FearOfPhysics.com: What Causes Phases of the Moon

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What Causes Phases of the Moon

Sometimes, when you look up and see the moon, it's a full circle. Other times, it's part of a circle. The different "shapes" of the moon that you see are called "phases" of the moon. The moon is shaped like a ball--this never changes, so what's with the different shapes that you see? Take a look at these video clips. The left clip shows what the Sun, Earth, and Moon might be doing, as viewed from outer space. The right clip shows you what you would see if you were looking up from your green city.

What the sun, moon, and earth are doing.

What you would see from your green city
(The Sun's light is coming from the right)

So here's where the phases of the moon come from: Unless it moves behind the Earth, half of the moon (that faces the Sun) is always lit by the Sun. That's why the moon always appears like a bright round ball in the left video. But look at where your green city viewpoint is in the left video; sort of off to the side of the moon. You are not looking at the moon directly-- from the side, you see part of the lit side, and part of the dark side. How much of each depends on exactly where you are, and where the Sun, Moon, and Earth are. What the moon would look like from your green city is shown in the right video.

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