 Greetings and welcome to the introduction to astronomy. In this video we are going to talk about eclipses, and there are two types of eclipses that we want to look at, and that will be solar eclipses when the sun is blocked out by the moon, and lunar eclipses when the moon moves into the earth's shadow and is no longer illuminated by the sun. So both of these are very interesting effects that will occur just because of the geometry of the earth and moon and sun system. So let's get started here. First of all we want to look at why we get eclipses in the first place. So the big question here would be the why. Why are there eclipses that occur? Why do we ever see these? And eclipses simply occur when one object passes in front of another. We see eclipses in other cases, in the moons of Jupiter can eclipse each other. We see eclipses in other stars where a planet may pass in front of the star, blocking some of its light and causing it to eclipse. But in the case of here that we're looking at, the moon and the sun are both the same apparent size as seen from the earth. That means they are about half a degree in size. This makes the eclipses more interesting than they otherwise would be. If the moon were much smaller in past in front of the sun, yes it would block out some of the light, but it would be nowhere near as amazing as when it just barely blocks out all of the light from the sun. So because of this coincidence, now we know that the moon and the sun, this is the apparent size, not their actual size. The moon is actually a lot smaller, but is much closer. The sun is much larger, but further away, and that makes them appear to be the same size in the sky even though they are actually quite different. Now the two types of eclipses that we get are solar eclipses, which are when the moon passes in front of the sun and a lunar eclipse when the moon passes into the earth's shadow. So we see here an image of a solar eclipse, so this is an example of the solar eclipse here. When the moon, we can't see it, it's the dark portion right here, but this is the moon, the dark part, and we can see just a little bit of the atmosphere of the sun around it. So the sun itself, the surface of the sun is not visible here, but this is the sun's atmosphere that is visible when the rest of the sun is blocked out. So this would be an example of a solar eclipse. Now, why do we get eclipses, and what is the geometry of an eclipse? Let's take a look at those. And what we have here is, what is the geometry? Well, there's a couple of terms we want to look at. How are they set up? But the actual geometry of the eclipse means simply how things are positioned. So in this case, this would be an example of a lunar eclipse. In that case, the earth is in between the sun and the moon. The earth casts a shadow that then blocks out, keeps the sun's light from reaching the moon, and therefore makes it appear dimmer in the sky, and in fact blocks out all of that light. Now, when we look at the shadows, there are two types. We have the umbra, which is the darkest part of the shadow, and that is where sunlight is completely blocked. So if you were standing anywhere in this shadow, you would not be able to see the sun. The sun would be invisible, it would be blocked out by the earth. If you're in the penumbra, which is the region of partial shadow, then you would see only part of the sun. So if you're in this lighter shaded region, the sunlight is partially blocked. So if you were sitting there in the penumbra, you would then see the sunlight blocked, but only in part. The earth would be blocking just a portion of the sun, not the entire sun, as it is in the umbra. Now, this is the geometry that we see for a lunar eclipse. A solar eclipse looks a little bit different, and here's the example for a solar eclipse. We still have the two types of shadows that we're used to seeing, as we explained before. We have the penumbra and the umbra. And in this case, the moon now is in between the earth here and the sun here. So the moon is now moved in between the earth and the sun and blocks out the sun's light from reaching the earth. If you're in the umbra, here, the small dark portion, then you would get a total solar eclipse. The sun's light would be completely blocked from that portion of the earth. If you're in this little bit wider area that's a little lighter called the penumbra, the sun's light would only be partially blocked, so some of that sunlight would still get through. Now, what does this mean for the type of eclipses that we would get? Well, let's first look at the solar eclipses. And here's an example of a solar eclipse. I showed this image before. You can see the moon blocking out the sun's light, and we can see the outer edge of the sun visible. So there are three types of solar eclipses that we can get. The first would be a total solar eclipse, and that is the image shown here. That is when the entire face of the sun is blocked out. So we do not see any of what we call the surface of the sun. All we can see is its atmosphere, but the entire face of the sun is then blocked out. In a partial solar eclipse, only part of the sun would be blocked out. So let's look at an example of that one. This would be an example of a partial solar eclipse. Note here is the sun, and here is the moon, taking a bite out of the sun, blocking part of it. But this would be an example of a partial solar eclipse. Only part of the sun is being blocked. So the moon is taking a little bite here out of the sun and blocking some of its light. Now, the third type of eclipse that we can get, a solar eclipse, is an annular