 Greetings and welcome to Astronomy at Hack. As I've mentioned, we're going to be over the course of the winter break here. I'm going to be recording a couple videos on some special topics in astronomy just to give you a little bit of extra help and these will be useful for reviewing. And for those taking the class, you'll be useful to go back to them if you want to review a little bit more on any specific topics that I've covered. And what I'm going to talk about today is the, or is or are the phases of the moon. So the phases of the moon are what we see when we look at the moon. We see a different phase of the moon and there's a couple different phases. There's, first of all, there's the new moon, which is the one we don't see. We can't see the new moon directly because it's completely dark from our point of view. From our side, we're looking at it. We see only the darker side of the moon and we don't see anything illuminated. Then there is also a crescent phase, which is a very thin portion of the moon. Looks something like this and we'll look at it a little bit more detailed, but looks like a very thin moon. So that's when the moon is close to the sun, but far enough away that we can see a little bit. Then we have the quarter phase, which can be first quarter or third quarter and that's essentially a half a moon. So we see half the moon illuminated. And finally, or next, we have the gibbous phase, which is when more than half of the moon is then illuminated. So it's not quite full. Try to draw it there. It's a little hard. There's probably a better picture in your textbook, but it's not quite full. If it were full, you'd have a little bit more scattered around here. It's a little bit less than full. And full moon is, of course, the entire moon illuminated. And that's the complete surface of the moon. So these are the different phases of the moon and there's also two other parts that we can add in. As I said, you could have, in the quarter, there was the first quarter. I try to jump ahead a little bit too much there. Let's try that again. There is first quarter and third quarter. And in the crescent and gibbous phases, you can have a waxing crescent or a waning crescent. And you can have the same with the gibbous. You can have a waxing gibbous, a waxing gibbous, or you can have a waning gibbous. Now, when we look at that, waxing means it's growing bigger, and waning means it's growing smaller. So as we go in this direction towards full moon, we say that the moon is waxing. When we go in this direction, after we hit full moon and we go back towards crescent and new moon, then we say that the moon is waning. So we have a waxing moon as we go from the new moon through to full. Then once we hit full and go back towards new moon, we consider it to be waning. First quarter is here on the waxing side and third quarter is on the waning side. Now, I know that can be confusing. We're going to look at that a little bit more detail here with a diagram on the next page and look at a little bit more about the phases. So here we have a little diagram. The sun is way off to the right-hand side of the screen and then we have the earth here in the middle and we have different objects, different positions of the moon as the moon moves around the earth. Why we see the phases is because the moon is moving around the earth and we're seeing different portions of it illuminated. Let's look at a couple of those right now. If we look at here, when we look at this phase over here, what phase would this be? Well, divide the moon in half and we'll color in the section that's illuminated. Divide each of these moons in half right now while I'm doing it because note that the half towards the sun is all that's going to be illuminated no matter what we see. So that's always what's going to be illuminated and let's draw that in so as we fill that in here. So the red portion is what is actually illuminated and then we're going to do what we see. So the red portion here, that's the part. It's always the part towards the sun that's illuminated. So if we're looking at it from space, we don't see any specific phases. If we're looking down on it like this from above, then the moon is always going to be half illuminated. So that's when we look at it from Earth or from a different perspective that we actually see these phases. So those are the half of the moon that's always illuminated as shown in red there. Then we want to see what portion of that we're going to see at different points. So let's draw in in green here. Let's look at what we're actually going to see from the Earth. We're only going to see half of the moon and we're going to see the half here. So if we look at this phase, let's start on this side. So if we look here, we're only going to see the part that is facing towards the Earth. We're going to see this portion of the moon. So that's a full phase. We're seeing the entire disk of the moon illuminated. When we go over to this side, again we can only see the part that's pointing towards the Earth and that is then the new moon. The new moon because all of the part of the moon still half of it is illuminated but the only part that we can see is the side that is not illuminated. Now let's go up to the other ends here. On this side, again, we can only see the part, the half that's facing towards the Earth. So there's that and that and this would be first quarter and that would be first quarter. We're seeing half of the moon illuminated. If we come over here, we're going to see a similar thing. Again, half of the moon is illuminated. This would be third quarter. Now the slightly tougher ones. Here we're going to divide the moon in half again. Again, divide it right down the middle and what half can we see? Can you see there that you're going to be seeing most of the moon is going to be dark and only a little bit of it is going to be illuminated? This would be a crescent phase and it would be the waxing crescent phase. And if we go to this side, we'd get a similar thing. Again, most of the moon is dark. Very little of it is in red that we're showing here that is illuminated. So this would be again a crescent phase and in this case it would be the waning crescent phase. Okay, that leaves us with two more. So let's do a couple more here. If we look at this side and again drive right through the middle, we're seeing most of the moon illuminated but not completely. This would be the waxing gibbous phase and similarly when we divide this one we will see again most of the