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.
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.
>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.
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)
The moon is just a ball hanging in space, just like the Earth. The same principles apply. The Sun shines on it, so the half of the ball that is facing the sun it is lit up, and the back half of it is dark. The Earth sees this ball from different perspectives over the course of its monthly orbit. When we can see only the fully lit half, we call it a full moon. When we are only seeing the dark half, it’s a new moon. When we can only see a bit of the lit-up side, it’s a crescent moon.
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.
>We sometimes see the moon during the day for the same reason we see it at night — it is reflecting light from the sun — and its closeness to Earth makes it brighter than the daytime or nighttime sky. After the sun, the moon is by far the brightest celestial object we can see.
[Source](https://www.livescience.com/why-moon-visible-daytime)
Thank you all so much. This is the first time I’ve came to Reddit for a random question and I wasn’t disappointed. I knew it had to be something simple, yet one morning after getting nowhere near enough sleep the question popped into my head and I almost had a midlife crisis because I couldn’t come up with an answer LOL.
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