Transcription
In this lecture, we are going to talk about one of those fun topics of astronomy – the phases of the moon. Part of the reason why I think that the phases of the moon is such a fun topic is that it’s something that we’ve seen for most of our lives. We go outside at night, we see the moon in the sky, we realize, oh, it looks different now than it did a few days ago. More of it’s lit up, less of it’s lit up. We’ve seen the phases of the moon for many years. Now we’re going to learn in this lecture exactly how they work. What are the names for the different phases, how do they change from one day to the next.
Before we jump right into the phases of the moon, we need to talk a little bit about the moon’s orbit around the earth. The moon takes about 1 month to orbit the earth. Actually, it’s a little bit less, about 28 to 29 days. But that’s where we actually get the concept of the month. The concept of the month comes from roughly the time it takes for the moon to orbit the earth.
Now, notice something interesting here. We have a diagram of the moon making its orbit around the earth, and the sun will say is over here to the left. Notice as the moon orbits the earth, how the same side of the moon is always facing us. Here’s this guy living on the moon. As the moon goes around, the moon actually rotates at just the right rate, just the right speed, so that the same side of the moon always faces us. This is something you may have noticed before. When you go outside and you look at the moon, those dark spots, what we call the maria, we’ll talk about those later, in a later lecture, those dark spots are always in the same spot on the moon. All we ever see is one side of the moon, what we often call the “near side” of the moon.
Now, a lot of times we’ll say, well, if we’re always seeing the same side of the moon, that means the moon must not rotate. That’s actually not true. If you look at it from above, what you’ll see is the moon rotates once. This guy has gone around once, and the same amount of time it took the moon to go around the earth. We actually call this synchronous rotation when an object rotates in the same amount of time it takes to go around another object. Whenever you have synchronous rotation, the same side of the object, like the moon, will always face the earth.
So, we may hear about things like the near side and the far side of the moon. What you see here, the near side, is the side of the moon that is always facing the earth. The far side is the part of the moon that is always away from the earth. What’s sort of crazy about this when you think about it, for as long as we’ve had recorded history, even before recorded history on the earth, people have looked up, seen the moon, and this is the side they have always seen. These dark spots, the maria, that often make up what we’ll call the man on the moon or the rabbit on the moon. But it wasn’t until the late 1950s, 1960s, when we sent our first probe, or satellite, around the moon that we ever saw this side of it. It’s sort of crazy, our nearest neighbor in the sky, and it wasn’t until the 60s that we were able to see what was on the other side of the moon, to see that other side of the moon.
Now, in the diagram here, I’ve called it the far side of the moon. And you’ll notice that the near side and far side look quite different. We’ll talk in a later lecture about why they might be different from each other. You see, I called it the far side of the moon. You may have heard it by a different term. You may have heard this called the dark side of the moon. But let’s see if that’s actually a good name for it.
Alright, here we go, back to that diagram of the moon going around the earth. Now I want you to look, say, right here. When the moon is over here, the sunlight is coming in from the left here, and it lights up the side of the moon that’s facing us. This is what happens to be called a “full moon”; the entire moon is lit up. But notice half a month later, two weeks later, when the moon is over here, now the side of the moon that’s facing us, the near side, is actually dark. It’s not lit up by the sun, so we call it a “new” moon. But the part of the moon over here that is what we sometimes call the “dark” side of the moon is not dark at all. It’s being lit up by the sun. So, it’s not fair to call that side of the moon that is farther from us, that side of the moon that is not facing us, the dark side. It’s not the dark side. It actually gets just as much sunlight as the near side does. A much better term for it, the one we’re using in class, is the far side of the moon. Because, yes, this side of the moon is always farther away from us on the earth. But it gets just as much light as the other side of the moon does, just as much light as the near side does.
We can already get a little bit of a sense of the phases of the moon by looking at this diagram here. At any moment, as the moon goes around the earth, half of the moon is lit up by the sun. But depending on where the moon is in its orbit here on earth, we see different amounts of that lit side. Here we’re seeing all of the lit side; we call it the full moon. Here we’re seeing none of the lit side; we call it the new moon. Here we’re seeing half of the lit side, just this part right here. It’s what we call, it turns out, a quarter moon.
When we talk about the phases of the moon, we’re going to use this website here. This is a very important website because it gives us a model; it shows us what the phases of the moon look like as the moon is going around the earth. So, let’s check out this website here. Here we go. So, I’m just going to center it up in our box here and let’s start the animation. So, what you’re going to see here is the moon is going around the earth. It’s moving quite slowly at this point. And up here on the left, you’ll see the moon as it looks in the sky. And I’ll speed the animation up a little bit. What you’ll notice is, as the moon makes its orbit around the earth, more and more of the moon starts to get lit up until we get to a point where the entire moon is lit up. We call that the full moon. But notice what happens when we get past full moon. When we get past full moon, darkness starts to work its way from right to left across the moon here. Less and less of the moon seems to be lit up each moment until we get to the point now where at 3rd quarter, when we get to the point where there is no part of the lit side visible anymore. None of the moon seems to be lit up. We’re back to new moon. I’m going to play this loop just one more time and let you watch. Watch as we go through the different phases, and you’ll notice the names for the phases are actually written up here in the top right. So, watch this make one more loop around the earth. Remember, each loop is one month basically, 28 days. Okay.
