 Greetings and welcome to Astronomy at Hack. Over the winter break here, I have been recording a couple of different videos on some special topics in astronomy that may relate to both planetary, which is our Astronomy 103 course at Hack, and stellar astronomy, which is our Astronomy 104 class. And I'll be doing some topics. I'll try to post about one a week that will relate to either one or the other or both of these specific topics. The topic for today that I've chosen to do is lunar and solar eclipses. And lunar and solar eclipses are fascinating things. We just had a very nice lunar eclipse this past month. We had a nice one that was visible not from the eastern part of the United States, where we happened to be located. But if you're further west, you were able to see it. Or off into the Pacific and into Asia, it was very nicely visible. Now, what is an eclipse? An eclipse is actually a blocking, is actually the sun or moon being blocked by something. So something coming in between, something that is blocking out. So an example of a solar eclipse as to why they occur, a solar eclipse is the moon passes between the earth and the sun. So when the moon passes between the earth and the sun, it blocks out the sunlight. And they occur when the alignments are perfect. A lunar eclipse would occur when the earth passes between the sun and the moon. So we could either have the moon passing between the earth and the sun, the sun or the earth passing between the sun and the moon. The other possibility would be the sun passing between the earth and the moon. And if that ever happens, we are in very great trouble because it would get awful hot. So that's not an eclipse, but something that really could not occur. How often do they occur? Usually several times a year. And it will seem like much less than that in many cases, especially for solar eclipses, but there are several solar eclipses each year. There have been several this year that have occurred. So we've actually been able to see a number of solar eclipses, but they've all been very small ones. The problem with solar eclipses, we'll look at them in more detail, but they're only visible from a limited part of the earth's surface. So if we want to look in more detail, let's first look at lunar eclipses. What happens in a lunar eclipse? And in a lunar eclipse, as I told you, the earth has ended up in the middle. So the earth has gotten between, and we have the earth here, and you have the moon, which orbits around the earth here. And then way off in the distance over here is the sun. And the sun is casting its rays very far from the far distance, way off to the right-hand side of the paper here. And what happens is that the earth blocks out some of that sunlight. Wherever it is, it's going to block out some of that sunlight. So it casts a shadow out into space. So some of the sunlight goes here, and it forms a nice little cone in space and blocks out that light. So anything passing into this area of shadow, which we call the umbra of the earth's shadow, is blocked out from sunlight and becomes invisible. So as we watch a lunar eclipse, the moon will slowly disappear, like a bite is being taken out of it, that progressively gets larger and larger and larger until it disappears. Now the moon actually, at the point of eclipse, as it becomes completely into the earth's shadow, the moon actually turns red. It does not actually disappear completely. It looks red. So it's still there and still visible, but it's this very deep, blood-red color. And if you've ever had the chance to see a lunar eclipse, you've probably seen this phenomenon. Normally the moon is a bright, whitish-yellow almost color in the sky. But during a lunar eclipse, a total lunar eclipse, it actually looks red. This happened, this is because of the earth's atmosphere. The earth is not just a solid body like this, casting a shadow, but there's a thin atmosphere around the earth. And as the sunlight passes through this atmosphere, some of the sunlight gets bent into the shadow. So a little bit of the sunlight gets bent, and this would be red light that gets bent through the earth's shadow or falls on the moon and causing it to look red during a lunar eclipse. Now, the blue light gets scattered all out. The red light is the one that actually can make it through and makes it best through. The blue light tends to get scattered and that makes this light look redder, similar in the way that the sun sets look red. The sun isn't changing color as it sets low in the sky. It's just that all of the blue and the green light is actually getting scattered out into the atmosphere of the earth and coming down from all directions, giving us a very blue sky. Well, the same similar effect is happening here. The red light is what's making it through the earth's atmosphere. The blue and green light are not. So that's an lunar eclipse. And again, it's just the earth passing in between the sun way off to the distance on the right and the moon, the earth passes right in between. It's shadow blocks out the moon. But as I mentioned, the moon will actually look red in this case. It won't actually disappear. Now, we see something a little bit different when we talk about a solar eclipse, although we get the same, almost the