2.Topic 2-Video 2

Now, we're going to talk about the rotation and revolution of the Earth, and, some related concepts. We've already said that the Earth rotates on its axis. So, here's the Earth, and, this is the Equator. And, so the axis of the Earth is this imaginary line that runs through the center. And, the Earth spins about on its axis, like that. And, the rotation of the Earth causes day and night. It causes the Sun to rise and set. The Sun isn't going around the Earth. The Sun's really way back there, way back in the distance. And, as the earth rotates, the Sun appears to rise and set from our point of view. The same thing with the moon... The moon is here orbiting the Earth. But, as the Earth spins the moon appears to rise and set. And, the spinning of the Earth is a more significant factor, in the apparent motion of the moon, because, the moon goes completely around the Earth, once in a month. But, it goes... It rises and sets every day as the Earth rotates, or just about every day. And, the rotation of the Earth also causes the stars to rise and set. And, here's a picture taken with a time exposure, which means the shutter of the camera was left open. It looks like, for several minutes, may be half an hour or so. And, you can see on the road here. These are some buildings. And, along the road here, you can see these lights from headlights as cars drove by. And, the shutter was long enough for this entire drive, and, probably a lot longer, because, instead of seeing points of light in the sky, we see these arcs. As the stars moved across the sky, due to the rotation of the Earth. And, you can actually tell by the length of the arc, how long the shutter of the camera was open. And, you see these are actually in circular paths; all of these arcs. And, if you went down in this direction, you would find the center of the circle. It's out of view in this picture. But, the center of the circle would be the North Star. And, that's because the North Star is... is pointed to by the axis of the Earth. And, so if you're standing here, for example, looking out that way, you would see everything as if you were rotating about the North Star. All of the star trails in the sky would form circles. Another effect of the rotation of the Earth and the revolution of the Earth, along with the tilt of the Earth's axis is the Seasons. And, you can draw a diagram in your notes like I'm going to draw here. The angle at which the Sun's rays strike the Earth's surface, determines how much the surface of the Earth heats up. So, let's say, here's the Earth's surface, here. And... And, here's some incoming rays of sunlight. I'll draw them like this, and I'll try to draw them parallel, because, incoming rays are essentially parallel. Okay. So, these rays come in like this. And, this diagram would be the case in Winter. In the Winter, the Sun is not as high up in the sky. And, so the Sun's rays are coming in at a more shallow angle. And, if you imagine... well, think of a cross-sectional area of this amount of light here. So, the that's coming through this... you can think of that as a flat lens, or just a... imaginary section of space. The light coming through, that much area, is spread out over the Earth, over a larger area, because of that... because of the angle. In the Summer... Let's draw a diagram over here that represents Summer. In Summer, the Sun is higher in the sky. And, let's imagine it straight over head. So, these rays of Sunlight are coming straight down. And, now you can see that the light coming through, say this circle right here, a circle seen from the edge, is spread out over a smaller area. It's not... It's not spread out as much. So, the Sunlight is more concentrated. There's more energy per square meter striking the Earth's surface. So, the surface of the Earth heats up more in the Summer, because, the direct striking of the Sun's rays causes more energy per amount of area. Now, whether the light is striking at a shallow angle like that or a steep angle like that, is determined by the tilt of the Earth, and, where you are on the Earth, and the position of the Earth in its orbit around the Sun. So, let's draw the diagram again, and there's another place in, in your notes, you can draw this. This is the diagram, we're going to see a lot, with the Sun in the center, and the Earth going around it. And, so let's put the Earth... we'll draw the Earth in 4 different positions. Let's picture the orbit of the Earth is going around the Sun like that. So, this is a circle seen in a perspective view, at the Earth's orbit being approximately circular. So, here's the Earth over here, of course, not drawn to scale. And, let's draw it tilted a little bit. The axis... The axis of the Earth is tilted. So, here's our North pole and our South pole, like that. And, then let's draw another picture over here of the Earth. It's tilted like that. I'm going to redraw that one. Hold on just a second. Okay. Okay. And, then let's draw one... Let's draw one in back here. And, I'll make the one in back here a little bit smaller. And, the one in front here, a little big bigger, just to mentally aid in the perspective view. And, we'll draw some dotted lines here representing the orbit. Okay, so, the Earth moves around the Sun, like that. Okay. In the Northern Hemisphere, you can see... look over here at this picture, the rays of the Sunlight are coming in and they are striking directly on to the Northern Hemisphere. And, down here in the Southern Hemisphere, these rays of Sunlight coming in are striking at a shallow angle. Because of the tilt of the Earth's axis, the angle at which the Sunlight strikes the Earth is different in the Northern Hemisphere, and the Southern Hemisphere. So, you see North of the Equator, it's going to be Summer. The direct striking of the rays causes more heat to be transmitted to the surface. So, it's the Summer, in the Northern Hemisphere, right there. And, at the same time that it's Summer at North, it's Winter down South. Then 6 months later, the Earth is over here. And, you can see the incoming rays of Sunlight are striking the area below the Equator, rather directly. And, they are striking the area above the Equator at a shallow angle. So, in this diagram, or in this part of the diagram, the Earth is seen where it's a Winter in the Northern Hemisphere, and Summer in the Southern Hemisphere. And, over here, at this point, this would be in between. This would be Fall in the Northern Hemisphere. And, this would be Spring in the Northern Hemisphere. Now, if you're near the Earth's pole. Say, up here near the North Pole, or down near the South Pole, the light is striking at a shallow angle all the time. The Earth never changes its tilt or not much... And, it never... the North Pole never points toward the Sun. If it did, it would be very warm at the North Pole. And, the same with the South Pole. The poles are always cold. And, that's why there's snow, at those places, because, even in the Summer... Let's say it's right here. Summer in the Southern Hemisphere is still a shallow angle at which the Sunlight is striking the South Pole. So, the poles are always cold. And, that's why it has to do with the angle of the Sunlight striking the Earth. And, that's why we have seasons, and, why the seasons are opposite in the Northern and Southern Hemispheres.