 Greetings and welcome to the Introduction to Astronomy. In this week's Misconception in Astronomy, we are going to talk about the moon phases and understand what causes them and find out that they are not caused by Earth's shadow, which is a common misconception. So what causes the phases of our moon? Well, here we watch the moon go through its cycle of phases. And you can see that it will start out as a thin crescent, quickly become full, and then go back again to a thin crescent, very quickly in the image. But we'll note that it actually takes a full month. Now what we know that does not cause the moon's phases is the Earth's shadow. What we find is that we will always have one half of the solid surface of the moon that is illuminated by the sun. And we also find that half of the moon faces Earth. So let's take a look at this. Now the half facing the sun would be the half here on the right-hand side. And that would not matter which phase we're looking at here. We're looking at two examples. This half of the moon will always be illuminated. Now we also know that half faces the Earth. So the half facing the Earth would be this portion, so the part down below here. And in this case it would be angled a little bit more. Again, we want to go right through there and we would look at this portion facing Earth. Now the only part that will be illuminated is the portion, the illuminated portion of the moon that happens to be facing Earth at that time. So in this case, in this first case, we would see this portion of the moon meets both criteria. It is facing Earth and it is the illuminated portion. And there we would see a first quarter moon. Now in this case we see it's a little bit different. This part here is facing the sun, so it would be illuminated. This portion above the green line would then be the part that is facing Earth. And only this little portion here would be illuminated. So we would see a very thin crescent phase. So it is the combination of these two things that gives the moon its phases. Now to be clear, phases are not specific to the moon. Any object would go through phases. So if we were on the moon, and here is an image taken by one of the Apollo missions, watching the Earth rise over the lunar limb from orbit around the moon. And we note that our Earth is not in a fully illuminated phase. It is actually in what we would call a gibbous phase. We also have noted that the planet Venus will go through phases, as would the planet Mercury. Any planet closer to the sun than where you are would go through phases. So in these cases we would see phases elsewhere. Any object, any moon around another planet would also go through the same set of phases that our moon does. Now, it is true that the moon's Earth shadow does occasionally fall on the moon. This is what we call a lunar eclipse. And here we see the uneclipsed moon off to one side here. So there is the uneclipsed moon. And we see the moon much closer into Earth's shadow. So the Earth's shadow there is falling on the moon, but that is only in those very special cases when the Earth, moon and sun are exactly lined up that gives us a lunar eclipse. So let's go ahead and finish up with our summary. And what we looked at today is that the phases of the moon are not caused by the shadow of Earth. They are caused by how much of the illuminated portion of the moon's surface is visible from Earth at that time. Yes, it is true that the moon does occasionally pass through Earth's shadow, but this is only during a lunar eclipse, which does not happen every time, and is only for a short period of time over the course of maybe a couple of hours on just full moon and just those full moons when everything is lined up perfectly to give us an eclipse. So that concludes this lecture on the phases of the moon and that they're not caused by Earth's shadow. We'll be back again next week for another misconception in astronomy. So until then, have a great day, everyone, and I will see you in class.