eclipse. Now, an annular eclipse occurs when a ring of sun light is visible. So here's an image of it here. This would be an example of an annular eclipse. The moon is still here in the middle, but we get this ring of sunlight around the moon. And that is a ring or annulus, which is how it gets its name. An annular eclipse is not an annual eclipse. It does not happen every year. It is an annulus eclipse in that there is a ring of the actual sun still visible around the edge of the moon. So in a way, this is an example of a partial eclipse. This sun is not completely blocked out. Now, why do we get an eclipse like this? Well, the moon in these cases would actually be more distant, more further away. And what that means is that the moon is in an elliptical orbit. And sometimes it gets closer to the Earth. When something is closer to you, it has a larger apparent size. When something is further away, it has a smaller apparent size. And in the cases of an annular eclipse, the sun may be a little bit closer to the Earth or the Earth may be a little bit closer to the sun to be a little more precise. And the moon may be a little bit further away from the Earth. Because they are almost exactly the same size, in those cases, the moon is going to appear a little bit smaller than the sun and will not quite be able to block it out, giving us this type of annular eclipse. So solar eclipses are one type of eclipse that we can get when the moon passes in front of the sun. When the moon passes into the Earth's shadow, we get what we call a lunar eclipse. So this is an example of a lunar eclipse that we can see. And this would be an example of a total lunar eclipse. So in a total lunar eclipse, the entire moon has passed into the umbra of the Earth. That's the deepest part of the shadow recall. So the moon should disappear. No light from the sun is reaching it. However, that does not happen because the Earth has an atmosphere. And the Earth's atmosphere bends light. And it bends red light into the shadow. And when it does that, it actually doesn't. So the light, the shadow, is not completely dark. It does have some red light there. And that's why you sometimes get what we call the blood moon, where it looks as very deep dark red, not like any full moon would ever look. Here we see it. This is the deeper part of the umbra on this side, where it's a very deep red. This part would be closer to the edge of the umbra, but still within it. So this would be what we would call a total lunar eclipse. Now a partial lunar eclipse, in this case, we would see something a little more like this. And what do we see? Well, in this case, this is an example of a partial lunar eclipse when part of the moon is in the umbra. So the portion over here is in the umbra, but part of the moon is not within the umbra. This side is still being illuminated directly by the sun and will look a little bit brighter. So this is a partial lunar eclipse. You'll still get some of the reddish color here, especially if most of the moon is hidden within the earth shadow. If very little of the moon is hidden within the earth shadow, you would not be able to see the red, because it would be too much fainter compared to the rest of the moon. And the rest of the moonlight would then overwhelm it. So a partial lunar eclipse, like a partial solar eclipse, part, in this case, part of the moon is blocked by the earth. Now, the last type of lunar eclipse is what we call a penumbral eclipse. And that is something like this. And in this case, I've actually shown two pictures so that you can see the distinction. And it isn't very large. In a penumbral lunar eclipse, the moon only passes through the penumbra of the earth. So it's not passing through the part of the complete shadow. So we don't see near as distinct an effect when we get a total or a partial lunar eclipse. This is not eclipsed over on the right-hand side. And this is the penumbral eclipse here on the left-hand side. If you note, the upper left-hand side here is a little bit fainter. Overall, the whole moon would be a little bit fainter. But it's hard to notice many of the portions. But when you're getting here, you're getting closer to the umbra, but you have not quite reached it. So you're in a region of a little bit deeper shadow, but you have not reached any of the moon in the penumbra in this case. Now, what that means is that this is not the eclipse worth waiting up for, because you're not going to see a very distinct difference in the amount of the moon that is illuminated. It's going to look pretty much the same. It will get a little bit dimmer. And especially one side of it that closest to the umbra will look even dimmer than that, as it's getting very close to being eclipsed, but not quite. So those are the different types of eclipses that we can get. What we want to know, one of the things is how we can go about predicting eclipses. So how can we predict when an eclipse will occur? So the prediction we find out when we look at eclipses is that eclipses do not occur every month. Now, why is that the case? We might expect it because we do get a full moon every month, which is the time of a lunar eclipse, and we do get a new moon every month, which would be the time of a solar eclipse. So why do these not? Why do we not get an eclipse every month? Well, there are two reasons for this, is that the moon's orbit is tilted by about five degrees. The moon and the sun are about a half a degree in size. So that means that because the moon's orbit is tilted, as you see in the image here, the moon can often be well above the sun or well below the sun. And that means