I understand that it is the Earth’s shadow that causes moon phases. This works except for one problem, this morning the sun and the moon were out at the same time, which isn’t unusual except this time it dawned on me that if the whole Sun is out above the earth and the whole Moon is up above the Earth how is there still a shadow over the moon? Wouldn’t it have to be a full moon because the full sun is shining on it now? Or why wouldn’t the moon phases be horizontal instead of vertical?
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I’m afraid you understand incorrectly.
Imagine you’re in a dark room, and there’s a ball floating in front of you. You shine a torch on it. You can now see the whole side of the ball that’s facing you. Okay, so, you give the torch to your friend. They move behind you, still pointing the torch at the ball, and you can still see it.
Now, from this point, they’re going to slowly walk in a circle around you and the ball, still pointing the torch at the ball. When they are off to one side, you’ll be able to see half of the ball as a semicircle. When they move a bit further round, the bit you can see shrinks to a crescent. When they move behind the ball, one of two things will happen: if they are directly behind it, you won’t see the ball, but you’ll see the light from the torch glowing around it. If they’re a bit above or below, you just won’t see the ball very well.
This is basically the phases of the moon. It’s not the earth’s shadow on the moon. It’s just the angle between the earth (you), moon (ball), and sun (torch). When the ball perfectly blocks the torch, that’s a solar eclipse.
The only time the earth casts a shadow on the moon is during a *lunar* eclipse. This is like the torch being behind you. In that case, some of the light would probably spill through your hair and land on the ball, and you’d be able to see it pretty dimly.
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