How Earth & the Sun Affect the Phases of the Moon

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  • 0:02 The Changing Moon
  • 0:40 The Moon's Changing Shape
  • 2:25 A Little Experiment
  • 3:35 Synchronous Rotation
  • 5:38 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson will describe how the moon, Earth, and sun combine to form the phases of the moon. We'll also discuss the important concept of synchronous rotation.

The Changing Moon

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.

The Moon's Changing Shape

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!

Why the new moon is dark
New moon phase

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.

A Little Experiment

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.

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