you're not going to see the eclipse. If the moon is at its biggest tilt at five degrees, it's going to be way above the sun because the sun is only a half a degree in size. So unless you are very close to what we call a node of the orbit, which is when the path of the moon and the path of the sun actually intersect. So the sun's apparent path on the sky is shown here in yellow, the moon's is shown in gray, and when the moon and the sun are well above or below each other, we cannot have any eclipse. So during those times of the year, we do not have eclipses, but we do have eclipse seasons. And those are the times when you reach either the ascending or descending node, one of the two nodes of the orbit of the moon. And when those occur, we do get we will get eclipses. So a full moon or a new moon occurring around the time of the node is when we get an eclipse. Now, there is a pattern to when the eclipses occur. Not only do they occur at its seasons, but there is also a pattern that we call the serocycle. And eclipses will occur with a pattern of 18 years, 11 days, and eight hours. Now, what that means is that if an eclipse would occur today, you can expect a very similar eclipse to occur a little over 18 years later. However, because of this eight hours, that means it will occur a third of the way around the Earth from where you are right now. But if you add this up three times and you get then 54 years and 34 days. So that means about every 54 years if an eclipse occurred in a location today, 54 years from now, it would then occur. A very similar eclipse would occur. Not at precisely the same location and maybe not precisely the same type of eclipse. You might have had a total eclipse one time and you might be a little further away and only see a partial eclipse or an annular eclipse the next time. But you would see this recurring pattern of eclipses in the same part of the world every 54 years. So finally, how do we go about observing an eclipse? What do we want to do to actually look at an eclipse? And that depends on the type of the eclipse that you want to look at. For a lunar eclipse, it's no special equipment is needed, you can simply go out and watch the eclipse. These will always occur at night because they occur during the full moon and the full moon is only up when the sun is below the horizon. So you'll never see a lunar eclipse during the day only during the nighttime. But you don't need anything special to go and observe it. A solar eclipse, on the other hand, first of all the warning, it is never safe to look at the surface of the sun. It doesn't matter whether 100% of it is visible or 1% is visible. The surface of the sun is so bright that it can damage your eyes. So you never want to look unaided at the surface of the sun. There are a couple of things that you can do. You can make what is called a pinhole camera to look at the sun, which is when you put a pinhole in a piece of cardboard and you aim that towards the sun and then use another piece of cardboard to focus the image. So you can actually get an image like that. There are also solar filters that can be used. Regular sunglasses and things are not sufficient. They will not protect against the intensity of the sun's light. Solar filters will block out the vast majority of the sun's light and will also block out the most harmful types of sunlight. So these can be used on things like glasses or on telescopes to be able to allow you to look safely at the sun. But often, the best way to observe an eclipse is, the best way to observe an eclipsed sun is to find your local amateur group. And often they will have a setup available or set up something in a local park where the eclipse can be viewed. And they have the equipment, the proper equipment to be able to allow you to safely view the sun. Now, contrary to popular opinion, it is actually completely safe to view the eclipse sun. When the sun is totally eclipsed for that period of time, then the sun is very safe to view. But very carefully, because you need to know exactly when the eclipse starts and ends, you don't want to be staring at the sun and then have some of it visible. When only 1% or a couple percent of the sun is visible, it's no longer painful to stare at. And that means that you're more likely to stare at it than you are to stare at the uneclipsed sun. If you try to glance at the sun in a normal, bright day, you instinctively turn away because the brightness will hurt your eyes. And that's sort of a safety mechanism. However, in an eclipsed sun or a partially eclipsed sun, enough of the sunlight is blocked, then you may be able to look at it without hurting your eyes. But you can still be damaging your eyes because that image, that partially visible sun, can be burning its image into your retina. So you are always better using the specific equipment that is set up for this or using something like a pinhole camera to be able to safely observe an eclipse. So finishing up here, let's summarize what we've done. We've talked about two different types of eclipses. We had the solar eclipses where the moon passes in front of the sun and blocks out its light. We've had lunar eclipses where the moon passes into the Earth's shadow and the light reaching it is blocked out. We do not get an eclipse every month, but there are predictable patterns as to when an eclipse will occur. So an astronomer can easily tell you when the next eclipse will occur in any given area. So that concludes our discussion of eclipses, and we'll be back again next time for another topic in astronomy. So until then, have a great day, everyone, and I will see you in class.