Now that you’ve seen those moon phases, let’s actually put some terms to the different phases of the moon. We say that the new moon is when none of the lit side is visible; the moon seems completely dark. We’re not seeing any of that lit side. Right after the new moon, we get what we call a crescent. This is when we see less than half of the lit side. Going back to our simulation here, you can, for instance, see a crescent. We’ve got a nice crescent moon here. Less than half of the moon seems to be lit up. The crescent moon is sort of like that moon you see maybe in the Dreamworks logo. You’ve got the guy fishing off the moon. That sort of banana shape. That is an example of a crescent moon.
Well, what comes next after a crescent moon? After a crescent moon, we have sort of one of the trickier names, what we call a ‘quarter moon”. A quarter moon is when you see half of the lit side. So, we bring the moon over here. Here we are; we’re seeing half the lit side. That’s what we call a quarter moon. Let’s try and figure out where that name comes from. Sort of weird that half of the lit side visible means a quarter moon. But if you look at the moon’s orbit around the earth, what you see is when the moon starts here at new moon, when it goes from here to here, it has gone through the first quarter – one quarter of its orbit around the earth. When it gets up to here, it’s gone through 3 quarters – 3 quarters of its orbit around the earth, 3 quarters of a month. That’s where we get this term, quarter, from.
So, what comes after the quarter phase? So, after the quarter phase, you have what’s called “gibbous”. A gibbous moon means you see more than half of the lit side. So, at every step, you’re lighting up more and more of the moon. And then the final stage, we call the “full” moon. We see the entire lit side. With our diagram here, here we are. A quarter moon, moving around. Now we’re at a gibbous moon, so more than half of the moon is lit up. Keep going all the way around until the whole moon is lit up. What we have is a full moon. Just to review, starting here at new moon. Everyday, a little bit more and more of the moon is lit up. We go through crescent, we go through half of it lit up – the quarter, then more than half, gibbous. And then finally, the whole thing lit up, the full moon.
Now, what happens next? Once we go past full moon, darkness starts to work its way across the moon. But if you look at the phase, let’s look at this. Well, we have more than half lit. So, what is that – a gibbous. Keep going in time, less and less lit until we have exactly half lit, a quarter moon. Keep going more, less than half lit, a crescent moon. Keep going all the way until we’ve made one complete loop and we’re back to new moon again. So, we can think about this really as a step. We’re going from new moon down to full moon, and then we’re going from full moon back up to new moon. How long does this take? Well, to go from new moon to full moon is about half a month, so 2 weeks. And to go from full moon back to new moon is also half a month, about 2 weeks. So, 2 weeks down, 2 weeks back. We can think of it as about a week for each big step. So, new to quarter is a week, quarter to full is a week, full back to quarter another week, and quarter back to new, a 4th week.
Now, look at the diagram here, you’re going to see that there are some names attached. Instead of just calling it quarter, people say crescent; it’s called a waxing crescent, or a waxing gibbous, or a waning gibbous, or a waning crescent. So, what do these things mean? Whenever we see waxing, waxing means the moon is waxing means more and more of the moon is lit up every single day. If the moon is waning, it means less and less of the moon is lit up every single day. So, when we start at new moon, we have a waxing moon for 2 weeks, more and more of the moon lit up. And then after full moon, we have a waning moon for 2 weeks, less and less of the moon lit up until you get back to a new moon. Here we have a slide showing this. Waxing, more lit each day. Waning, less lit each day.
Now, if I just showed you a picture of a phase of the moon, how could you tell if it was waxing or waning? Well, if you go back to our picture here comparing waxing, we’ll just compare crescents for a second. Here’s a waxing crescent. Here’s a waning crescent. What’s the difference? Yeah. Which side is lit up? In the waxing crescent, as well as a waxing gibbous, it’s the right side of the moon lit up. But in the waning phases, it’s the left side of the moon lit up. So, you can just look at which side of the moon is lit up, and that will tell you if the moon is waxing or if the moon is waning.
One other trick you can tell you can use to tell whether the moon is waxing or waning has to do with the fact that the moon is not always in the sky at the same time every single day. We’ll see this more in a second, but for now, just go with this little trick. If the moon is visible in the sky at sunset, you can see the moon in the sky at sunset when it has to be a waxing moon. So, for instance, right after new moon, you look to the west right at sunset, you’ll see a nice crescent moon, the right side lit up. That’s a nice waxing crescent moon.
You’ll want to add one more thing here about the phases of the moon. You’ll see that we have the quarter moons. This one, which is really the waning quarter, is sometimes called the 3rd quarter. And this one here, which is called the waxing quarter, is also sometimes called the 1st quarter. On a test, I’ll be happy if you put waxing quarter or 1st quarter, and the same thing if you put waning quarter or 3rd quarter. Either one is fine with me. But if you’re reading it online or in a book, normally we say 3rd quarter instead of waning or 1st quarter instead of waxing.