same type of alignment. Now we have the moon passing in between the earth and the sun. So here's our earth. And there's the moon orbiting it, nothing to scale. And again, the sun is way off here in the distance. Shining its light. And now it's the same effect. The moon is going to cast its shadow on the surface of the earth. So the moon casts its shadow on the surface of the earth. The moon being a lot smaller casts a much smaller shadow. So what we have is, at this point, this is what we call the path of totality. That is where you happen to be. If you happen to be there, you will get to see a total eclipse. Now, if you're a little further away from that, you're not completely out of luck. Because we have, as we mentioned the last time, we have, this is the umbra of the moon's shadow. What I didn't mention for the earth, because it really isn't noticeable in terms of those eclipse, in terms of a lunar eclipse. But there's also another part of the shadow. And that's what we call the penumbra. So what we have is not only this shadow, but we also have another part of the shadow that will cause another. So what we can see is that there's another part of the shadow. And we draw it in here. Do this as, let's do a different color. We'll draw it here in green to make it stand out a little bit. But this part in the green is called the penumbra. And this is the region of partial eclipse. So we'll be able to see a partial eclipse of the sun. Whereas in the umbra, that is the region of total eclipse. And we will see the sun completely blocked out. It will become nighttime. And it will be night. It'll be dark. The stars will come out. Crickets will chirp if I can speak properly today. And you will be able to see. And it will be just like nighttime for a few minutes, if you happen to be in the path of totality. But as you see, it's a very small region, only a few miles across. It's not a whole area that will see it. So even just a certain area, you can be only a few miles away from it and not see a total eclipse. And you have to block out almost all of the sunlight in order to see any any eclipse. In order to see it. In order for things to actually get dark, for things to change. What you'd normally see if you were to look at it, you would see a chunk of the sun taken out. So you might see something like, here's your sun, and you might see a big chunk of the sun sort of taken out. But it will not get that much darker and not enough that you'd actually notice it. You will see this if you happen to look at the sun, but unless you're actually into this path of totality, unless you happen to be in exactly the right spot, you won't be able to see the eclipse. Now eclipses do not occur all every month. They will occur, for example, a solar eclipse will occur only at the time of new moon. So the only possible time that a solar eclipse can occur is when the moon is new. That'll happen about 12 times a year. So there's about 12 chances for a solar eclipse. But it's even less than that because the earth, the moon, and the sun have to be lined up perfectly. And they are not always done. They do not always line up perfectly. For example, you might have, and we'll draw this again, not to scale, but you may have the earth here. And you might have the moon up here and the sun way off over in the distance here. But you may have it so that the moon's shadow, you know, is cast up here and never actually reaches the earth's surface. This is the possibility that happens most months. It's only when the moon actually happens to be exactly lined up so we can take this moon and move it and it happens to line up so that it casts the shadow again, somewhere on the earth. This happens most of the months. This is most of the time. But during the seasons, what we call the eclipse season, is the time when the earth, the moon, and the sun are all lined up. If we happen to get a new moon, sorry, that was supposed to be eclipse season, eclipse seasons, if we happen to get a new moon, then we get a solar eclipse visible somewhere on the earth. If we get a full moon, then we would have a lunar eclipse somewhere on the earth. That doesn't mean it would be visible from where you happen to be. And you can find there are very good charts that show you when the next good eclipses will be for those of us living in the eastern part of the United States where I happen to be. There's not a lot of good eclipses in a couple years. We've got a couple about two to three years to wait before we get a really nice good eclipse that we can see. Other part, but there will be eclipses. Eclipses do occur each year. So every year there is an eclipse someplace. Sometimes they occur in the middle of the ocean. Sometimes they'll occur just at the very edges of the earth, down in Antarctica or up in the Arctic. So they occur in various places just depending on the exact positioning of the sun, the earth, and the moon. So I hope this has been a little helpful in sort of reviewing eclipses and giving you a little bit better idea of how eclipses work and what the positioning is and how they go about. So sort of use it as a supplement to what we've covered in class and what you may cover in your textbook. So until next time, have a great day, everybody. And I will see you in class.