Earth is almost 93 million miles, that's 150 million kilometers, from the Sun. So if gravity is weaker when objects are farther apart, then in order for gravity to be keeping these structures bound together, they must be really massive. And they are.
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Galaxies and gravity Earth in space Middle school Earth and space science Khan Academy.mp3
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We know that things in space are really far apart. The Earth is almost 93 million miles, that's 150 million kilometers, from the Sun. So if gravity is weaker when objects are farther apart, then in order for gravity to be keeping these structures bound together, they must be really massive. And they are. The Sun has a mass of about 2 nonillion kilograms, that's 10 to the power of 10. And the Milky Way galaxy, which is about a quintillion kilometers in diameter, has a mass of about 1.5 trillion times that of the Sun. So thanks to their high masses, objects like our Milky Way galaxy and our nearest neighbor, the Andromeda galaxy, which are the two largest galaxies in the local group, can be pulled together by gravity even across millions of light-years.
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Galaxies and gravity Earth in space Middle school Earth and space science Khan Academy.mp3
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And they are. The Sun has a mass of about 2 nonillion kilograms, that's 10 to the power of 10. And the Milky Way galaxy, which is about a quintillion kilometers in diameter, has a mass of about 1.5 trillion times that of the Sun. So thanks to their high masses, objects like our Milky Way galaxy and our nearest neighbor, the Andromeda galaxy, which are the two largest galaxies in the local group, can be pulled together by gravity even across millions of light-years. In fact, gravity between our two galaxies is the same as gravity between the two galaxies. Gravity between the two galaxies is so strong that it's actually pulling the Milky Way and Andromeda galaxies together into an eventual collision. But eventually in this case means almost 5 billion years from now, so no need to worry.
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Galaxies and gravity Earth in space Middle school Earth and space science Khan Academy.mp3
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So thanks to their high masses, objects like our Milky Way galaxy and our nearest neighbor, the Andromeda galaxy, which are the two largest galaxies in the local group, can be pulled together by gravity even across millions of light-years. In fact, gravity between our two galaxies is the same as gravity between the two galaxies. Gravity between the two galaxies is so strong that it's actually pulling the Milky Way and Andromeda galaxies together into an eventual collision. But eventually in this case means almost 5 billion years from now, so no need to worry. To review, in this video we covered how the Earth is part of the solar system, and our solar system is part of a galaxy, and our galaxy is part of a group. Galaxy groups, galaxies, and solar systems are all bound together by the same force, that of gravity. Gravity is a force which tries to pull objects with mass closer together, and it is weaker for less massive objects or for objects that are farther apart.
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Phases of the moon Middle school Earth and space science Khan Academy.mp3
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How would you keep track of how much time had passed? Well, you could look to the moon. Humans have used the moon to keep track of time for thousands of years. It isn't a coincidence that the word moon is related to the word month in Old English. The moon is Earth's only natural satellite. A natural satellite is a naturally occurring body that orbits a planet. Some of the planets in our solar system have more than 50 satellites or moons, but the Earth just has one.
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Phases of the moon Middle school Earth and space science Khan Academy.mp3
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It isn't a coincidence that the word moon is related to the word month in Old English. The moon is Earth's only natural satellite. A natural satellite is a naturally occurring body that orbits a planet. Some of the planets in our solar system have more than 50 satellites or moons, but the Earth just has one. The moon does not generate its own light. We can see the moon from the Earth because it is partially lit by the light from the sun. In fact, moonlight is just sunlight reflected from the moon onto Earth.
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Phases of the moon Middle school Earth and space science Khan Academy.mp3
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Some of the planets in our solar system have more than 50 satellites or moons, but the Earth just has one. The moon does not generate its own light. We can see the moon from the Earth because it is partially lit by the light from the sun. In fact, moonlight is just sunlight reflected from the moon onto Earth. The moon takes about 27 days to make a full orbit around Earth, and as it does, the lit part of the moon appears to change shape to us here on Earth. These shapes are called the moon phases, or lunar phases. So why do we see different lunar phases?
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Phases of the moon Middle school Earth and space science Khan Academy.mp3
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In fact, moonlight is just sunlight reflected from the moon onto Earth. The moon takes about 27 days to make a full orbit around Earth, and as it does, the lit part of the moon appears to change shape to us here on Earth. These shapes are called the moon phases, or lunar phases. So why do we see different lunar phases? Well, the 27 days it takes the moon to orbit the Earth once is the same amount of time it takes the moon to rotate on its axis once. This means that the same side of the moon is always facing us. At the same time, the sun always lights up half of the sphere of the moon.
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Phases of the moon Middle school Earth and space science Khan Academy.mp3
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So why do we see different lunar phases? Well, the 27 days it takes the moon to orbit the Earth once is the same amount of time it takes the moon to rotate on its axis once. This means that the same side of the moon is always facing us. At the same time, the sun always lights up half of the sphere of the moon. However, the side of the moon that's facing us isn't always the same part of the moon that the sun lights up. This causes the moon to appear to have different shapes or phases in the sky depending on the time of the month. Let's take a closer look at the moon's phases.
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Phases of the moon Middle school Earth and space science Khan Academy.mp3
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At the same time, the sun always lights up half of the sphere of the moon. However, the side of the moon that's facing us isn't always the same part of the moon that the sun lights up. This causes the moon to appear to have different shapes or phases in the sky depending on the time of the month. Let's take a closer look at the moon's phases. First, we have the new moon, which happens when the moon is closest to the sun in its orbit. During this phase, the lit side of the moon is completely facing away from Earth, so it looks like the moon has disappeared. In a day or two, we'd be able to see a little sliver of moon in the sky.
