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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The phases of the moon are not caused by Earth’s shadow. When the Earth’s shadow falls on the moon, that’s an eclipse.
The phases of the moon are cause by the fact that only half the moon is illuminated by the sun, and sometimes you see the illuminated half, sometimes the dark half and sometimes a little bit of each.
[Here’s a blog post that illustrates that.](https://alex.strinka.net/blog/why-does-the-moon-have-phases.html)
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