Let me just say, the Pixar movie Onward was fun. It’s your classic kids go on quest/gain new confidence story, but set in a world with elves and dragons and smartphones—you know, magic stuff like that.

Also, I am aware that it isn’t “real,” which means that they can make up whatever rules for physics they want. But that’s not going to stop me from thinking about real physics.

One of the things you see quite often in the movie are these two moons in the sky. They look cool, but they are also kind of troubling (at least to me). In case you haven’t seen the movie, here is my sketch of the two moons:

I’m assuming these are actual moons showing actual phases—but they are all wrong.

What Causes the Phases of the Moon?

It just so happens we have a real moon in our sky. You should go out and look at it sometimes. If you do, you will notice that its appearance changes over the course of a month. Sometimes it’s a big luminous disc and sometimes it’s just a sliver. But why is that?

You’d be surprised how many people think they know the answer, but actually don’t. Ask them what causes the varying phases of the moon, and here are some of the common (wrong) answers you will hear:

  • The Earth’s shadow on the moon
  • The rotation of the Earth
  • Clouds (the logic here escapes me)

Here’s the real answer: The phases of the moon are indeed caused by a shadow, but it’s essentially the shadow of the moon on itself. When light from the sun hits the moon, it illuminates only half of the moon’s surface, such that one side is bright and one side is dark. You could say the dark side is that way because the moon is blocking the sunlight. It’s sort of like the moon’s shadow.

A diagram will help. Here is a view of the Earth and moon along with the light from the sun (not drawn to scale).

Illustration: Rhett Allain



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