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Phases of the moon Middle school Earth and space science Khan Academy.mp3
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Let's take a closer look at the moon's phases. First, we have the new moon, which happens when the moon is closest to the sun in its orbit. During this phase, the lit side of the moon is completely facing away from Earth, so it looks like the moon has disappeared. In a day or two, we'd be able to see a little sliver of moon in the sky. Over the next few days, the crescent moon will appear to get bigger and bigger. When the moon appears to get bigger from one day to the next, we say that the moon is waxing. So this phase of the moon is called the waxing crescent moon.
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Phases of the moon Middle school Earth and space science Khan Academy.mp3
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In a day or two, we'd be able to see a little sliver of moon in the sky. Over the next few days, the crescent moon will appear to get bigger and bigger. When the moon appears to get bigger from one day to the next, we say that the moon is waxing. So this phase of the moon is called the waxing crescent moon. You might notice that sometimes we can still see the rest of the moon in the shadow. This is because the Earth reflects sunlight onto the moon, just like the moon reflects light onto Earth. Eventually, the moon appears to change shape so much that it isn't a crescent anymore, but a half circle in the sky.
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Phases of the moon Middle school Earth and space science Khan Academy.mp3
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So this phase of the moon is called the waxing crescent moon. You might notice that sometimes we can still see the rest of the moon in the shadow. This is because the Earth reflects sunlight onto the moon, just like the moon reflects light onto Earth. Eventually, the moon appears to change shape so much that it isn't a crescent anymore, but a half circle in the sky. This is called a first quarter moon. There are two ways to think about why this phase is called a quarter moon, even though it looks like the moon is half illuminated. The moon is a sphere, so we can only ever see half of the moon from Earth.
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Phases of the moon Middle school Earth and space science Khan Academy.mp3
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Eventually, the moon appears to change shape so much that it isn't a crescent anymore, but a half circle in the sky. This is called a first quarter moon. There are two ways to think about why this phase is called a quarter moon, even though it looks like the moon is half illuminated. The moon is a sphere, so we can only ever see half of the moon from Earth. Like a quarter moon, the moon forms a right angle with Earth and the sun. This means that we see a half-lit portion of the half of the moon that's always facing us. It's half of a half, so it's a quarter.
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Phases of the moon Middle school Earth and space science Khan Academy.mp3
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The moon is a sphere, so we can only ever see half of the moon from Earth. Like a quarter moon, the moon forms a right angle with Earth and the sun. This means that we see a half-lit portion of the half of the moon that's always facing us. It's half of a half, so it's a quarter. Also, a first quarter moon occurs when the moon is a quarter of the way through its new cycle. Next, there's the waxing gibbous moon. The word gibbous comes from the Latin word meaning humpback.
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Phases of the moon Middle school Earth and space science Khan Academy.mp3
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It's half of a half, so it's a quarter. Also, a first quarter moon occurs when the moon is a quarter of the way through its new cycle. Next, there's the waxing gibbous moon. The word gibbous comes from the Latin word meaning humpback. Once the moon is farthest from the sun in its orbit, the full sunlit side of the moon faces Earth. We call this phase the full moon. But, the moon is not done yet.
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Phases of the moon Middle school Earth and space science Khan Academy.mp3
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The word gibbous comes from the Latin word meaning humpback. Once the moon is farthest from the sun in its orbit, the full sunlit side of the moon faces Earth. We call this phase the full moon. But, the moon is not done yet. It's only finished half of the cycle. Next we have the same phases, but in reverse. As the moon appears to get smaller and smaller, we say that it's waning.
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Phases of the moon Middle school Earth and space science Khan Academy.mp3
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But, the moon is not done yet. It's only finished half of the cycle. Next we have the same phases, but in reverse. As the moon appears to get smaller and smaller, we say that it's waning. The full moon appears to shrink, and then we see a waning gibbous. Then there's the third quarter moon. This happens when the moon is three quarters of the way done with its cycle, and the moon forms another right angle with the sun and the Earth.
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Phases of the moon Middle school Earth and space science Khan Academy.mp3
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As the moon appears to get smaller and smaller, we say that it's waning. The full moon appears to shrink, and then we see a waning gibbous. Then there's the third quarter moon. This happens when the moon is three quarters of the way done with its cycle, and the moon forms another right angle with the sun and the Earth. The moon appears to shrink even more, and it becomes a waning crescent. And the cycle starts anew, with another new moon. Even though the moon completes an orbit every 27 days, the lunar phases actually repeat about every 29.5 days.
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Phases of the moon Middle school Earth and space science Khan Academy.mp3
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This happens when the moon is three quarters of the way done with its cycle, and the moon forms another right angle with the sun and the Earth. The moon appears to shrink even more, and it becomes a waning crescent. And the cycle starts anew, with another new moon. Even though the moon completes an orbit every 27 days, the lunar phases actually repeat about every 29.5 days. This is because the Earth is revolving around the sun while the moon completes its orbit, so the moon has to travel a little extra to catch up. The moon isn't just something beautiful to look at in the sky, or just an easy way to keep track of time. The moon's gravity controls the tides, which are the rise and fall of water in oceans, lakes, and rivers.
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Phases of the moon Middle school Earth and space science Khan Academy.mp3
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Even though the moon completes an orbit every 27 days, the lunar phases actually repeat about every 29.5 days. This is because the Earth is revolving around the sun while the moon completes its orbit, so the moon has to travel a little extra to catch up. The moon isn't just something beautiful to look at in the sky, or just an easy way to keep track of time. The moon's gravity controls the tides, which are the rise and fall of water in oceans, lakes, and rivers. Tides allow for unique ecosystems, like tide pools, to exist, and we can use the tides to create electricity and tidal power plants. The moon also helps keep Earth's axis stable. Without it, our planet would wobble more dramatically on its axis over long periods of time, which would change up our weather and our seasons.
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