# How do moon phases work?

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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 earth’s shadow does not cause moon phases. The moon’s own shadow does, the same way that earth’s shadow makes it night on one side of earth. The moon goes around the Earth every 27ish days. The angle of the sun in relation to the moon when you see it determines how much of the side you can see is illuminated.

The earth’s shadow on the moon is a lunar eclipse/blood moon.

The Earth’s shadow is not what causes Moon phases.

At any given time, half the Moon is lit up by the Sun (just like half the Earth is). If that side of the Moon faces the Earth (which happens when the Earth is roughly in between the Sun and the Moon), you get a full moon (or occasionally a lunar eclipse when the vertical axis lines up perfectly too). If that side of the moon faces away from the Earth, you get a new moon (and occasionally a solar eclipse when the vertical axis lines up perfectly too). If it’s somewhere between those extremes, you get a crescent, quarter, or gibbous moon.

>I understand that it is the Earth’s shadow that causes moon phases.

That’s not true.

The phases are the moon’s shadow on itself. Consider you have a sphere, and you shine a flashlight on it. Then you can walk around and different shapes will be illuminated.

There’s only 2 options here: either there’s a solar eclipse, or the Sun isn’t *directly* behind. If the latter, it will cause a shadow, purely because of our line of sight.

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)