 Greetings and welcome to the Introduction to Astronomy. In this lesson, we are going to talk about the moon, our closest neighbor in space, and tides, and the tides that are caused by the moon. So we have two things that we're going to be looking at in this lesson, both very closely related to our moon. The phases, of course, are what we see when we look at the moon. The moon is not always fully illuminated. When we look at the sun, we always, unless there's an eclipse, see a full sun. In the case of the moon, we see various phases, and we can sometimes see the moon as a crescent or a full phase in various phases in between. So let's start out by looking at those. And what we see is that the moon has a set of phases that it goes through. And as we look here, the moon phases are caused by the illumination of the sun and its position around the earth. And there are two things that are important here. First of all, half the moon is always illuminated by the sun, and half the moon is always visible from the earth. So if we look at our diagram here, we can see that no matter what the phase of the moon is, half of the moon is illuminated. So in a full phase, we are seeing that illuminated phase, that half. And in a new phase, half of the moon is still illuminated, but that is now the half that is pointing away from the earth, so we do not see it. So the moon phases are caused by these two things. They are caused by the fact that half the moon is always illuminated, and half the moon is always visible from the earth. Now what is not drawn in here is what part of the moon is visible from the earth. So you can imagine drawing a line straight across the moon, and that would show this half is the part that is visible from the earth. So this is the part that we can see. So at a first quarter moon, half of the moon is illuminated, and we see half of that, so we see a quarter of the moon as being illuminated. And we also have pointing towards us a quarter of the moon that is not illuminated. Now if we looked at a full moon here, we draw the same line, just like this now, pointing straight towards, directly towards the earth, or perpendicular to the earth, I'm sorry. And then the illuminated phase is now the part that is pointing towards the earth, and we would have the full phase. In between, we could get a crescent phase. So if we did this up for this one, and drew a nice line through that, now we see that less than half of the moon is illuminated, at least that we see. Again, fully half of it is illuminated, we just only see a small fraction of it. That's what we would call a crescent phase. So when the moon goes through its cycle of phases is as it orbits around the earth. So as the moon goes around the earth, nothing changes. Still half of it is illuminated, and half of it is pointing towards the earth. But the percentage of the illuminated part that we see from the earth changes. So if we start at new moon here, and work our way through, let's look at the first set of phases. These are what we call the waxing phases. Waxing phases are what occur when the moon is getting more and more illuminated each day. So that's the first set of phases as we start with the new moon here. So as we go from new moon, and as the moon orbits around the earth, it will get progressively more and more illuminated. Here we saw nothing. That's what the new moon phase means. Here we see a small portion of it. Here we see a quarter of the moon. Here in the gibbous phase, we now see more than half of the moon illuminated. And up to the full phase where we see the entire set of the moon illuminated. So these are the waxing phases, and if you see the moon out in the evening, you can watch it go through these phases over the course of just a couple of weeks. It takes the moon about 14 days to go from new moon, not being visible, to full moon. So it only, you can see a significant changes over the course of just a few days. Now the other thing we see are what we call the waning phases. The waning phases here is when the moon becomes less and less illuminated each day. Again that we start at full moon here and work our way back around to new moon. This also takes about 14 days. And in this case the moon will get less and less illuminated. So starting with full moon where we see the fully illuminated portion. The waning gibbous we see more than half of it illuminated. Third quarter we see exactly half. And then the waning crescent we see only a small portion of the moon illuminated by the sun. So we see less and less in the terms of the waning phases. So going waxing is when the moon is getting more illuminated each day. Waning is when it is getting less and less illuminated each day. And the entire cycle of phases takes about one month. So that's where we get our month from the moon and from this phase cycle of the moon. Now let's look at this entire cycle here. Here is an example of a calendar from 2005 looking at the phases of the moon. And this gives the entire set of phases here that we can see from the moon. There are two different phase cycles that we look at. The cycle of phases is what we see and that is what we call the synodic month. The synodic month is 29 and a half days. That is what we use as our month. That is the time that it takes the moon to go through a cycle of phases. And in this case you go from new moon on the 8th here and go through to the next new moon on the 6th of the following month. That would be about 29 and a half days in May that has 31 days. So going from the 8th to the 6th of the next month would then take you through this cycle of phases. However there is another month that we use as well and that is the sidereal month. The sidereal month is 27.3 days and that is the actual revolution period of the moon. How long does it take the moon to go around the earth once? Now the phases that we see, I've mentioned them when I showed the previous slide, but the phases that the moon will go through starting off with the new phase when it's not visible at all, then becoming a thin and then a thicker crescent, a first quarter phase, a gibbous phase when it's more than half, so this just means more than half is illuminated. The full phase when we see the entire illuminated phase of the moon and then we go back through it again the opposite direction in the waning phases, going from the gibbous to the quarter to the crescent and back to the new phase and starting it all over again. So what phases are visible does depend on the time of day and the positioning of the moon. So let's look at when we'd be able to see these various phases and the phase will depend on that positioning. So a new moon, for example, is invisible because it's always in the same direction as the sun, meaning that it will rise with the sun and set with the sun. So it is up when the sun is up and it's also pointing the darker side of the moon towards the earth, so it's not illuminated and therefore it is not visible. A crescent phase will always be close to the sun in the sky. So if you look here, if you're looking at a crescent phase in this section, the sun is off where the arrows are showing. So the sun is way off there, but the crescent has to always be relatively close to the sun. You will always see a crescent phase in the morning hours. So you'll see this right before sunrise or right after sunset. It is always close to the sun in the sky. The quarter phase is halfway around the sky from the sun, so it's a little further