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Video transcript

- [Instructor] In a previous video, we asked ourselves a very important question. As the moon has its 28-day cycle around the Earth, we talked about how a new moon is when the moon is between the earth and the sun, and so from the Earth's point of view, or from the point of view of someone standing on Earth, you're seeing the side of the moon that is not lit up. But an obvious question is why doesn't that block out the sun every time we are in a new moon position? After all, the sun would be out here, 93 million miles away, and so wouldn't the moon block out the sun in that scenario? And we also talked about when the moon is on the other side of the Earth, when we would typically see a full moon, the Earth is between the moon and the sun. Why doesn't the Earth block the light from the sun that's being reflected on the moon? So the big question is, why don't we see a solar eclipse every new moon and why don't we see a lunar eclipse every full moon? If we wanna ask the same question, looking at the scale of the Earth and the moon, we can see it right over here and so this would be the new moon position where the moon is between the Earth and the sun. The key explanation for why we do not have a solar eclipse every time we are in the new moon position is that the rotation of the moon around the Earth is not in the exact same plane as the rotation of the Earth around the sun. It actually turns out that the plane of the moon's rotation is at a five-degree angle with the plane of the Earth and the sun. If this is the sun here, and this is not drawn to scale by any means, this is the Earth, and so the Earth is in its orbit, something like that, the moon's orbit does not sit exactly in this plane, it does not sit exactly in this plane, the moon's orbit is at a five-degree angle, is at a five-degree angle to this plane, so depending what time of year you are at and where the moon is in its cycle, the moon will often sit above or below the plane of the Earth's orbit, so for example, from the vantage point of the Earth, the moon will vary up to five degrees above the plane of Earth's rotation around the sun, often known as the ecliptic, and five degrees below that. And so you can see here, the shadow of the moon will only fall on the Earth when the moon is crossing through the plane of the rotation of Earth around the sun. Many times, the moon might be here or here or here and in those times, the shadow will not hit the Earth. Now what you do see here, depicted in this picture, would be a solar eclipse. Now there's two things that I've drawn here and this is important for understanding a solar eclipse, what you see in those yellow lines, that's the umbra of the moon, so if you are in, if you're at that point right over here in this picture, then the moon will completely block out the sun and it turns out depending on the solar eclipse, where that umbra is hitting the Earth, it might only be a few hundred miles where the moon completely blocks out the sun. What you see in this dark blue color, that is the penumbra, that is the outer shadow and if you're in one of those areas, you will see the moon partially block out the sun. Now this is the exact same reason, the fact that the plane of its rotation is at a five-degree angle with the plane of rotation of the Earth around the sun, that's also the reason why we don't see a lunar eclipse every 28 days. Here we see different scenarios where the moon is on the other side of Earth from the sun, when the moon is what we would call a full moon position, and as we can see, it can vary five degrees above the plane of the Earth and the sun or five degrees below that and you're only going to get a scenario of a lunar eclipse when the moon happens to fall in the shadow of the Earth and here once again, between the yellow lines, you see the umbra, and between the blue lines, you see the penumbra, which would be kind of a partial shadow. So next time you look up at the moon, hopefully you are armed with a lot more information to think about how it's oriented with respect to the Earth. In fact, now when you look at the moon at night, you can usually look at the moon and tell which direction the sun is in and based on that, you can even think about what time of day it is and you could figure out east, west, north, and south. And we've talked a lot about the sun, in fact, this entire video and the one before it is all about light from the sun. And just to finish off with a little bit of awe, let's just appreciate how large and how far the sun is. As we mentioned before, this is a scaled representation of the Earth and the moon, the moon being roughly 240,000 miles away from Earth. If we were to try to model the sun here, it is 400 times as far, depending on your screen size, it would be a football field to the right of your screen and you would have something roughly the size of a beach ball to represent the sun. If you wanted to just compare the sizes of the sun to the Earth and the moon, here you go. The sun is huge.