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Intro to Moon phases

The Moon and the sun look roughly the same size in the sky because although the sun’s diameter is ~400 times greater than the Moon’s, the sun is ~400 times farther away from the Earth as the Moon is! The Moon goes through phases because as it rotates around the Earth, different parts of the Moon are made visible to us from the sun’s light. Created by Sal Khan.

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Video transcript

- [Narrator] When you look at the moon in the night sky, it might seem reasonably close. But it's actually incredibly far away. Right over here is a scaled picture of the distance between the earth and the moon. Earth has a diameter of approximately 8,000 miles. While the moon has a diameter of approximately 2,200 miles. So, a little bit more than one fourth the diameter of earth. Now the distance between the two is 239,000 miles. Which you can imagine is incredibly far. Now one thing that is very interesting, this is why the moon looks like it's the same size as the sun in the sky, even though it is one four hundredth of the diameter, is the ratio between the distance to the moon and the diameter of the moon is roughly the same as the ratio of the distance of the sun to the diameter of the sun. So, for the moon, that ratio, the distance to the moon, 239,000 miles over the radius of the moon, 2,200. It's approximately the same as the ratio between the distance from earth to the sun which would be 93 million miles over 865,000 miles in diameter. So this number is roughly 400 times larger than that number. And this number is roughly 400 times larger than that number. But what's neat is, and this depends on where earth is in its orbit, but this is approximately equal to 108. This gives me goosebumps whenever I think about it because it didn't have to be this way. That the moon and the sun, even though they have these vastly different diameters, that they look roughly the same size in the sky because the ratio of the distances is comparable to the ratios of their diameters. And this is why the number 108 is actually an auspicious number in some cultures, especially in Hinduism. Now, when you're discussing the moon and especially the moon as viewed from earth, one of the obvious questions is why does the moon go through phases? Why do we see different portions of the moon lit up from day to day? This right over here is a diagram from NASA's website. And what you see here, clearly this is not at scale. This picture over here, this inner picture, the size of the moon and the earth is roughly at scale, but clearly the distances between them are not. You don't see that 239,000 miles between them. In this inner circle, what you see is that the moon and the earth is always lit up from the right. So, it's assuming that the sun is 93 million miles in that direction. And it is lighting up both the earth and the moon from the right. Now, as the moon rotates around the earth in its 28 day cycle, and if you're wondering, gee a 28 day cycle seems awfully close to a month, that is not a coincidence. The notion of a month comes from the cycles of the moon. In fact, even the word month and the word moon have the exact same root in old English. And in proto Germanic. They are essentially the same word. But what you see is the moon goes in this 28 day cycle, when the moon is roughly between the earth and the sun, the lit up half of the moon is away from what we can see here on earth. And so, we see the non lit up side which would be a new moon. Now, as the moon goes, this is viewing from above the earth, this would be the North Pole right over here, as it goes counter clockwise as viewed from above the earth, we start to be able to see a little bit of that half of the moon that gets lit up. So, when the moon is in this position, we see, us from earth, from this vantage point, we're able to see a little bit of the lit up side. When the moon is over here, we're able to see half the lit up side and half the non lit up side. And that's called a first quarter moon. And that keeps on going. When we're halfway through our cycle here, at a full moon, the earth is between the moon and the sun and so from our vantage point, we are able to see the entire lit up side. And that's why it is a full moon. And then we keep going all the way until we get back to a new moon. Now, one question that might be bothering you, it bothered me for many years, is as soon as I understood this, the cycle of the moon, how the moon has this 28 day cycle as earth rotates around the sun, I always wondered well the new moon, it looks like the moon is between the earth and the sun, why doesn't it block out the sun every time we have a new moon? Why don't we have a solar eclipse every 28 days? Similarly, for the full moons, when we're in this position, it looks like the earth is between the moon and the sun. Why doesn't earth's shadow block out the sun so that we have a lunar eclipse instead of a full moon and we would have one of those every 28 days? Why don't we see that? Think about it yourself. What's a plausible explanation? In the next video, we'll actually talk about why eclipses, both solar eclipses when the moon blocks out the sun, and lunar eclipses when the earth's shadow falls on the moon, why they're not happening every 28 days.