moon is illuminated. Very little but it's still not. We're not quite to full. This would be a waning gibbous phase. So really the phases of the moon just depend on what positioning we're seeing from the moon or what part of it is illuminated from around the sun. Now if we look at the earth here we have a couple little points marked. We're looking down at the top of the earth. At this point what time would it be? This would be at noon. This would be at midnight. This would be essentially at 6 a.m. and this would be 6... oops, sorry, backwards there. That would be 6 p.m. This would be 6 p.m. between noon and midnight and between midnight and noon you get 6 a.m. So using this we can actually look at what phase of the moon you would be able to see at a given time. So let's see what we can do here. If we have, for example, if it was noon, if we're at noon on the earth we can never see a full moon because if we were at noon we can only see, where on earth we can see what's here. So we can see any phases of the moon that are above the horizon. This black line would then represent our horizon. So we can see anything that's above it. We could see a crescent phase of the moon possibly at noon and sometimes you will note the moon there out in the middle of the day and that can be quite visible. And you could see a new moon except the new moon isn't visible because the whole illuminated side is not visible from us. The only time we see a new moon is during a solar eclipse. A total solar eclipse, if the moon happens to pass directly in front of the sun then it would be visible in that it's blocking out the sun. But we can't see the full moon at noon. It's quite impossible. There is no way at noon time to see the full moon here on the earth. Or the gibbous phases. We will never see those at noon. As this rotates I'm going to erase it here for a second. Let's clear that off. And so if we draw in our horizon for a different time, let's draw in a different horizon to look at this a little bit differently. But if we draw in a different horizon here, let's say what time is it now? This is the middle of the afternoon. This should be about 3 p.m. And again, what we'd see is if we're looking there at 3 p.m. as this rotates around as the horizon, as the earth rotates this appears to move at 3 p.m. Then the new moon would be in the sky. The first quarter moon, the waxing gibbous, if 3 p.m. the waxing gibbous moon would be just rising, the waning crescent would be just setting. The new moon would be visible in the sky. A waxing crescent or a first quarter these would all be visible at 3 in the afternoon depending on the actual phase at the time. But things like the full moon, the waning gibbous in the third quarter would not be visible at that time. So if you draw your horizon in and set up your time and just say, okay, we're here, here's my horizon, I'm right here, that's about 3 p.m. I can tell what kind of phases, what kind of phase of the moon is. Now we can use this for a number of different questions to ask and to determine what different phases would be visible at which times. So as we clear this part off again, we could do another type of question. Let's say that we see the waning gibbous moon is just rising. So it's just rising. What time of day is it? And we can do that with this diagram. We can actually figure that out. If the waning gibbous moon is just rising, then we're here because it's just rising above the horizon and the horizon is going this way. So what time is it if the waning gibbous moon is just rising? And you just have to look and see what time it is and you can look at the thing here. Well, let's see, that's in between 6 p.m. and midnight. So the waning gibbous moon would be rising around 9 p.m. So around 9 in the evening, if you see a moon just rising on the eastern horizon, it is a waning gibbous moon. Let's do another one. So we see that, we see the waning gibbous moon there. Okay. Let's say that we see, let's say it's 6 a.m. Yeah, let's do 6 a.m. What phase of the moon is just rising? So what phase of the moon is rising at 6 a.m.? Well, at 6 a.m. we draw in our horizon here and it's rotating here. So what's going that way? And what's going that way? The new moon is just rising here at 6 a.m. The new moon is just rising just coming up over the horizon as the horizon goes this way. The new moon is just coming into view. Of course, you won't be able to see it because it's not visible. And the full moon would just be setting. It's just going below the horizon. So you can use this for a number of different questions. This type of diagram, and there's a nice one of it in your textbook, that you can use for a number of different questions like this that we can ask to help to enhance your understanding of the moon and what phases are visible at what time. So you can look at it there and you can see that at certain times all you have to do is draw in the horizon of the earth and actually see what phases of the moon possibly be visible at specific times. Or if you know a specific time, you draw on the horizon for that time and you can tell which phase of the moon happens to be rising or setting. So the phases of the moon are one of those things that you can see very easily. They're very easy to go out and look at. You can see the moon. You don't need a telescope. You don't need binoculars. You just have to walk out and look at the moon and it's there and you can actually see it. You can see it very easily and you can watch it over the course of a month. You can see this entire set of phases. If you just go and look out at the moon each day and try to determine actually how the moon, you can even see when the moon is rising and see how it changes over just the course of a month, it doesn't take a very long time to actually be able to see this. So I hope this has helped to give you a little bit better understanding of, first of all, what the different phases of the moon are and how they're visible from the earth, how we see the different phases and then also be able to determine what phases of the moon are going to be visible at different times of day. So that's something that's very interesting to be able to know, to be able to look and see what type of phases the moon will be visible. We'll be recording again a few more of these over the course of winter break for you to give you some special topics just to look at and then be useful as the reviews for the coming semester. So until next time, have a great day everyone and I will see you in class.