Back To CourseBasics of Astronomy
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The moon, Earth's only natural satellite, is very hard to miss in the night sky, unless it's not there, of course. You've certainly seen how the moon cycles through different phases. Sometimes it's very bright and large, a full moon, and other times it's a beautiful crescent moon. The moon has different phases for a reason. It is its relationship to the sun and Earth that makes it seem like the moon changes shape all of the time and quite dramatically so over a period of time. How the moon, Earth, and sun combine to cause these phases of the moon will be outlined for you now.
When the moon orbits the Earth, it always has the same side facing us. We never get to see the far side of the moon. That's why whenever you look up at the moon, you see the same patterns on its surface as the night before. The reason the moon changes shape as it cycles through all of its phases is thanks in part to the sun shining upon the different areas of the side of the moon that we can see from Earth.
So here's how this works: at any given point, the sun will illuminate one half of the moon. However, since we only have one half of the moon facing us, the same half all of the time, the moon appears to change shape based upon how much of the sunlight is illuminating our side of the moon compared to the far side of the moon!
Look on your screen. During the new moon, the phase of the moon where we cannot see the moon at all, the half of the moon that's illuminated by the sun is the far side of the moon, the half of the moon we never see. Therefore, if the half of the moon that's illuminated is the side that we never see, does any light fall on our side of the moon? No! The sun can only illuminate half the moon at any given time! This means we see nothing during the new moon, unless there's a solar eclipse.
On the flip side, during the full moon, all of the sun's light falls on the half of the moon that faces us. Therefore, we see a big, pretty, bright moon in the night sky. As a result, how much of the moon we see during its month-long cycle depends on where the moon is positioned in its orbit.
To help you better understand how the moon, Earth, and sun interact to create the phases of the moon, you can create your own little experiment. Take a round object, like a baseball, and have that represent our moon. Now, stand near a light source. Ensure it's the only light source in the room. At first, face this light, representing our sun, stretch out your hand directly in front of you, as well.
At this point, the far side of the baseball is fully illuminated by the lamp and the side facing you is completely dark. This is the new moon. Now, turn 90 degrees to the left to mimic the orbit of the moon. As you slowly turn to the left, more and more of the baseball's side facing you becomes illuminated, mimicking the changing phases of the moon. If you rotate another 90 degrees, only the side facing you will be illuminated, representing the full moon.
That's the gist of it I want you to remember, for this lesson at least. Another lesson will describe these phases for you, from start to finish, in a lot more detail, and we'll use this example once more to polish off all of the phases.
But before I let you go, I need to discuss one last important thing that my little experiment oversimplified on purpose, so that you wouldn't be overwhelmed with too much info at once. You might think that the reason why we only see one side of the moon is because the moon doesn't rotate on its axis and hence only has one side facing us all of the time. But this cannot possibly be the case.
The way the moon rotates is quite unique. It's called synchronous rotation, a kind of rotation where it takes exactly as long for the moon to rotate on its axis as it takes for it to make one orbit around the Earth.
Let's take a look at the same image as we did before, but one where we now allow for the moon to spin on its own axis as it orbits the Earth.
Synchronous rotation further allows you to see why no side of the moon is in continuous darkness. Any point on the moon will spend two weeks (half a lunar orbit) in darkness and then two weeks in sunlight, during its orbit around the Earth precisely because of its rotation during its orbit. You can now explain why the name 'the dark side of the moon,' erroneously used for the far side of the moon, is a misconception.
The moon, Earth's only natural satellite, is very hard to miss in the night sky, unless it isn't there, of course. The moon undergoes phases due to its relationship with the Earth and sun.
During the phase of the moon where we cannot see the moon at all, the new moon, only the side of the moon we don't see is illuminated by the sun. As the moon orbits and rotates, more and more of the side we do see becomes visible to us, all the way up to the maximum, the full moon, when only the side we see is illuminated.
Although it may seem like we only see one side of the moon because it doesn't rotate, this isn't true. The moon does rotate and that is actually the reason why we only see one side of the moon. This is called synchronous rotation, a kind of rotation where it takes exactly as long for the moon to rotate on its axis as it does for it to make one orbit around the Earth.
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Back To CourseBasics of Astronomy
28 chapters | 325 lessons