away. The gibbous phase, you'll never see right after sunrise or after sunset. It's much further away. It's in the opposite portion of the sky. So it is more than halfway around, more than that quarter way around from the sky, from the sun. And the full phase that we see is always opposite to the sun in the sky, so completely opposite. So the quarter phase is halfway around from where the sun would be. You go halfway to the quarter phase and completely around would take you to the full phase. It rises and sets opposite to the sun, so when the new moon is setting, the full moon will just be rising. And that would mean that the full moon would be setting right at sunset. So the different phases depend, the timings that we see here depend on when they would be visible. So if you are looking for a phase, if you're looking for the phase that is for example overhead at 3 p.m., that would be a waxing crescent phase. The phase overhead or closest overhead at midnight would be a full phase. At 6 a.m., it would be a quarter phase. So that's one way that you can tell whether it's first quarter or third quarter. The first quarter moon would be visible at 6 p.m., high in the sky. So if you're seeing it in the evening, in the late afternoon, evening, around sunset time, then you're seeing the first quarter phase. If you are seeing the quarter moon early in the morning, then it would be the third quarter phase. And you can also note by from here that as you're looking from the earth, opposite sides will be illuminated. During the first quarter phase, as you're standing here on the earth looking outward, you would see the right-hand side of the moon illuminated. The third quarter phase, you would see the left-hand side of the moon illuminated. So let's look a little bit about the rotation and revolution of the moon. The moon rotates and revolves at exactly the same rate. So these two are precisely the same. And that is very important. We sometimes think that the moon does not or does not rotate, and it does. Half of the moon is never visible from the earth because it rotates and revolves with exactly the same period. And that means that one side of the moon is always facing the earth. That's the side we always see. The other side of the moon is never seen and in fact was not seen until 1959 when the Luna 3 spacecraft flew around the moon and took images and sent those back to give us the first view of the other side of the moon. So even though it's our closest neighbor, it is not something that we knew about until recently. Maybe a misconception here. Is there a dark side to the moon? Well, yes, half of the moon is always illuminated and half of it is not illuminated. But there is not a side of the moon that never gets sunlight. All of the moon gets sunlight over the course of the month and all of the moon gets darkness. So maybe a better terminology is the far side of the moon that is always on the other side from the earth. So the side that we do not see. But in reality all of the moon gets the same amount of sunlight over the month because of this synchronous rotation that occurs that keeps the moon always pointing its same side towards the earth. Now the other thing we wanted to talk about here are the tides. Now tides are caused by two different things. They are caused by the moon and the sun. So they are caused by what we call a differential gravitational force in the moon and the sun. So what happens here is if you look, first of all, let's just look at the lunar tides. The most of the tides that we get here on earth are caused by the moon. So when the moon is close to the earth, if you know the force of gravity is equal to a gravitational constant times the masses of the two objects divided by the distance squared. So if something is a little bit closer, the gravitational force is a little bit stronger. If it's a little further away, the gravitational force is a little bit less. So in our image here, this is the earth. So this is our earth. And imagine the moon off to the right. This edge of the earth is a little bit closer than this edge of the earth. So the gravitational force here is a little stronger, shown by the larger arrow. And the gravitational force here is a little bit weaker. In the, if we want to look at that as a net force, what is happening relative to the center of the earth, then we subtract out this arrow at the center of the earth. And that means that what this differential force does is it exerts a force towards the moon on this side and away from the moon on this side, giving us the different tides. So giving us a high tide here and a low tide here, a high tide over here, and a low tide here. So the forces are pulling that, pulling the water away. Now why is it the water that gets pulled? And that's only because the water actually flows. The water is able to flow relative to this force, whereas the solid rock is not going to move near as much. So the water being able to flow then gives us high and low tides. Essentially here we get a high tide every 12 hours or so. Now the sun is important in this too. So let's look at another image here that shows that. And we have what we call the spring and the neeptides. So there's two different sets of tides here because the sun also produces tidal force, but only at about half the strength of the lunar tides. So what happens here is that the moon is producing a tide. It's pulling on the earth. And during a spring tide the moon and the earth are pulling together so that they will, sorry the moon and the sun are pulling together so the tides will be added. So the lunar tides and the solar tides will then be added together and give you a much stronger tide. So at the new and full moon when the moon and the sun are lined up together in a straight line then they will give you a much stronger tide than we would otherwise see. Now if we turn this around a little bit and let this look towards the other set of tides at the neeptides then when it's in the quarter phase, so now when we see the moon and the sun at right angles to each other, so now the moon is this direction and the sun is this direction, their gravitational forces are working opposite each other and they are going to produce tides that are weaker than normal that we call the neeptides. So spring tides are just high tides that are stronger than normal. Neeptides would be high tides and low tides that would be weaker than normal. So finishing up here we want to look at what we've covered here. First of all the phases of the moon depend on really the positioning of the sun and the moon in the moon's orbit around the earth. Recall half of this moon is always illuminated and half of it is always pointing towards the earth. Those two together give us the phases. Specific phases are visible at specific times of day. In the late afternoon you cannot see a third quarter moon. It will never be up, it is only up in the early morning hours after midnight. A full moon can never be seen during the middle of the day because of its positioning around the sun and for example a crescent phase always has to be close to the sun in the sky. And then we talked about tides and how they're caused by the differential pull of the moon and the sun on the earth and it is the water that moves in response to the excess force. It is the water that can actually is able to move. Well that concludes our lecture here on the phases of the moon and tides. 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.