Why Do We See Different Phases of the Moon?

For tens of thousands of years, people have been looking up at the moon and wondering, why does it look different every night? And why do these different shapes, which we call phases, repeat themselves on a regular basis every month or so? To understand, we need to zoom out and take a look at this from a wider angle. Before we start, we need to know a few pieces of background information. Firstly, the same face of the moon always faces the earth. So when we look up at the moon, we're always seeing the same side. Before humans first went to the moon, no one even knew what the other side looked like. Second, half of the moon is always lit by the sun, despite what it looks like from earth. One half is always lit, and one half is always dark, although it's not always the same side that's lit or in darkness. Okay, but the phases. It turns out that because one half of the moon is always lit, and the same face of the moon always faces the earth, the shadow from the sun rotates around the moon as it orbits the earth. This means that we see different amounts of the moon lit up and in darkness every day. And because the moon is a sphere, we end up seeing some pretty cool shapes. Let's look at this in more detail. The first phase is called the new moon, which takes place when the moon is located between the earth and the sun in space. We're basically looking at the back of the moon, and for that reason we only see the dark half. The other half is still lit, it's just facing the sun so we don't see it. Because the moon is between the earth and the sun, it's only visible from the side of the earth that's facing the sun, or daytime. During the new moon, the earth, moon, and sun are pretty much in a straight line in space. For this reason, the moon stays near the sun in the sky, rising at sunrise and setting at sunset. Because it's up for most of the day, and because we're looking at the dark half, it's really hard to see the new moon without the help of a telescope. After a couple days of the moon's orbit around the earth, the illuminated face of the moon begins to grow, which appears as a curved line to us. Because of its curved shape, and because the lit portion is growing, we call it the waxing crescent moon, where waxing means growing, and crescent means crescent. About 7 days after the new moon, or 25% of the moon's orbit around the earth, the moon is now at 90 degrees off to one side. At this point, we're looking directly at the line that separates the light half from the dark half. Now because the moon is a quarter of the way through the cycle, we call it the first quarter moon, even though clearly we can see half of the moon. I don't know, we gotta do better with these names, guys. The first quarter moon rises around noon and sets at midnight. As the moon enters the second quarter of its orbit, the illuminated face that we see from earth begins to grow even further. This is called the waxing gibbous moon, where gibbous means bulging outwards or growing. Because waxing also means growing, this phase is basically called the growing growing moon. Anyway, at this point, the waxing gibbous moon rises in the late afternoon and is up for most of the night. Because the moon is now up most of the night, and most of its face is now illuminated by the sun, it's starting to get really noticeable in the sky. This is one of the reasons for the common misconception that the moon only comes out at night. When the moon has reached the halfway point of its orbit, it's now 180 degrees from the sun, and the earth is effectively sandwiched between the sun and the moon in space. As we look back at the moon, we're looking at its fully illuminated face. This is of course called the full moon. Because the moon is on the opposite side of the earth to the sun, it rises at sunset and sets at sunrise. It's up all night. As the moon continues its rotation around the earth after the full moon, the lit portion begins to shrink. Remember, the first half of the moon's phases was called the waxing moon. We call the second half the waning moon, which means to decrease in size. The next phase after the full moon is another gibbous moon, this time the waning gibbous moon, because now the lit up face of the moon is shrinking. After progressing through the the moon is again at 90 degrees off to one side of the earth, this time on the other side. This is called the third quarter moon because the moon is three quarters of the way through its orbit. The third quarter moon rises at midnight and sets at noon. The moon continues its orbit around the earth, rising and setting later and later, and again reaching the curved shape of its final phase. This time it's called the waning crescent moon. Finally, after 29.5 days or about a month, the moon has now traveled all the way around the earth and has arrived back where it started, sandwiched between the earth and the sun at the new moon. The cycle of phases is complete. So to recap, the moon has phases because one side of the moon is always lit and one side is always dark. As the moon rotates around the earth, we constantly see different sections of the moon lit up and in darkness, which results in the phases that we know and love. Thanks for watching! If you're interested in learning more, there's a link in the description to an activity sheet that you can try at home. And, if you like the video, please hit like and subscribe to my channel. I'm going to be posting more videos just like this one soon. I'm Mr. Harris, class dismissed.